Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Quote from: saturated on October 20, 2024, 02:38:50 AMI'm gonna say yes it's correct as it looks like a bipolar power supply but we better wait for verification

And I would think it is the capacitor on the left

You are correct. I followed the schematic back to the HT caps, and it was in fact the cap on the left. Now that the issue with the caps has been remedied. I'm having issues biasing the amp.

Based on a thread here in the forum. I should see 40/50mA across the two .6 ohm 10 watt resistors. However, I'm only seeing as high as 20mA with the bias pot nearly dimed clockwise.

However, it doesn't seem that the amp sounds like it should regardless of where I set the trim pot.

What am I missing here??

Any bias experts here in the forum regarding the Randall RG100-ES??

Quote from: Tassieviking on October 21, 2024, 02:13:31 PMI presume you are measuring either the positive or negative side of the amp, one 0R6 resistor is on the positive rail and one is on the negative rail.
If you use a 2 channel cro and run the cro on both resistors you would find one half sinewave on each resistor.

Quote from: RG100ESROX on October 20, 2024, 11:32:02 AMBased on a thread here in the forum. I should see 40/50mA across the two .6 ohm 10 watt resistors. However, I'm only seeing as high as 20mA with the bias pot nearly dimed clockwise.

However, it doesn't seem that the amp sounds like it should regardless of where I set the trim pot.

What am I missing here??

Any bias experts here in the forum regarding the Randall RG100-ES??

Quote from: g1 on October 23, 2024, 09:05:54 PMI don't know where that bias info came from but it sounds a bit suspect to me.
And the term 'both' resistors always leads to some confusion.  Sometimes it means 'each' and sometimes it means the 2 in series.
Better to just stick with measuring each and stating it that way.
Lots of amps bias with 10 to 20 mA current per output transistor.  10mA would be 6mV across each 0R6 resistor.  I think that may be sufficient to get out of crossover distortion, which is all we care about when biasing SS amps.  Any more than that will not help the tone in any way, and will run the output transistors hotter than they need to.
Set the bias to the minimum mV reading across each 0R6 (no signal applied).  What reading do you get?
Now get a nice clean sinewave at the output like the first pic in post #24, but with less amplitude, maybe 100mV peak to peak.  Do you still have a nice clean sinewave like that, or is there a crossover notch in the middle of the sinewave?
For solid-state amps, we just increase the bias til the crossover notch just disappears, no more is needed.

Quote from: RG100ESROX on October 25, 2024, 01:46:20 AM1) Can I power up the amp without any of the power transistors installed to take voltage readings...?

Answer 1):  Yes, I don't see a problem with that in this circuit.

2) Does this amp require a speaker to be plugged into it, or must a dummy load be used?

Answer 2):  No speaker or load required

3) Can I plug headphones into the direct out (DI) jack WITHOUT a speaker plugged in?? Or, must there be a dummy load attached in conjunction with the headphones?

Answer 3): no load required

4) Can the 120 VAC accessory outlet on the rear panel be wired at the power switch so that it can be controlled with the switch as well as the amp?

Answer 4):Yes, but the extra current will be through the amp's circuit breaker, so depends on what you want to plug in there

5) Any advantage to using an attenuator with this amp. (I've been a tube amp guy for so long I don't know the answer to this.)
Answer 5) can't see any advantage to this, will just make the amp run hotter which is worse for it

6) Dumb question, but...should I be getting a resistance reading between the + and - of the input jacks? I'm assuming so, but .4 ohms is kind of like a short, no? I'm getting about .4 ohms on the Hi input and 137K on the Lo input.

Answer 6):  The input jack's switch contact shorts it out when nothing is plugged in, to avoid hum and noise

7) Why am I getting a reading in both directions on a few of the diodes on the board?? Shouldn't I see an open in one direction??

Answer 7): parallel resistance in the circuit may give a reading in reverse-bias direction, and lower than normal reading in forward biased direction.  When in doubt, disconnect one end of diode from the circuit.

Quote from: Loudthud on October 25, 2024, 08:05:21 PMLooks like a thermister, marked TRM on the schematic.

Quote from: Loudthud on October 25, 2024, 11:00:41 PMBreaking the circuit would blow the Mains fuse or trip the circuit breaker. Thermisters and diodes are used to keep the bias current of the output (and driver) transistors the same as they heat up.

Quote from: Loudthud on October 26, 2024, 01:02:02 AMThese things are very difficult to replace, you'll need to get the exact part number and vendor from Randall, or beg them to sell you the part. Say a little prayer that the part isn't bad.

Quote from: RG100ESROX on October 25, 2024, 04:39:06 PMAlso, without the power transistors in, my channel LEDs don't work. Just want to be sure this is to be expected??

I'm also getting some higher than normal voltages around the bias pot without the power transistors. Is this also to be expected??

Quote from: g1 on October 26, 2024, 03:18:33 PM

Quote from: RG100ESROX on October 25, 2024, 04:39:06 PMAlso, without the power transistors in, my channel LEDs don't work. Just want to be sure this is to be expected??

I'm also getting some higher than normal voltages around the bias pot without the power transistors. Is this also to be expected??

Neither of these issues make sense to me.  With power transistors removed, main supply voltages may increase a bit but not much.  That should not affect the channel lights or the bias circuit.

Quote from: saturated on October 27, 2024, 11:35:29 AMI'm trying to figure out what you did

grounded my O-Scope to the Hi input jack

you connected the scope probe or the ground clip on the scope probe

Also...dude...stuff happens don't stress out about it

Quote from: g1 on October 27, 2024, 08:43:21 PMSee if the hum is DC or AC at the output.  If it's DC that is more serious.  If it's AC you may have just lost a ground somewhere (like a burnt or blown ground trace).
Any kind of DMM that has black probe connected to ground is potential trouble.  I only use fully floating battery powered DMM's.
Sounds like basically you grounded out which ever end of the 0.62 resistor you connected the black probe to.
Also, bias is only checked under no-signal conditions, and is a DC voltage reading at emitter resistors of output transistors.  So no scope or signal generator should be connected when checking idle current.
If looking for crossover distortion, that is with signal gen. connected to input, and scope connected at amp output.  It is not a bias measurement.

Quote from: g1 on October 27, 2024, 08:51:18 PMOne meter probe (black) to chassis, one meter probe (red) to the 'tip' connection of the output jack.  Set meter to DC volts, then check again with meter set to AC volts.

Quote from: g1 on October 27, 2024, 08:51:18 PMOne meter probe (black) to chassis, one meter probe (red) to the 'tip' connection of the output jack.  Set meter to DC volts, then check again with meter set to AC volts.

Quote from: saturated on October 28, 2024, 05:47:42 PMNice find  :tu:

but is that not a resistor ?

Maybe not I wonder what that diode symbol means

Quote from: saturated on October 28, 2024, 10:54:51 PMThat is some good work.  I would chill out and wait for the big dogs to help you get this sorted out  :dbtu:

Quote from: RG100ESROX on October 28, 2024, 10:17:41 PMWell, no sound. Other than the somewhat  diminished hum/buzz that I had previously.

I tested the thermistor and it worked very well when heat was applied. So, I don't think this is my issue.

I changed both TIP32C's and the one TIP31C. And both 1N5484's located near the bias trim pot.

Any thoughts??

What should I be checking next??

Quote from: phatt on October 29, 2024, 07:22:40 AM

Quote from: RG100ESROX on October 28, 2024, 10:17:41 PMWell, no sound. Other than the somewhat  diminished hum/buzz that I had previously.

I tested the thermistor and it worked very well when heat was applied. So, I don't think this is my issue.

I changed both TIP32C's and the one TIP31C. And both 1N5484's located near the bias trim pot.

Any thoughts??

What should I be checking next??

1N5484 is suspect,, No such diode can be found in my quick search.
The Schematic says 1n914 which are just small signal diodes.
Using the wrong part at that point will likely end in smoke.
don't frig anymore until you build the light bulb limiter.

This is a DC Amp which means, All the Tr's sit at different DC potentials so if something fails it can take out many parts in a flash of magic smoke.
Now I'm not the expert here but here is a brief run down.

Unlike a Valve amp there are no coupling caps between active Tr's, hence the term DC coupled (no Cap to decouple the DC of each section) The limiter will save $$ and lots of wasted time.

The input is similar to the PI input to Valve power amp, used differently.
The idea is to keep the bases of Q8 & Q9 And the Spk output at close to Zero Volts as possible. Q10 is the Voltage Amp. (I think these Tr's see the biggest voltage and I think they tend to run hotter than output at times)
Q11 & Q12 are predrivers for the Darlington pair outputs, formed by Q13 & Q15 and Q14 & Q16. The diode string and trim pot from Q10 is the bias circuit. the thermister usually mounted to heat sink tracks the heat and slightly alters the bias as the amp heats up.
Google differential SState power amp designs or something like that .. Phil.

Ed I just found this; https://www.eeeguide.com/bjt-power-amplifier-with-differential-input-stages/

Quote from: phatt on October 30, 2024, 09:03:13 PMYou need to get priorities in perspective.
forget the preamp the power amp has blown so please focus on that first. Fix one issue at a time otherwise you just make life hard for you as well as those here trying to help you. 8|

If it's all too hard to follow instructions then it's a fair bet one or more Tr's have blown so you could just replace them from Q10 onwards, with equivalent or higher rated. Q8 & Q9 are likely ok as they don't have to pass big current.

Then with no load and light bulb limiter bring up the voltage and check DC voltages shown on schematic. The bases of Q8 & Q9 as well as the speaker out should be very close to zero volts. (i.e. under 100mVDC)

****Be aware this circuit has No Short Circuit Protection like later designs so even a momentary short at speaker out will likely blow transistors****
 No need to ask me how I found that out  :duh
This is a very simple circuit so a good learning curve for you.

The simple fact that the trace burnt out tells you there has been a critical short causing big current to flow and melt the copper trace.
Take it one step at a time.
Hint listen to G1,, He has the experience to help you. :tu:
Phil.

Quote from: g1 on November 03, 2024, 09:41:22 PMSometimes, as the voltage is being brought up with variac, there will be current at various voltages just due to variance in the supply rails as they are coming up.
So, if you are bringing it up at different speed than you did last time, you may not get glow where you did previously, or vice versa.
I guess you are still showing -20VDC at output?
Next step would be to record DC voltages for E,B, and C of transistors in power amp section (Q8 thru Q16).

