Hello, this is my first time posting on the forum, I have been using related posts on here to help me so far. Apologies if I have posted this in the wrong place.
I have a Fender 112 deluxe plus that was blowing the internal 2a fuse on start up. The amp was also making a very loud buzz with all volumes at 0.
I disconnected the speaker and checked the output transistors. I found a shorted pair of tip142/tip147 so I replaced all four transistors using new micas and heat compound. I pulled and checked the heatsink diodes which all tested ok with multimeter. I also replaced the .47 emitter resistors for each output transistor. I checked the diodes in the power supply and Qs 3-9.
The channel switching was also playing up - turning on the clean and overdrive channels at the same time. I replaced the u4 ic after finding it shorted with a new 4560D and powered the amp up with a light bulb limiter. This seemed to have solved that problem and the channel switching works again. The bulb was glowing very dimly with no sign of shorts and fuse not blowing, so I powered off and reconnected the reverb pan - still all good. Then I powered off and reconnected the speaker - the amp is still making the same very loud buzz. I have gone through the entire board with a multimeter checking for continuity between each solder joint and all seems well. I reflowed the solder on a number of joints including the filter caps just to be sure. The 4 ohm speaker measures as 3.97ohms, so I assume ok. Could this be filter cap/power supply cap related or an issue with the transformer? Or am I missing something else e.g. switching issues in the jacks? I know these amps are not worth much but I would like to get it running again.
Any help would be greatly appreciated!
Thanks,
Andy
https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Fender-Deluxe-112-Plus-Schematic.pdf
SO the amp works with no speaker? At least not blowing fuses? Measure for DC voltage on the output. Or look at the speaker cone when it makes the loud hum. Does the cone move one direction and stay there? That is DC on a speaker. It will damage a speaker so no more than a couple seconds please.
DC on the output with no load will not draw large excess current. Put a speaker or load on it and it will.
Thanks Enzo,
Yes, the amp powered on ok without the speaker (using the lightbulb limiter) and did not blow the fuse.
When I connected the speaker the noise was apparent but did not blow the fuse before I turned it off. The lightbulb remained at a dim glow throughout.
I will check for dc on the output and let you know.
OK, if there is no DC, then my next suspects are the main filter caps. The two standing side by side near the heat sink. I tend to think they are OK, but may have cracked their solder.
The main rails are 40v each, are both up to that? Or is one substantially lower, like 25 or something? That would indicate large ripple and lack of filtration. Or certainly you can meter or scope for ripple.
Hi Enzo,
Here is the reading across the speaker leads.
Thanks,
AndyYou cannot view this attachment.
Test point readings
Tp3 39.10vdc
Tp4 -39.10vdc
Tp5 16.35
Tp6 -16.42
Tp1 30.22vac
Tp2 30.25vac
If TP 3,4,5,6 are right - and they are - then TP1,2 pretty much had to be.
OK, so no DC on output and rails are right. You might look for ripple on the supplies, but looks like that isn't the deal.
Plug the guitar into the POWER AMP IN jack and dial it to zero. Hum still?
The buzz is still present with all volumes on 0 and guitar plugged into the power amp in.
I meant the volume on the guitar itself, we were bypassing all the preamp. I will assume you did that.
That puts the hum in the power amp. CAn you scope or otherwise measure the ripple on the 40v rails?
Q7 is the mute JFET at the input of the power amp. SHort across it, does that kill the hum? Or remove Q7. Same question.
Hello, yes that was with all volumes at zero - amp and guitar.
Unfortunately I don't have a scope.
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When shorting q7, should I wire across joining the three legs of the fet?
The test points for 7 and 8 are a long way off - does this also suggest a preamp issue?
@ FenderD112Plu, Regards Test point 7 & 8
Those AC test points need to be taken with a signal at input.
Read Note 5 bottom left corner for clues.
Phil.
2mv hum is not a lot
The JFET is across the signal path. I just want to know what happens when you short from r59/C33 to ground. NO need to short the gate. While the JFET itself can be noisy, I was thinking your noise is either coming into the power amp through that path or it is after that point
Thank you both for clarifying those points for me.
@ Enzo, buzz still happening with r59 shorted to ground.
Check supplies for ripple. You don't have a scope so you can measure AC volts at TP's 3,4,5,6
Thanks g1,
Ac voltages
Tp3 0.1174
Tp4 0.1421
Tp5 0.0025
Tp6 0.0023
When you reinstalled the PCB you may have inadvertently disconnected the Chassis Com from circuit com.
If the amp is otherwise working I would be checking that you have Circuit common in contact with Chassis.
If that connection is open it will cause hum.
Phil.
Hi Phil,
The chassis ground is attached and I have checked the continuity of the pots and they are grounded to the chassis.
R1 is not showing a continuity to the chassis - is that as expected?
The schematic does not show the PA jack tip connected to R1. The tip is pin 2, which when plugged connects through auxiliary contacts 5 and 6 down to R99, which is the power amp input. R1 does not connect to ground, it runs through the other auxiliary contacts and then back over to the input jacks. It is part of the mute circuit.
It's a long shot but,,, maybe check you have continuity from Mains plug earth pin back to chassis. For that matter try another wall outlet they can indeed fail on rare occasions? (if earth pin in an outlet fails equipment still works,, just no earth :o )
Phil.
Thanks Phil,
Continuity between earth pin and chassis is good and I have tried plugging into different plug sockets with same result.
Tp16 and Tp17 give the expected voltages of +25.7 vdc and -25.7 vdc.
Hello, does anyone have ideas about how I should proceed next? Is getting hold of a scope the best option?
A scope may not help much if it's ground hum.
I'm assuming the amp is working and passing signal except for the hum?
Well as there is no ripple on the supply and voltages are within spec then the most likely issue is one of 2 possibilities.
1/ open ground connection somewhere.
If the star ground node of main supply is not connected back to chassis correctly then it can cause hum issues. (Which is the node between C47 & C48 or Conn CP4)
You can test simply by shorting the main Common back to chassis with a wire. If the hum stops then you found the issue,, if hum increases then you just created a ground loop,, which brings us to,, 2/ Something is grounded that should Not be grounded, which would cause a ground loop hum.
Ground plane issues can be a nightmare to track down.
sometimes in design phase one has to use an alligator clip to find the best ground path before committing to a layout.
Often components like input jacks, FX loops and speaker outputs, even the PCB need to be *Isolated from chassis*
This will depend on how the circuit common was designed which is hardly ever noted on schematics.
Tiss easy to loose these little isolation washers or forget to