Quote from: RG100ESROX on November 03, 2024, 10:36:05 PM

Quote from: g1 on November 03, 2024, 09:41:22 PMSometimes, as the voltage is being brought up with variac, there will be current at various voltages just due to variance in the supply rails as they are coming up.
So, if you are bringing it up at different speed than you did last time, you may not get glow where you did previously, or vice versa.
I guess you are still showing -20VDC at output?
Next step would be to record DC voltages for E,B, and C of transistors in power amp section (Q8 thru Q16).

Voltages of EBC Q8~Q16.👍🏻

Are these readings with references to ground?

Also, please note. I am out of town until Wednesday evening. So, look for these voltage readings Thursday morning at the earliest.

Thanks again for all of your help.

Much appreciated.

-Jay

Quote from: g1 on November 04, 2024, 03:53:48 PMYes, DC volts to ground.
Good luck to Utah in the NHL game Tues. (old Jets vs new Jets).  :)

Quote from: Jazz P Bass on November 07, 2024, 12:29:15 PMThose voltage readings are not good.
Especially the -18 volts on the output bases.

Quote from: g1 on November 07, 2024, 09:17:48 PMFor now, focus on the TIP32 nearest to the trimpot (Q10).
You show +30V at the emitter.  It is a PNP transistor, so if good, the base can only be 0.7V more negative.  Yet you show -18V at the base, which is impossible if the transistor is good.

Quote from: g1 on November 08, 2024, 02:06:42 PMThe trim pot should be 250R, not 250K.  You should be able to measure it in circuit.  And there should be a resistor, R51 in parallel with it (value somewhere between 47R and 100R).
What do you mean, 'not able to measure resistance', is it giving you 'OL' indication?  Otherwise, a reading of zero ohms indicates a short circuit.  Always state the reading the meter shows.

Quote from: g1 on November 11, 2024, 02:32:52 PMDo diode check for E-B on three transistors above.  With black lead to base, then with red lead to base.

Quote from: RG100ESROX on November 12, 2024, 01:05:54 AM2nd TIP32C black to B/E = .284v
Red to B/E = .284v

Quote from: g1 on November 11, 2024, 02:32:52 PMDo diode check for E-B on three transistors above.  With black lead to base, then with red lead to base.

Quote from: g1 on November 12, 2024, 11:51:46 AM

Quote from: RG100ESROX on November 12, 2024, 01:05:54 AM2nd TIP32C black to B/E = .284v
Red to B/E = .284v

Disconnect either the base or emitter from the circuit.  Re-check.

Quote from: RG100ESROX on November 12, 2024, 12:00:22 PM

Quote from: g1 on November 12, 2024, 11:51:46 AM

Quote from: RG100ESROX on November 12, 2024, 01:05:54 AM2nd TIP32C black to B/E = .284v
Red to B/E = .284v

Disconnect either the base or emitter from the circuit.  Re-check.

2nd TIP32C with base removed from board/circuit black to B/E = .596v
Red to B/E = .OL

Quote from: g1 on November 12, 2024, 08:44:32 PMThose readings are good.  Yet it was giving you an impossible voltage reading in circuit (for a good PNP).
It's possible it is breaking down under voltages higher than your meter can provide on 'diode check' function.
Might be easiest to just replace it if you have one.

Quote from: g1 on November 13, 2024, 12:20:00 PMThen re-do the voltage readings on it (and see post #79).

Quote from: g1 on November 13, 2024, 01:52:11 PMThat's better, but as far as I know it is Q10.  Did you mean to say that, or is there a reason you called it Q12?

Are Q8 and Q9 replacements or the original parts?

Quote from: g1 on November 13, 2024, 08:56:49 PMThe DC voltages are way off, so no point in testing audio.  There is -18V at the output.
I can only go by the schematic.  It is up to you to find the parts that correspond to it, by following what the transistors are connected to according to the schematic.  From the connections and circuit components, you should be able to locate which transistor is which.
If you are unable to read schematics, there is not much more I can do.  Not meaning to be rude, but this is not a repair that can be accomplished just by random parts replacement.

Quote from: g1 on November 13, 2024, 09:07:16 PMYou need to get the DC voltages right.
Something in the bias string is not working.  Q10 is not turning on.  Possibly because Q8 is not on.  You had given voltages for one of the small transistors, but not sure if it was Q8 or Q9.  Q8 connects to R41 and base of Q10. 
Measure e,b,c voltages for Q8.
Measure voltages through the bias string (the bias string consists of:  R47, Q10, D7, D8, trim pot, R48, R49).

Quote from: g1 on November 13, 2024, 10:49:08 PMI need the voltages from the points I listed in post #99, according to the schematic.
I don't have a board layout so the picture doesn't get me anywhere. 

Quote from: RG100ESROX on November 13, 2024, 02:30:13 PMI have replaced all of the transistors from Q8 thru Q16. So, they're all new.

I believe Q8 and Q9 are the 2N5484s near the bias trim pot???

Quote from: g1 on November 13, 2024, 11:18:22 PM

Quote from: RG100ESROX on November 13, 2024, 02:30:13 PMI have replaced all of the transistors from Q8 thru Q16. So, they're all new.

I believe Q8 and Q9 are the 2N5484s near the bias trim pot???

I just caught this.  Q8 and Q9 should be TIS98, not TIS58 or 2N5484.  They should be bi-polar transistors, not Fets.  What were the originals and what is in there now?

Quote from: g1 on November 14, 2024, 01:43:43 AMIt's not possible for the power amp to work with Fet's (2N5484) in the Q8 and Q9 positions.  So either you are mistaken, or it has never worked for you.  :)
The power amp can't work until there are proper bi-polar transistors in those locations.

Q12 is the TIP32 with NO heatsink, R54 is the brown 100R below the black connector in the photo.  Straight down from there below the diode and cap is R48 the 1K.  The end of R48 that connects to Q12 base should also connect to one of the trim pot lugs.
If you don't have the connections, you will have to look for broken or burnt traces on the other side of the board.

Quote from: Loudthud on November 14, 2024, 03:45:32 AMWere the 2N5484s put into the board without crossing the legs or twisting the part around ? It seems like the TIS98s would have the Base as the middle leg, but the 2N5484s would have the Gate on one end.

The 2N5484 is rated at 25V BV(GSS) so not likely to survive in the power amp circuit.

Quote from: g1 on November 14, 2024, 02:02:41 PMWith series components in a circuit, it usually does not matter the order of them.
So with the 2 diodes and the trim pot, it's possible the order on the schematic does not match the board layout.
The schematic shows the trim pot below the 2 diodes, but it could be placed between them, or above, and it would still work the same.
If you are seeing a connection between the trimmer and Q10 collector, I guess the trimmer may be schematically above the 2 diodes.

Sort of like this:

bias string.jpg

Quote from: g1 on November 19, 2024, 08:36:24 PMI think at your last check the voltages were pretty close to the values shown in the ovals on the schematic? (aside from bias string due to schematic differences)
Verify around 1.2V at Q11 base and -0.6V at base of Q12.
If those are good, check voltage drops across R55,56,57,60 and see what kind of range you can get for those values by adjusting trim pot.

Quote from: g1 on November 20, 2024, 12:34:23 AMI think you are getting Q10 and Q12 mixed up.

Quote from: g1 on November 14, 2024, 01:43:43 AMQ12 is the TIP32 with NO heatsink,

Quote from: g1 on November 20, 2024, 02:18:13 PMYes. From before  :)

Quote from: g1 on November 14, 2024, 01:43:43 AMQ12 is the TIP32 with NO heatsink,

Also, from the schematic you can see that Q10 gets positive supply (at emitter), Q12 gets negative (at collector).

Post voltages for the various points along the diode string, then we can figure out the order (because it doesn't match schematic).
DC volts at both sides of each of the 2 diodes, and at both sides of the trim pot.

Quote from: g1 on November 20, 2024, 09:23:35 PMLooks like you are almost there.  Just need a bit more current through the bias string which will bring the Q11 base voltage up a little.  It's around half a volt and should be around a volt.

Are you still on the variac and bulb?  Supply voltages should be around 40V and I think you are still around 30.  That may be the reason.

So get the AC line voltage up where it should be and see if you get any change with the bias trimmer.
If not, you can try changing that resistor that is in parallel with the trim pot. 

Schematic shows "47 to 100 ohms".  You have 47 there so you could try replacing with 100.

Quote from: saturated on November 21, 2024, 06:56:46 AMThis has gotten way past my pay grade  :lmao: but seems like there is an open somewhere maybe go back and check anything that was installed/removed/replaced and or pinouts of said components (?)

Good luck guys  :dbtu:

Quote from: g1 on November 21, 2024, 11:18:37 AMYes, a dim bulb is ok.  There is a bit of current used to run the power supplies.  Bring it up to 120V.
I don't expect the Q12 base voltage to change much, as it can only go max approx. 0.7V difference from the center point (this is because of the forward biased B-E junction of Q12).
But Q11 base voltage should change more as you move the trimmer. 
Then, as the voltage difference between Q11 base and Q12 base increases, you should start seeing the output transistors start conducting, which will put some voltage across those .27 and .6R emitter resistors.

Quote from: g1 on November 21, 2024, 04:02:40 PMAt Q12, did you mean -16.5mV rather than V ?
If so, try doing the resistor swap at R51.

Quote from: g1 on November 21, 2024, 04:30:57 PMYes that's a good thing.  Everything seems stable and approx. where it should be.
Try the resistor swap and see what it does for the voltage across those big emitter resistors.

Quote from: g1 on November 21, 2024, 05:41:01 PMThat is across the resistor (one probe to each end) ?
Did you try turning the trim post through it's full sweep?

Quote from: g1 on November 21, 2024, 07:46:10 PMMeasure the voltage at Q11 base, with trimmer fully CCW, then with trimmer full CW.
Then do same for Q12 base.

Quote from: g1 on November 21, 2024, 08:26:33 PMThat seems like enough range.
Time to check the voltages at the output transistors.

Quote from: g1 on November 21, 2024, 09:17:17 PMQ13 base should be connected to Q11 emitter.  Why aren't the voltages the same?

Quote from: g1 on November 21, 2024, 10:15:55 PMQ11 collector connects to the + supply rail, so I think you are mistaken.
You showed around .45V at Q11 emitter before, that's what should be at Q13 base.  Find out why it's not.  Triple check voltages at Q11 and Q13, and their pinouts.

Quote from: g1 on November 22, 2024, 01:39:21 PMThe schematic is not wrong. 
Don't use continuity beeper, some will beep as high as 200 ohms.
Measure ohms on resistance range.  A good connection will measure .1 ohms or less.  This is in addition to whatever your meter shows when you short the 2 probes together.

Not sure what happened to Q11 emitter voltage, as you had it there before (around half a volt).

Quote from: Loudthud on November 22, 2024, 05:26:39 PMThat signal is about 3KHz. Is it coming from a generator, or is the preamp creating it ?

Quote from: RG100ESROX on November 22, 2024, 02:14:49 PMJust so I am clear. What order do the power transistors go in??

Bottom side view from left to right??

Quote from: g1 on November 22, 2024, 05:42:00 PM

Quote from: RG100ESROX on November 22, 2024, 02:14:49 PMJust so I am clear. What order do the power transistors go in??

Bottom side view from left to right??

From left to right in your photo:  Q15,13,14,16

Quote from: Loudthud on November 22, 2024, 07:19:47 PMWhat is the DC at the output with no signal ?

Because this thing has a pretty high output impedance, the signal amplitude at the output as well as the DC offset could change considerably when you connect a dummy load.

Quote from: g1 on November 23, 2024, 01:58:50 PMStill can't get any mV reading across the 0R68 power resistors?
Those are the only 2 that will show idle current.  Q15 and Q16 will be off when idling.
The '97 model uses the same basic power amp but they have removed the 22R resistor that is across D8.
I would try removing that one and see if you can then get a reading across R55,R56.  Start with the trimmer full CCW.
If you can, and the pot is now too sensitive, change the resistor across the pot back to 47R (instead of the 100R you put there).

Quote from: g1 on November 23, 2024, 04:03:54 PMWe are now adjusting bias, not making the amp work.
It will work without the output transistors running idle current, and may well have been doing so before you ever touched it.
You found originally that it could only be turned up to 25mV across the 0R68, but it may have been set at zero.
Changes to the drivers and output transistors may have resulted in no longer being able to get any reading at all, due to variance of the new parts.
The reason I looked to the 22R was that you were not getting sufficient voltage difference between the driver bases (Q11 and Q12).
With 2 diodes in the bias string, normally there would be around 0.7V drop across each.  But with the low resistance of 22R across D8, the voltage across D8 would be reduced.
Looking at the '97 schematic confirmed my suspicion, as they have removed it for use with the more modern output transistors. (see attached)
Your scope shot in post #143 shows no evidence of crossover distortion, and that was before you had any mV reading across the 0R68's.  Crossover distortion would show as a notch in the sides of the sinewave.
It will usually be most evident at low volume levels, so try doing that same test on the scope, but with very low output, like 50mV peak to peak or something like that.
I don't think it will matter whether a load is attached, but you are probably ok to try that now too.

Quote from: g1 on November 23, 2024, 06:35:00 PMThere is no signal there.  Just noise and ripple from the power supply.  Are you still injecting 3Khz?  What level of 3K were you injecting before, and what now?

Quote from: g1 on November 23, 2024, 08:17:11 PMIs that at the output of the amp?  It looks good, but would like to see at a much lower level.
Drop your source voltage from .5V to .05V if you can.
That should drop the output voltage to about 1/10th of what it is now (4Vpp).

Quote from: g1 on November 23, 2024, 09:03:32 PMDo you have a load connected?  Otherwise the outputs shouldn't be getting hot.

Quote from: g1 on November 23, 2024, 09:17:18 PMNo, if you are getting heat you should not connect load or speaker.
Not sure why you are getting heat on the power resistors with no load.
Possibly your meter is not registering the real DC voltage across the 0R68's ?
How good is your meter, especially at low mV levels?

Quote from: g1 on November 23, 2024, 09:37:12 PMWell, previously you said there was 0V DC at the speaker output.  How can there be 42V at and emitter resistor?  Is it open, or do you have DC on the output now.

Quote from: RG100ESROX on November 22, 2024, 08:02:19 PM

Quote from: Loudthud on November 22, 2024, 07:19:47 PMWhat is the DC at the output with no signal ?

Because this thing has a pretty high output impedance, the signal amplitude at the output as well as the DC offset could change considerably when you connect a dummy load.

Voltage at the output is -0.023VDC with no signal.

Quote from: RG100ESROX on November 23, 2024, 09:42:23 PMWith reference to chassis ground on the positive side of the two far right power resistors, I get about 42.0VDC. Across the resistors there was 0.0V.

I'll check for DC on the output, and check the voltage of the two resistors again...

-42.0VDC on the far right .62R and .27R +, and  -.020mV on the far left power resistors +.

On the output I have  -.022VDC.

Quote from: g1 on November 24, 2024, 12:42:43 PM

Quote from: RG100ESROX on November 23, 2024, 09:42:23 PMWith reference to chassis ground on the positive side of the two far right power resistors, I get about 42.0VDC. Across the resistors there was 0.0V.

I'll check for DC on the output, and check the voltage of the two resistors again...

-42.0VDC on the far right .62R and .27R +, and  -.020mV on the far left power resistors +.

On the output I have  -.022VDC.

+42V vs -42V at the right resistors is radically different.  Triple check all your info before you post.  It's impossible to work with faulty data.

If there is no DC across any of the .68 or .27 power resistors, there should be no heat.
Do they get warm idling with no signal?

Quote from: g1 on November 24, 2024, 08:27:35 PMNot sure why they would run cooler now since they are showing voltage, but it sounds like an improvement.
I'll post what I was writing anyway.

Some of those 5W resistors are for the power supply and will get hot when on.  As they are radiating heat, the closest 0R68 10W will get warm too.
If that is the only one of the four 10W warming up, then no worries.

Also, if you still have your jumpers in the footswitch jack, you should remove them.  You should only use LED's there, not straight jumpers.  The footswitch adds LED's in series.  Straight jumpers allow too much current which will stress the front panel LED's and make two of the 5W resistors run hotter than they should.

Do another scope shot like post #158, but turn your volume control down a bit so you only have 0.1V (100mV) Vpp at the scope.  Keep the trimmer at full CCW.

Quote from: g1 on November 24, 2024, 10:22:10 PMThat is just power supply ripple again.  You can see on the scope readout that it is approx. 120Hz.
The first pic says 1Vpp at 0 Hz, so not sure what is going on there.
Get your 3Khz back up to a level where the scope says 3Khz.  Reduce the amp volume while increasing the scope sensitivity, so it shows a larger waveform from top to bottom.  Try to get around .1Vpp at 3Khz.

Quote from: g1 on November 25, 2024, 10:33:02 AMHook the scope up to the signal generator.  Make sure you have signal there.
If so, connect gen. to amp and scope at the FX send.  Do you get signal?

Quote from: g1 on November 25, 2024, 01:24:03 PMIt says .02V on that scope shot.  Is that what it is set to put out, or is that the most it can do?

Quote from: g1 on November 25, 2024, 03:37:05 PMThat's fine if you had the gen. set for low level output.
Check the amps power supply (DC)voltages, shown as  A,B,C,D,E on the schematic.

Quote from: g1 on November 25, 2024, 08:26:20 PMThose all seem good.
Where are you getting the voltages you were concerned about in post #177 ?
Have you tried scoping at the master volume pot lugs to see if signal is getting that far?
Use a gen. level that was giving you good output before.

Quote from: Loudthud on November 25, 2024, 10:48:27 PMThose Voltages in the preamp are close enough for Rock and Roll. About as good as you can get with JFETs and fixed (not pots) resistors.

Quote from: Loudthud on November 25, 2024, 11:06:11 PMMyself, I would look at the output with a scope and drive it to clipping with no load. If the amp survives, try it with a dummy load resistor. If that's OK, hook up a speaker and Rock ON !

Quote from: Loudthud on November 26, 2024, 01:12:11 AMBias current is usually adjusted with no signal, then look at the output with a scope and a small signal to confirm there is little to no notch where the sine wave crosses zero Volts. Sometimes a dummy load will disturb the DC readings across the emitter resistors in the power amp if there is DC Voltage on the output. In that case, disconnect the dummy load before adjusting the bias. Then reconnect the load to check for crossover notch. In some cases you might adjust the bias with a small signal to observe the crossover notch and set the bias to the lowest value that eliminates the notch. If you are really diligent, you might check the bias when the amp is hot after being played for some time to see if the temperature compensation is working as it should. You should see no notch, but bias current may have drifted up of down with the change in temperature. That's OK, don't adjust the bias unless you see a gross notch.

Clipping is when the input signal is too big for the power amp to reproduce without distortion on the peaks. The peaks of the sine wave are "clipped" off so the wave starts to look more like a square wave. This insures that the transistors in the power amp can withstand the Voltages that they might see in normal use, not just the Voltages with no signal.

The next test is with a dummy load. This test makes sure the transistors can withstand the current required for the amp to produce full power. If the power amp can pass this test, it us usually good to go.

Quote from: RG100ESROX on November 26, 2024, 02:22:31 AMQ: Should I be concerned with the millivolts across the .62R resistors while biasing the amp?? Does this matter?? What's considered to be too high of a voltage (if there is?)

Quote from: RG100ESROX on November 26, 2024, 02:22:31 AMOr am I only concerned with the notch in the sine wave?? Which there is very little of at min bias adjustment.

Quote from: RG100ESROX on November 26, 2024, 02:22:31 AMAlso, I used to get -.6v on the bottom lug of the trim pot which I don't have anymore. It's now a positive voltage. Starting at .876V at the minimum bias setting. It only increases with the adjustment of the bias pot.

Quote from: RG100ESROX on November 26, 2024, 02:22:31 AMI also have a bad hum that gets worse as the bias is increased.

Quote from: RG100ESROX on November 25, 2024, 11:44:15 PM

Quote from: Loudthud on November 25, 2024, 11:06:11 PMMyself, I would look at the output with a scope and drive it to clipping with no load. If the amp survives, try it with a dummy load resistor. If that's OK, hook up a speaker and Rock ON !

When you say drive it to clipping. You mean adjust the bias up to the point where it just starts to clip?

How much clipping are we talking about here??

We're not going to be measuring any voltages to bias the amp??

Quote from: J M Fahey on November 26, 2024, 11:12:26 AM

Quote from: RG100ESROX on November 25, 2024, 11:44:15 PM

Quote from: Loudthud on November 25, 2024, 11:06:11 PMMyself, I would look at the output with a scope and drive it to clipping with no load. If the amp survives, try it with a dummy load resistor. If that's OK, hook up a speaker and Rock ON !

When you say drive it to clipping. You mean adjust the bias up to the point where it just starts to clip?

How much clipping are we talking about here??

We're not going to be measuring any voltages to bias the amp??

No PLEASE!!!
Completely unrelated and you will overheat/blow your amp.

Do not mess with bias which clearly you do not understand.
Sorry. 

Be safe.

Quote from: g1 on November 26, 2024, 01:43:44 PMYou had around 15mV across the .68 with the trimmer set somewhere around mid-point I think.
That is a safe level.  Leave it there and proceed with the crossover notch info/questions that have been discussed.  Post scope shots.
The difference in voltage at the trim pot was probably due to the removal of the 22R across D8.  You don't need to worry about that, unless you find DC on the output, which would indicate a problem.

Quote from: g1 on November 26, 2024, 06:53:23 PMPlay it.

Quote from: saturated on November 26, 2024, 07:17:05 PM

Quote from: g1 on November 26, 2024, 06:53:23 PMPlay it.

200w.gif

 :tu:

Quote from: saturated on November 27, 2024, 09:18:34 AMAre they supposed to be insulated like with mica  xP

Quote from: Miyagi_83 on November 27, 2024, 06:36:32 AMYou most probably know all of this and you probably think the same, but the thing that I would do is the following:
1. reread this thread,
2. write down (I'd do that on paper) all suggestions provided by the pros (g1, Loudthud, et al.),
3. try to recall everything that led to the amplifier's working properly, or at least to an acceptable degree.

Once you've discovered where you fouled up, maybe it'll be possible to undo the changes that caused the amp to misbehave.

Unfortunately, I can't offer much assistance because solid-state amplifiers are still beyond me.
Good luck!
M.

Quote from: RG100ESROX on November 27, 2024, 11:57:29 AMAt this point, the amp seems to be fine with the exception of it sounds like it has crossover distortion in it

Quote from: Miyagi_83 on November 27, 2024, 01:41:15 PM

Quote from: RG100ESROX on November 27, 2024, 11:57:29 AMAt this point, the amp seems to be fine with the exception of it sounds like it has crossover distortion in it

Let me check. I'll be back...

Yep. She sounds just fine from the FX send into another amp. That's a relief.

So, we now know that the preamp section is fine, and the issue is in the power section???

Great idea!!

Quote from: g1 on November 27, 2024, 02:33:31 PMYou are going to blow up your amp if you try to bias it for tone.
Stop replacing parts that you have not proven defective.

Post data everytime you make an adjustment.  Scope shot of the waveform, and DC readings across the .68 resistors with no signal.

Start by turning the bias all the way down and posting a pic of the scope showing the crossover notch so we can be sure we are talking about the same thing.
Get an output the same level as post #192 but turned the trimmer full CCW so you get the crossover notch.  Post the picture of the scope.

Quote from: RG100ESROX on November 27, 2024, 01:46:43 PMYep. She sounds just fine from the FX send into another amp. That's a relief.

So, we now know that the preamp section is fine, and the issue is in the power section???

Quote from: RG100ESROX on November 27, 2024, 11:57:29 AM... if I unground the 'effects return' jack. My sound goes away, and I get a pretty good hum out of the speaker.

Quote from: Miyagi_83 on November 27, 2024, 04:21:07 PM

Quote from: RG100ESROX on November 27, 2024, 01:46:43 PMYep. She sounds just fine from the FX send into another amp. That's a relief.

So, we now know that the preamp section is fine, and the issue is in the power section???

That's how I understand it.

Quote from: RG100ESROX on November 27, 2024, 11:57:29 AM... if I unground the 'effects return' jack. My sound goes away, and I get a pretty good hum out of the speaker.

You mean you disconnected the ground connection on the return jack, correct? If so, the hum could be a normal thing, kinda like if you plugged a cable into the input, but no guitar. I'm just speculating here, though.

Quote from: RG100ESROX on November 27, 2024, 03:04:53 PMQ: Is there a reason my sine wave is so jittery??

Quote from: g1 on November 27, 2024, 06:50:29 PMIs that scope shot in post #206 supposed to be showing the crossover notch?  Because there is none there.

Quote from: Miyagi_83 on November 27, 2024, 05:45:36 PM

Quote from: RG100ESROX on November 27, 2024, 03:04:53 PMQ: Is there a reason my sine wave is so jittery??

What do you mean?

Quote from: RG100ESROX on November 27, 2024, 07:34:14 PMI have to pause the live sine wave because it is jittery on the screen. It's not steady.

Quote from: RG100ESROX on November 28, 2024, 01:06:11 AMThe return jack IS grounded when nothing is plugged into it.

Quote from: Miyagi_83 on November 28, 2024, 01:31:22 AM

Quote from: RG100ESROX on November 28, 2024, 01:06:11 AMThe return jack IS grounded when nothing is plugged into it.

Ok, another question.
By 'ungrounding' the return jack you meant plugging something into it, right? Not disconnecting the return jack ground by, for example, unsoldering it?
If that's the case I'd like to clarify something.

The return jack is very often (usually?) a switching jack configured so that when a cable is plugged into it, it breaks the connection between the preamp and the power amp. It's as if you put an SPDT switch in series with the preamp out and power amp in and flipped it off. The return jack, unlike the PREAMP input jack, does not ground its input signal with nothing in because that would shunt the preamp out to ground.

Now, if your signal generator is connected to the PREAMP input AND there is some plug in the loop return jack, you will only get a faint signal at the output, if any at all. Simply because the connection is broken, as if our imaginary SPDT switch were open.
*Unless you've got a parallel loop which doesn't seem to be the case here.

Quote from: RG100ESROX on November 28, 2024, 01:42:09 AMNow I know why the amp made a pretty big humming noise

Quote from: RG100ESROX on November 28, 2024, 01:06:11 AMYes. Both. Snake AND float.

Quote from: Miyagi_83 on November 28, 2024, 02:31:50 AM

Quote from: RG100ESROX on November 28, 2024, 01:42:09 AMNow I know why the amp made a pretty big humming noise

Exactly. When you plug a cable into the loop return, most likely, there is nothing to attenuate the input noise, like a gain pot, tone controls etc. because probably also the master volume potentiometer is in the preamp section.

Now that you know that your preamp is good, I suggest you do the following for signal testing your power amplifier (correct me if I'm wrong, someone):
- get a spare jack socket, a patch cable, and a 10-22k linear (B taper) potentiometer;
- connect the pot so that the wiper is tied to the hot terminal of your socket and one of the extreme lugs is tied to the ground terminal. Breadboard, solder, alligator clips, whichever you find convenient. This way you will be able to attenuate the signal from your generator to low levels, which g1 suggested earlier;
- plug the cable into the jack socket;
- hook up your generator's signal clip to the free lug on the potentiometer and the ground clip to ground of the whole setup;
- finally, insert the patch cable into the return jack. This way the preamp won't be in the signal path and so it won't introduce its distortion.
EDIT: In case I'm being unclear, I've drawn a diagram for it.

Quote from: RG100ESROX on November 28, 2024, 01:06:11 AMYes. Both. Snake AND float.

I don't know if this is going to make any difference, but why not try? Have you tried adjusting your scope's parameters, such as volts per division or milliseconds per division?

Hope this helps,
M.

QuoteThank you so much for investing to such a degree in a problem that isn't even yours.

QuoteHopefully you will be available Sunday or Monday to respond to my replies regarding my findings.

Quote from: Miyagi_83 on November 28, 2024, 04:41:37 PMJay, as regards your sine wave dancing around the screen, read this post (https://www.ssguitar.com/index.php?topic=5431.msg42881#msg42881) and Loudthud's reply just under it.

QuoteThank you so much for investing to such a degree in a problem that isn't even yours.

That's how I understand a forum - a place where people discuss things, including trying to solve other people's problems. Although, actually, the thanks should go to g1, IMO. After all, he's been involved in this thread the most.
*This is not to suggest that you have ever forgotten to thank him.

QuoteHopefully you will be available Sunday or Monday to respond to my replies regarding my findings.

I should be available, but remember that I reside in a timezone that's 8 hours later than yours ;) What's more, solid-state power amplifiers are terra incognita for me, but I'll try to help you to the best of my knowledge, hopefully, with supervision from the more experienced people here.

Happy Thanksgiving to you and yours!
M.

Quote from: Miyagi_83 on November 28, 2024, 02:31:50 AM

Quote from: RG100ESROX on November 28, 2024, 01:42:09 AMNow I know why the amp made a pretty big humming noise

Exactly. When you plug a cable into the loop return, most likely, there is nothing to attenuate the input noise, like a gain pot, tone controls etc. because probably also the master volume potentiometer is in the preamp section.

Now that you know that your preamp is good, I suggest you do the following for signal testing your power amplifier (correct me if I'm wrong, someone):
- get a spare jack socket, a patch cable, and a 10-22k linear (B taper) potentiometer;
- connect the pot so that the wiper is tied to the hot terminal of your socket and one of the extreme lugs is tied to the ground terminal. Breadboard, solder, alligator clips, whichever you find convenient. This way you will be able to attenuate the signal from your generator to low levels, which g1 suggested earlier;
- plug the cable into the jack socket;
- hook up your generator's signal clip to the free lug on the potentiometer and the ground clip to ground of the whole setup;
- finally, insert the patch cable into the return jack. This way the preamp won't be in the signal path and so it won't introduce its distortion.
EDIT: In case I'm being unclear, I've drawn a diagram for it.

Quote from: RG100ESROX on November 28, 2024, 01:06:11 AMYes. Both. Snake AND float.

I don't know if this is going to make any difference, but why not try? Have you tried adjusting your scope's parameters, such as volts per division or milliseconds per division?

Hope this helps,
M.

Quote from: RG100ESROX on November 29, 2024, 12:48:37 PMIs this what you had in mind???

Quote from: Miyagi_83 on November 29, 2024, 12:56:47 PM

Quote from: RG100ESROX on November 29, 2024, 12:48:37 PMIs this what you had in mind???

Almost. There should be no red wire between the pot and the jack plug.

Quote from: RG100ESROX on November 29, 2024, 02:22:57 PMHow important is it that the pot be a log (B) taper??? All I have at home are audio taper pots.

Quote from: Miyagi_83 on November 29, 2024, 03:28:38 PM

Quote from: RG100ESROX on November 29, 2024, 02:22:57 PMHow important is it that the pot be a log (B) taper??? All I have at home are audio taper pots.

It's not going to explode if you use a log (A, audio) pot. The difference lies in the rate at which the resistive divider is formed as you turn the knob. In an ideal linear (B) potentiometer the resistance value will change, as the name suggests, linearly. A log potentiometer will reach about 10% of its value at 50% rotation. Do a search for 'log pot graph' / 'log pot curves' or something along those lines. A picture is worth a thousand words.

Quote from: g1 on November 29, 2024, 07:59:48 PMThe floating/dancing/jitter on the scope sounds like a triggering issue.  The scope should have a few choices for trigger source.  For example, some have norm, auto, or line to choose from.  Try the different trigger options and see if you can get the waveform to lock.

The waveform you posted before did not have any crossover distortion, and that was with the trimmer all the way down.  None of the other pics have shown any crossover notch either, so I would just leave the trimmer set to minimum.
I think maybe your meter is not accurate at the low mV levels, and maybe it is biased hotter than what the meter is showing.  Do you have access to a better quality meter to try?
When you hear hum that gets worse as you turn up the bias, that is probably due to increased loading of the power supply.  Proper bias levels should not add any kind of hum.
Crossover distortion would only be noticeable at very low volumes, like on the decay of a note.  It would sound like a bit of fizzyness or graininess. This is not what I am hearing. It's more like having too much distortion, and not the good kind.
Other than that, any audible effect you are hearing from changing the bias is due to loading down the power supply and making more 120Hz ripple, which modulates the signal in different ways.  So you don't ever want to adjust bias in a solid state amp by ear.

Here is a picture of the crossover notch, which I don't think you have had at any time with your amp.

classb01.gif

Quote from: phatt on November 30, 2024, 11:07:10 PMAt RG100
From post11,
When you say;
  "It used to sound just like the tone in the Dokken 'Just Got Lucky' (guitar only) video."

Well there are a lot of things besides the amplifier that can alter the outcome.
so,,, the Q you need to ask yourself is,
What was your whole setup before it all went south?

Did you change speakers? Did you change PU's or electronics in guitar?
Many things besides an amplifier can easy rob you of sound quality.
Just changing speakers can alter tone in a DRAMATIC way. 4 G12's in a box will sound very different to a single G12 in an open back cab
Some speakers might flab more so that others.

At post 13;
You mentioned you have taken a speaker out of the Randell cab and mixed in another speaker. That will change the sound big time when compared to the original Quad setup.
How about going back to the setup you had BEFORE it went wrong,,,then start all over again.

Dirty or corroded connections can cause signal loss, guitar cords can slowly cause signal loss due to broken strands. It seems to me that you have made a false assumption,, ""as G1 has noted there is no crossover distortion.""
And besides biasing ANY amp hotter does not make for better tone.
A Bias pot is NOT a Friggin GAIN knob. It has nothing to do with the gain of the amp, it just sets the balance between the 2 output devices.

Yes there is a lot of hype regards to biasing Valve rigs hotter to get the Mojo sound. well they don't understand what they are talking about. I have shown a few players in real time the sonic result of hot verses cold bias.
Here is what happens, with hot bias the amp starts to break up on 4.
With cold bias it starts breakup on 7.
So hot or cold the amp sounds the same. only Q left to ask the owner is how often do you want to replace your power valves.

The SS amp design you have is not going to give you power amp Dirt or sag as they don't work the same way. Valve amps have a soft power supply while SS use a stiff rigid PSU but you can design them to sag a bit with some mods.

If the Amp is working,,,My advice, go back and Establish just where it all went wrong. Because it's a fair bet something else might be the culprit. 8|
Phil.

Quote from: phatt on December 02, 2024, 07:12:39 AMThanks Jay (I assume) I appreciate the effort you have gone to, to explain it all.
I know it must be very frustrating for you. :grr

I had a hair line crack hidden under a solder pad in an amp that drove me nuts for a very long time. It was by chance that I turned the amp on just before I turned on the bench light one evening and saw a tiny flash on the circuit board which led me to the answer to the intermittent bad hum.
The crack was one of the main filter caps solder joints. The slightly over filled solder was covering the Crack in the copper track.
Phil.

Quote from: RG100ESROX on December 02, 2024, 11:12:35 PMI expected to be home by now, but, my dad took a nasty fall yesterday. So, I will be staying with the folks a little longer than expected.

However, I will be running home tomorrow morning to retrieve my amp, soldering station and tools, so that we can continue this adventure without too much delay.

Thanks for everyone's patience, guidance and help. Most appreciated.

-Jay

Quote from: RG100ESROX on December 02, 2024, 01:26:53 AM

Quote from: g1 on November 29, 2024, 07:59:48 PMThe floating/dancing/jitter on the scope sounds like a triggering issue.  The scope should have a few choices for trigger source.  For example, some have norm, auto, or line to choose from.  Try the different trigger options and see if you can get the waveform to lock.

The waveform you posted before did not have any crossover distortion, and that was with the trimmer all the way down.  None of the other pics have shown any crossover notch either, so I would just leave the trimmer set to minimum.
I think maybe your meter is not accurate at the low mV levels, and maybe it is biased hotter than what the meter is showing.  Do you have access to a better quality meter to try?
When you hear hum that gets worse as you turn up the bias, that is probably due to increased loading of the power supply.  Proper bias levels should not add any kind of hum.
Crossover distortion would only be noticeable at very low volumes, like on the decay of a note.  It would sound like a bit of fizzyness or graininess. This is not what I am hearing. It's more like having too much distortion, and not the good kind.
Other than that, any audible effect you are hearing from changing the bias is due to loading down the power supply and making more 120Hz ripple, which modulates the signal in different ways.  So you don't ever want to adjust bias in a solid state amp by ear.

Here is a picture of the crossover notch, which I don't think you have had at any time with your amp.

classb01.gif

The floating/dancing/jitter on the scope sounds like a triggering issue.  The scope should have a few choices for trigger source.  For example, some have norm, auto, or line to choose from.  Try the different trigger options and see if you can get the waveform to lock.
I do have a couple of choices for the trigger. I will try them and see if the sine wave stops moving around.

The waveform you posted before did not have any crossover distortion, and that was with the trimmer all the way down.  None of the other pics have shown any crossover notch either, so I would just leave the trimmer set to minimum. Will do. I'll leave the trim pot set to FCCW.
I think maybe your meter is not accurate at the low mV levels, and maybe it is biased hotter than what the meter is showing.  Do you have access to a better quality meter to try?
No. I wish I did, but I don't.
When you hear hum that gets worse as you turn up the bias, that is probably due to increased loading of the power supply.  Proper bias levels should not add any kind of hum.
Crossover distortion would only be noticeable at very low volumes, like on the decay of a note.  It would sound like a bit of fizzyness or graininess.
Other than that, any audible effect you are hearing from changing the bias is due to loading down the power supply and making more 120Hz ripple, which modulates the signal in different ways.  So you don't ever want to adjust bias in a solid state amp by ear. Understood. Thank you.

Here is a picture of the crossover notch, which I don't think you have had at any time with your amp. I did have this notch in my sine wave at one point, and it looked just like your illustration. If I adjusted the bias about 5mA or so, it would go away. Why I no longer have it is beyond me. If it reappears, I'll take a photo and post it here. Thank you for all of the great info and illustration. Much appreciated. I should be home by tomorrow evening. Is there anything specific you want me to try when I get home? I am going to try 'Miyagi's' suggestion using the trim pot and injecting the sine wave at the FX return jack. If you'd rather I not try this, please let me know. Otherwise I'll go ahead with it.

Quote from: RG100ESROX on December 04, 2024, 06:09:03 AMAlso, the amp is somewhat functional. We just need to figure out what's causing the extra unwanted distortion. The sine wave showed no sign of Xover, so It has to be something else that's causing this issue. But what? What in the power section would/could be the cause? The transistors are all new. Most of the resistors are new. I've meticulously checked continuity between all of the components...

Quote from: Miyagi_83 on December 04, 2024, 06:33:29 AM

Quote from: RG100ESROX on December 04, 2024, 06:09:03 AMAlso, the amp is somewhat functional. We just need to figure out what's causing the extra unwanted distortion. The sine wave showed no sign of Xover, so It has to be something else that's causing this issue. But what? What in the power section would/could be the cause? The transistors are all new. Most of the resistors are new. I've meticulously checked continuity between all of the components...

Just a thought, have you tried a different speaker cabinet?

Quote from: Miyagi_83 on December 04, 2024, 06:43:53 AMOk, so it's not the speaker. What about mechanical rattle in the amplifier's chassis or guitar?

Quote from: Miyagi_83 on December 04, 2024, 07:10:42 AMWhat about these two plugs (marked with pink-ish arrows)? Shouldn't they be connected?

Quote from: RG100ESROX on December 04, 2024, 07:31:39 AMThey are. I disconnected that one so I could photograph the board better.

Quote from: g1 on December 04, 2024, 02:50:55 PMIf you connect generator to the FX return jack, can you get full power with a clean sine wave into a dummy load?
If so, what signal level is required from the generator to get there?

Quote from: Miyagi_83 on December 04, 2024, 03:50:24 PMJay, the dummy load will get hot. It soaks power in, so that's the way it is. If you haven't done it yet, put a heatsink on it.

If you plug your signal generator into the FX loop return, the preamp should make no difference. Personally, however, I would turn every knob on the preamp down to zero in case any noise were to bleed through. Maybe I'm paranoid, I don't know.

I don't know what circumstances you are under at the moment, but there are a few questions and requests that you haven't responded to.

From g1
1. If you connect generator to the FX return jack, can you get full power with a clean sine wave into a dummy load?
If so,...
2. what signal level is required from the generator to get there?

From me
3. Are you sure the distortion is not mechanical rattle? You know, from a loose screw, bolt or nut on a transistor, socket, switch, etc.? Or maybe something in the room is vibrating? If it's definitely not mechanical...
4. Could you record a sample of the distortion you're talking about so the more experienced guys can get an idea of what we're dealing with? Even with your phone perhaps?
*I, as an inexperienced guy, would like to learn from that as well.

Quote from: RG100ESROX on December 05, 2024, 03:15:38 PMWith initial power up this morning, and with a 8Ω dummy load plugged in. My idle bias current rose extremely fast and way too high to proceed (trim pot set to FCCW.)

Could this be an indication of the issue with the amp?

How to proceed?

Quote from: Miyagi_83 on December 05, 2024, 03:23:50 PM

Quote from: RG100ESROX on December 05, 2024, 03:15:38 PMWith initial power up this morning, and with a 8Ω dummy load plugged in. My idle bias current rose extremely fast and way too high to proceed (trim pot set to FCCW.)

Could this be an indication of the issue with the amp?

How to proceed?

Did it rise at idle or with signal applied?

Quote from: RG100ESROX on December 05, 2024, 05:08:03 PM

Quote from: Miyagi_83 on December 05, 2024, 03:23:50 PM

Quote from: RG100ESROX on December 05, 2024, 03:15:38 PMWith initial power up this morning, and with a 8Ω dummy load plugged in. My idle bias current rose extremely fast and way too high to proceed (trim pot set to FCCW.)

Could this be an indication of the issue with the amp?

How to proceed?

Did it rise at idle or with signal applied?

With the .1mA 1KHz signal applied...

I checked and this happens with or without a signal applied.

If I remove the dummy load. The idle current with or without an injected signal is about 5mV.

Quote from: Miyagi_83 on December 06, 2024, 12:23:39 AMWhat is different from the previous setup when it was more or less functional?

Quote from: RG100ESROX on December 06, 2024, 03:01:57 AMAND...we're injecting the signal in at the FX return

Quote from: RG100ESROX on December 05, 2024, 09:39:40 PMIf I remove the dummy load. The idle current with or without an injected signal is about 5mV.

Quote from: Miyagi_83 on December 06, 2024, 04:58:39 AM

Quote from: RG100ESROX on December 06, 2024, 03:01:57 AMAND...we're injecting the signal in at the FX return

This shouldn't make a difference other than the signal being processed (amplified, clipped, shaped etc.) when going through the preamplifier. You can plug your guitar straight into the loop return and play it like that, no problem. Not that you will like the results, but you can. I have done this a number of times.

Did you build that attenuator to control the signal level going into the return jack?

Also, do you have another dummy load resistor to try?

Have you tried playing the amp at your parents' house before you took those alarming measurements?

Quote from: RG100ESROX on December 05, 2024, 09:39:40 PMIf I remove the dummy load. The idle current with or without an injected signal is about 5mV.

If I understand it correctly, when you inject a signal, it's no longer idle. Idle is when NO SIGNAL is being applied, so the device is not amplifying anything except noise, it's idle.
When you put a signal in, the output devices will conduct, so the current through the output devices and anything in series with them will rise. Correct me if I'm wrong, anyone.

Quote from: g1 on December 06, 2024, 12:10:47 PMDo not proceed with signal if you have a problem with idle current (no signal).
Get that sorted out first.
Disconnect anything plugged in to input or FX return.
It is critical that the (-) side of the output jack does not get connected to chassis ground (because R63).  Sometimes the test equipment can do this.
Make sure there is no DC at the output.  If so connect the speaker.
With no signal, do you hear much hum?  Does the idle current through the emitter resistors rise?

As Miyagi stated above, when the amp is running properly, delivering power to the load will increase the current through the output transistors.  This is normal, and not idle current, but working current.

Quote from: RG100ESROX on December 06, 2024, 05:36:34 PM

Quote from: Miyagi_83 on December 06, 2024, 04:58:39 AM

Quote from: RG100ESROX on December 06, 2024, 03:01:57 AMAND...we're injecting the signal in at the FX return

This shouldn't make a difference other than the signal being processed (amplified, clipped, shaped etc.) when going through the preamplifier. You can plug your guitar straight into the loop return and play it like that, no problem. Not that you will like the results, but you can. I have done this a number of times.

Did you build that attenuator to control the signal level going into the return jack?

Also, do you have another dummy load resistor to try?

Have you tried playing the amp at your parents' house before you took those alarming measurements?

Quote from: RG100ESROX on December 05, 2024, 09:39:40 PMIf I remove the dummy load. The idle current with or without an injected signal is about 5mV.

If I understand it correctly, when you inject a signal, it's no longer idle. Idle is when NO SIGNAL is being applied, so the device is not amplifying anything except noise, it's idle.
When you put a signal in, the output devices will conduct, so the current through the output devices and anything in series with them will rise. Correct me if I'm wrong, anyone.

Q:Have you tried playing the amp at your parents' house before you took those alarming measurements? YES. I RECORDED A SOUND DEMO OF THE DISTORTION I AM GETTING FROM THE AMP. TRUST ME, SOME MIGHT SAY IT SOUNDS FINE, BUT IT'S NOT. IT'S THE BAD KIND OF DISTORTION SITTING ON TOP OF THE GOOD DISTORTION.


Q:Did you build that attenuator to control the signal level going into the return jack?
I'M SORRY...I COMPLETELY FORGOT ABOUT THIS THING. MY APOLOGIES. I WILL NEED TO BUILD IT. SO, NO, IT WAS NOT USED WHEN INJECTING THE SIGNAL INTO THE FX RETURN. ONCE I HAVE THE POT BUILT, I'LL TRY THIS AGAIN AND GET BACK TO YA. THIS IS WHY THE IDLE BIAS CURRENT WAS SO DAMN HIGH. GEEEEEEEZE....


Q:Also, do you have another dummy load resistor to try?
I DO NOT. JUST THE ONE.

Quote from: g1 on December 06, 2024, 12:10:47 PMDo not proceed with signal if you have a problem with idle current (no signal).
Get that sorted out first.

I FAILED TO USE THE ATTENUATOR WHEN INJECTING THE SIGNAL INTO THE FX RETURN. THIS IS WHY THE IDLE CURRENT WAS SO HIGH.


Disconnect anything plugged in to input or FX return. DONE.
It is critical that the (-) side of the output jack does not get connected to chassis ground (because R63).  Sometimes the test equipment can do this. NOTED.
Make sure there is no DC at the output.  If so connect the speaker.
With no signal, do you hear much hum?  Does the idle current through the emitter resistors rise? I WILL CHECK THIS AND POST BACK.

As Miyagi stated above, when the amp is running properly, delivering power to the load will increase the current through the output transistors.  This is normal, and not idle current, but working current. THIS QUESTION IS FOR MY INFO. WHAT KIND OF CURRENT ARE WE TALKING ABOUT HERE? WHAT'S TYPICAL AT SAY A HIGH BEDROOM LEVEL VOLUME?

Quote from: RG100ESROX on December 06, 2024, 05:36:34 PMQ:Also, do you have another dummy load resistor to try?
I DO NOT. JUST THE ONE (disregard how you see the dummy Load box connected in the photo. it's for the image of the box only.)

Quote from: RG100ESROX on December 06, 2024, 05:36:34 PMI AM ASSUMING THAT THIS WILL WORK IN PLACE OF THE 22K POT THAT I WAS SUPPOSED TO BUILD...? I BROUGHT MY RESISTOR SUBSTITUTION BOX WITH ME. I'LL START OUT AT THE 1M, AND ADJUST ACCORDINGLY FOR THE OUTPUT SIGNAL INJECTED AT THE FX RETURN?? AND YES, I AM GOING TO HAVE TO DO SOME CREATIVE WIRING IN ORDER TO USE THE SUBSTITUTION BOX.

Quote from: RG100ESROX on December 06, 2024, 05:36:34 PMYES. I RECORDED A SOUND DEMO OF THE DISTORTION I AM GETTING FROM THE AMP. TRUST ME, SOME MIGHT SAY IT SOUNDS FINE, BUT IT'S NOT. IT'S THE BAD KIND OF DISTORTION SITTING ON TOP OF THE GOOD DISTORTION.

Quote from: RG100ESROX on December 06, 2024, 07:29:33 PMThe ground connection that connects to the tip, is completely disconnected with the phono plug plugged into the FX return jack.

Quote from: RG100ESROX on December 06, 2024, 05:36:34 PMYES. I RECORDED A SOUND DEMO OF THE DISTORTION I AM GETTING FROM THE AMP. TRUST ME, SOME MIGHT SAY IT SOUNDS FINE, BUT IT'S NOT. IT'S THE BAD KIND OF DISTORTION SITTING ON TOP OF THE GOOD DISTORTION.

Quote from: Miyagi_83 on December 07, 2024, 02:52:55 AM

Quote from: RG100ESROX on December 06, 2024, 05:36:34 PMYES. I RECORDED A SOUND DEMO OF THE DISTORTION I AM GETTING FROM THE AMP. TRUST ME, SOME MIGHT SAY IT SOUNDS FINE, BUT IT'S NOT. IT'S THE BAD KIND OF DISTORTION SITTING ON TOP OF THE GOOD DISTORTION.

I've listened to your demo again just now. I'd like to ask you to post your preamp settings for it, if possible of course.

EDIT: Also, as difficult as it may be to do using words, could you please describe the distortion you hear?

Quote from: RG100ESROX on December 07, 2024, 03:09:46 AMPreamp settings were: P10-B0-M0-T10 (and pulled for clipping)-V3~4-G10

The distortion I'm hearing sounds like if you took a distortion pedal, and on top of a good sounding amp with the gain set to 10, you add just a tad bit more distortion on top of the amps gain, and it sounds like you've turned the gain up just a little too much to the point where it no longer sounds good any longer. It's just too much distortion. The only other way I know of describing this distortion is, kind of like Xover distortion. It does not sound like a good distortion while you're playing. It's just....too much distortion.

Sorry, it's just one of those things that's hard to describe, but we all know what 'Good' distortion sounds like. Well, this is just too much distortion to sound good.

Quote from: Miyagi_83 on December 07, 2024, 05:01:14 AM

Quote from: RG100ESROX on December 07, 2024, 03:09:46 AMPreamp settings were: P10-B0-M0-T10 (and pulled for clipping)-V3~4-G10

The distortion I'm hearing sounds like if you took a distortion pedal, and on top of a good sounding amp with the gain set to 10, you add just a tad bit more distortion on top of the amps gain, and it sounds like you've turned the gain up just a little too much to the point where it no longer sounds good any longer. It's just too much distortion. The only other way I know of describing this distortion is, kind of like Xover distortion. It does not sound like a good distortion while you're playing. It's just....too much distortion.

Sorry, it's just one of those things that's hard to describe, but we all know what 'Good' distortion sounds like. Well, this is just too much distortion to sound good.

I understand it's not easy to describe tone in words. You've done a pretty good job anyway, afaic.

Another question, how are YOU (you specifically) realizing channel switching?
*Looking at the schematic, the switching works by simply shunting either channel's output to ground, so only one of them puts out signal.

Quote from: g1 on December 07, 2024, 02:17:43 PMLike Miyagi stated, idle current is with nothing plugged in to any input.  So you do not need to check voltage across those resistors any time a signal is applied.

Your load box is connecting the amp output jack to chassis ground, via the scope BNC connecter.  That is bad as it shorts out R63, which is part of the feedback loop and will affect the sound.  It may also cause instability which could be messing with the idle current.

Use your scope probe.  Connect scope ground to chassis, and the scope probe to the red binding post on your load box.
When bringing up the voltage with variac, you may get idle current surges like that acros the 0R62's until it reaches a stable voltage.  Are you sure it was not doing that before?

Quote from: RG100ESROX on December 07, 2024, 06:41:02 PMI measured the DC at the speaker output jacks, and it read -.88mV...

Quote from: Miyagi_83 on December 07, 2024, 07:56:09 PM

Quote from: RG100ESROX on December 07, 2024, 06:41:02 PMI measured the DC at the speaker output jacks, and it read -.88mV...

Did you take this reading just now? If so, I understand that you put the theory to the test and it turned out to be true, right?

Quotehow do I do that since the front panel controls do not function while we're plugged into the FX return jack. Do I just increase the signal level? If so, to what level? How many volts? I'm not clear on this

Quote from: RG100ESROXSine wave (is it just me, or does this sine wave look pretty bad? Is that cover distortion we're seeing?)

Quote from: Miyagi_83 on December 07, 2024, 08:55:17 PM

Quotehow do I do that since the front panel controls do not function while we're plugged into the FX return jack. Do I just increase the signal level? If so, to what level? How many volts? I'm not clear on this

First, bias should be right so there's no crossover. Once that is done, and, if I remember well, it is, move on to the next steps.

Btw, did you place that resistor in series with the signal generator like I suggested earlier, in reply #258?

Do this experiment:
Unplug your signal and resistor box combo from the amplifier. Disconnect your scope from the amp too, just in case.
Now, hook up your scope probe to the output of the SG / resistance box, the point where it says 'to plug' on the diagram I posted in reply #258.
Set the signal level on your generator to, say, 1V. What can you see on the scope screen?
Now change the resistor value on the substitution box. What can you see on the scope? Try again and look at the scope.
 
This is your makeshift volume control now. Use it to adjust the signal going into the amp's return jack. Then, on your oscilloscope, watch the waveform coming out of the speaker jack and see when clipping starts. Full clean power is just before the onset of clipping, as I understand it. Then post the signal voltage needed to get there, as requested by g1.

Quote from: RG100ESROX on December 07, 2024, 09:50:39 PMHere is the result of the signal input into the FX return, biased just out of xover at 16mV,  at the highest resistor sub box setting (1M) with a 10K in series with the sub box.

QuoteAs you can see, we have clipping at the highest resistance setting with a .1mV/1KHz injected signal.

Quote from: Miyagi_83 on December 08, 2024, 02:49:37 AM

Quote from: RG100ESROX on December 07, 2024, 09:50:39 PMHere is the result of the signal input into the FX return, biased just out of xover at 16mV,  at the highest resistor sub box setting (1M) with a 10K in series with the sub box.

The oscilloscope is hooked up to the speaker output / dummy load box, right? YES.

QuoteAs you can see, we have clipping at the highest resistance setting with a .1mV/1KHz injected signal.

Highest or lowest? I'm asking because the file name says 'lowest'. SORRY. IT'S THE HIGHEST. RESISTOR SUB BOX WITH THE 10k RESISTOR IN SERIES IS SET TO 1M OHM. SO, WE'RE MAXED OUT ON HOW MUCH WE CAN ATTENUATE THE SIGNAL USING THE RESISTOR SUB BOX. I DO HAVE A 5W 220K RESISTOR I COULD USE IN PLACE OF THE 10K THAT IS IN SERIES WITH THE SUB BOX.

Quote from: g1 on October 23, 2024, 09:05:54 PMSet the bias to the minimum mV reading across each 0R6 (no signal applied).  What reading do you get?
Now get a nice clean sinewave at the output like the first pic in post #24, but with less amplitude, maybe 100mV peak to peak.  Do you still have a nice clean sinewave like that, or is there a crossover notch in the middle of the sinewave?
For solid-state amps, we just increase the bias til the crossover notch just disappears, no more is needed.

Quote from: Loudthud on November 26, 2024, 07:42:24 AMA crossover notch might not be exactly at the zero crossing but at a slightly higher Voltage where the second pair of output transistors turn on at around +/- 3V.

Quote from: Miyagi_83 on December 08, 2024, 02:57:31 PMSo perhaps a 100k pot would be a better idea so as not to load down the signal generator too much? Or bigger, like 250k?
EDIT: Or maybe try that 220k resistor instead of the 10k, Jay, and see how your signal behaves.

Quote from: RG100ESROX on December 08, 2024, 04:09:46 PMI just tried an 220K 5W resistor...and while it helped, I still have clipping on the sine wave at Vpp  .06V.

Quote from: RG100ESROX on December 08, 2024, 04:33:58 PMQ: Before I broke the amp. I used to get -.6V on the bottom lug of the bias trim pot. Just like it shows on the schematic. Since I broke the amp. The voltage at that location is a +.6V (or around +.588V FCCW) instead of a negative -.6V.

Quote from: g1 on December 08, 2024, 04:49:00 PMWe had established that the trim pot is not located as shown in the schematic.  Then you turned around and said that it was, after all.
But following the traces in your pictures, it is not.  That is why the voltages do not follow.
And if you mark down the voltages on the bias string, it makes it clear that the schematic shows it in a different location.
So why don't you go ahead and record those bias string voltages again, to settle the issue.
Both sides of D7,D8,trim pot,R48,R49.

Quote from: RG100ESROXPreamp settings were: P10-B0-M0-T10 (and pulled for clipping)-V3~4-G10

Quote from: Miyagi_83 on December 09, 2024, 05:32:35 PMJay, regarding your preamp settings for the distortion demo

Quote from: RG100ESROXPreamp settings were: P10-B0-M0-T10 (and pulled for clipping)-V3~4-G10

How did you have reverb set? The RCA connectors for the reverb have not been plugged in, and the reverb setting is always at zero.
I'm just thinking out loud now, BUT... Good. I like the idea.

If there is something wrong with the power amplifier's power supply (negative rail measures positive now, according to the image you attached earlier), there might have been a fault in the reverb driver's supply which caused a voltage drop across the op amp, which in turn led to clipping in the op amp itself, which was then blended with the signal. Does that make sense? Yes. However, it occurred to me at some point a while back that maybe the OpAmp was to blame. So, I removed it as well as replacing it with a known good IC4558, and it did not change anything.
Could you measure the voltage at pins 4 & 8 of the 1458? Sure. Last time I did. The voltages were as spec'd on the data sheet for it. I'll check em again just be sure they're still to spec.
And be sure to examine the power supply: rectifier, filter caps, Zener diodes and so on.
M. What am I looking for specifically? Voltages? What is the best way to test the diodes in a circuit? Or does it matter that they are in circuit, and will test fine regardless?

Also, I did replace the rectifier (see attached photo) with one of a higher amperage rating at one point a while back. I don't think this would have anything to do with what's going on, or am I wrong? It should be no different than the one that was in there, except it'll handle more current.

IC4558:
PIN 4 = -12.45VDC
PIN 8 = +12.45VDC

Quote from: Miyagi_83 on December 09, 2024, 05:59:12 PMDisregard the pink arrows. I just couldn't find a pic without them. Sorry.

Where does the wire marked with the green arrow go? I can't see it clearly in this picture.

That black wire connects the + side of the left filter can, and - side of the right filter can to ground. (see attached photos)
 

Quote from: RG100ESROX on December 09, 2024, 06:52:06 PMThe RCA connectors for the reverb have not been plugged in, and the reverb setting is always at zero.
...
 Yes. However, it occurred to me at some point a while back that maybe the OpAmp was to blame. So, I removed it as well as replacing it with a known good IC4558, and it did not change anything.
...
IC4558:
PIN 4 = -12.45VDC
PIN 8 = +12.45VDC

QuoteWhat am I looking for specifically? Voltages? What is the best way to test the diodes in a circuit? Or does it matter that they are in circuit, and will test fine regardless?

QuoteAlso, I did replace the rectifier (see attached photo) with one of a higher amperage rating at one point a while back. I don't think this would have anything to do with what's going on, or am I wrong? It should be no different than the one that was in there, except it'll handle more current.

Quote from: RG100ESROX on December 09, 2024, 07:41:03 PMThat black wire connects the + side of the left filter can, and - side of the right filter can to ground. (see attached photos)

Quote from: Miyagi_83 on December 10, 2024, 12:31:11 AM

Quote from: RG100ESROX on December 09, 2024, 06:52:06 PMThe RCA connectors for the reverb have not been plugged in, and the reverb setting is always at zero.
...
 Yes. However, it occurred to me at some point a while back that maybe the OpAmp was to blame. So, I removed it as well as replacing it with a known good IC4558, and it did not change anything.
...
IC4558:
PIN 4 = -12.45VDC
PIN 8 = +12.45VDC

My idea was that if the voltage across the op amp was too low, it would clip at less input signal, even if the device itself was in perfect condition. For example, if you power an operational amplifier with 9VDC, it will not put out a signal larger than 7-8 Vpp (less?) because signal can't go further than power supply rails, and in op amps it's a little less than that. So, the input signal will be amplified until it hits the power supply rails where it will be clipped off.
Nonetheless, we can assume that the reverb driver is powered properly, and the extra distortion was generated elsewhere, especially that the reverb pot was turned down to zero, so nothing should have been able to sneak into your signal. I'll keep thinking about it, though.

QuoteWhat am I looking for specifically? Voltages? What is the best way to test the diodes in a circuit? Or does it matter that they are in circuit, and will test fine regardless?

First of all, orientation, especially on components that you replaced, especially polarity-sensitive ones. In circuit, most probably you won't be able to test them correctly because they interact with each other.
First, however, I would do voltage tests around the power supply, points marked A, B, C, and D on the schematic (although C and D seem fine, judging from your op amp test).
For some reason your negative rail (point B) is positive and I'm trying to wrap my head around it. What about the positive rail (point A)?

Maybe test ground connections too?

QuoteAlso, I did replace the rectifier (see attached photo) with one of a higher amperage rating at one point a while back. I don't think this would have anything to do with what's going on, or am I wrong? It should be no different than the one that was in there, except it'll handle more current.

Yeah, a beefier rectifier will do no harm. It's like installing a drain pipe of a larger diameter, it's going to do the same job, but it will be able to take more water. Unless it's dud of course, but yours probably is fine.

Quote from: RG100ESROX on December 09, 2024, 07:41:03 PMThat black wire connects the + side of the left filter can, and - side of the right filter can to ground. (see attached photos)

Thank you. So, that's a ground lug there, underneath the black wire going to the pilot lamp? Yes; it is. Make sure it makes good contact with the chassis.

Where is the transformer center tap connected?

I'll check. Thanks for all of your help and input. It's most appreciated.

I'll try and find the post where g1 requested the voltages of the 'A' through 'E' points on the schematic. I will double check them again to see if they remain the same as previously posted.

M.

Quote from: RG100ESROX on November 25, 2024, 06:51:19 PM

Quote from: g1 on November 25, 2024, 03:37:05 PMThat's fine if you had the gen. set for low level output.
Check the amps power supply (DC)voltages, shown as  A,B,C,D,E on the schematic.

VOLTAGES REQUESTED:

A =   41.90VDC
B = -41.60VDC
C =  12.61VDC
D = -12.60VDC
E =   25.38VDC

Quote from: Miyagi_83 on December 10, 2024, 04:32:41 AM

Quote from: RG100ESROX on November 25, 2024, 06:51:19 PM

Quote from: g1 on November 25, 2024, 03:37:05 PMThat's fine if you had the gen. set for low level output.
Check the amps power supply (DC)voltages, shown as  A,B,C,D,E on the schematic.

VOLTAGES REQUESTED: I will get these voltage readings for you later this morning.

A =  41.90VDC
B = -41.60VDC
C =  12.61VDC
D = -12.60VDC
E =  25.38VDC

I found them. 2 weeks ago they were fine, so you might have made a mistake in your labelling, but do check again, just to be sure.

Edit: BTW, do you have another voltmeter on hand that you could try? I do, and I have determined that they both provide the same voltage readings down to .01 volts. The incorrect readings might be caused by a number of factors. Faulty or poor-quality test equipment, faulty component(s) in the circuit, or human error come to mind. Oh, we know all about this...  (-;

Quote from: RG100ESROX on December 10, 2024, 10:00:55 AM

Quote from: Miyagi_83 on December 10, 2024, 04:32:41 AM

Quote from: RG100ESROX on November 25, 2024, 06:51:19 PM

Quote from: g1 on November 25, 2024, 03:37:05 PMThat's fine if you had the gen. set for low level output.
Check the amps power supply (DC)voltages, shown as  A,B,C,D,E on the schematic.

VOLTAGES REQUESTED: Here ya go!

A =  41.90VDC  = +41.18VDC
B = -41.60VDC  = -41.22VDC
C =  12.61VDC  = +12.45VDC
D = -12.60VDC  = -12.45VDC
E =  25.38VDC  =+25.19VDC

I found them. 2 weeks ago they were fine, so you might have made a mistake in your labelling, but do check again, just to be sure.

Edit: BTW, do you have another voltmeter on hand that you could try? I do, and I have determined that they both provide the same voltage readings down to .01 volts. The incorrect readings might be caused by a number of factors. Faulty or poor-quality test equipment, faulty component(s) in the circuit, or human error come to mind. Oh, we know all about this...  (-;

Quote from: RG100ESROX on December 10, 2024, 02:41:28 PM Here ya go!!!

Quote from: Miyagi_83 on December 10, 2024, 03:19:24 PM

Quote from: RG100ESROX on December 10, 2024, 02:41:28 PM Here ya go!!!

Thank you, sir.
I understand that point A is roughly the same as point B, only positive. This means it was simply mislabelled in the bias-string-voltage pic posted earlier. Therefore, I would like to kindly ask you to measure and write down the voltages along the bias string again, but please don't rush it, focus, and note down everything correctly so we don't spend time looking for solutions to non-existent problems.
Looking forward to it.
M.

Quote from: RG100ESROX on December 11, 2024, 01:27:02 PMAnyway, let me know what you think, and if you need any further clarification or additional voltage readings.

Quote from: Miyagi_83 on December 11, 2024, 03:53:15 PMIs there a meaning to the short red line below the trim pot and the red dot between the yellow-filled oval and Q12 on the schematic excerpt you posted?

Quote from: RG100ESROX on December 11, 2024, 04:31:45 PM

Quote from: Miyagi_83 on December 11, 2024, 03:53:15 PMIs there a meaning to the short red line below the trim pot and the red dot between the yellow-filled oval and Q12 on the schematic excerpt you posted?

Yes. I was just denoting that I found it interesting that these two points in the circuit shared the same voltage reading....

Quote from: Miyagi_83 on December 11, 2024, 03:37:54 PM

Quote from: RG100ESROX on December 11, 2024, 01:27:02 PMAnyway, let me know what you think, and if you need any further clarification or additional voltage readings.

Jay, I'd like to ask you to measure voltages at the following points:
1. base and emitter of Q10
2. collector and emitter of Q11
3. collector and emitter of Q12

I suspect we'll need to rearrange the schematic. I suspect the same...

Quote from: RG100ESROX on December 11, 2024, 01:27:02 PMAs you can see...where I once had -.6 (as indicated on the schematic) is now +.521v. I don't even know how this is possible. But, this is the case.

Now, if I look at the back side of the PCB. I honestly do not see how the bias trim pot can be located in the circuit as indicated in the schematic. However, and again, I used to get -.6VDC on the bottom lug of the bias trim pot as it is indicated on the schematic before I broke the amp. So, how does it go from -.6VDC to +.521????

Quote from: Miyagi_83 on December 11, 2024, 04:54:01 PM

Quote from: RG100ESROX on December 11, 2024, 01:27:02 PMAs you can see...where I once had -.6 (as indicated on the schematic) is now +.521v. I don't even know how this is possible. But, this is the case.

Now, if I look at the back side of the PCB. I honestly do not see how the bias trim pot can be located in the circuit as indicated in the schematic. However, and again, I used to get -.6VDC on the bottom lug of the bias trim pot as it is indicated on the schematic before I broke the amp. So, how does it go from -.6VDC to +.521????

Are you absolutely, 100% sure that it used to be -.6VDC and not +.6VDC? Are you certain that your memory and / or notes serve you well on this one? If that particular leg of the trim pot is connected to the base of Q11, and it seems it is, at least judging by the voltage readings, according to the original circuit diagram there should be and should have been a positive voltage there. What's more, the original schematic says the voltage there should be roughly twice that, it says 1.2 V on the schematic, after all.

Quote from: Miyagi_83 on December 11, 2024, 04:54:01 PM

Quote from: RG100ESROX on December 11, 2024, 01:27:02 PMAs you can see...where I once had -.6 (as indicated on the schematic) is now +.521v. I don't even know how this is possible. But, this is the case.

Now, if I look at the back side of the PCB. I honestly do not see how the bias trim pot can be located in the circuit as indicated in the schematic. However, and again, I used to get -.6VDC on the bottom lug of the bias trim pot as it is indicated on the schematic before I broke the amp. So, how does it go from -.6VDC to +.521????

Are you absolutely, 100% sure that it used to be -.6VDC and not +.6VDC? Are you certain that your memory and / or notes serve you well on this one? If that particular leg of the trim pot is connected to the base of Q11, and it seems it is, at least judging by the voltage readings, according to the original circuit diagram there should be and should have been a positive voltage there. What's more, the original schematic says the voltage there should be roughly twice that, it says 1.2 V on the schematic, after all.

Quote from: Miyagi_83 on December 11, 2024, 05:38:50 PMOk, here's your voltage chart with added readings of requested transistors.
Looking at the numbers on the diagram, measurements for Q11 make sense to me. Those for Q12 don't. Take a look:
The collector of Q11 is connected straight to the +41V rail. According to your readings, the collector of Q12 is at +41V too, which suggests those two terminals are connected, but they shouldn't be. Q12 is in the negative leg of the power supply, so IMHO its collector should be negative in relation to ground. Unless there is a mistake in labelling of course.

Let's look at Q10 now. According to the schematic, its base is tied to the collector of Q8 and R41. R41 is tethered to the positive power supply rail. The original schematic shows a value of 38 V at the collector of Q8 and, consequently, base of Q10.

Personally, I would try to track the connections between different components on the PCB and compare my findings to the schematic so as to establish what is what and what connects to what. It'd be time-consuming and probably frustrating, I know, but I'd do that nonetheless. Perhaps it's unnecessary, I don't know. Let's wait for the more experienced players to chime in.

Here's my $0.02.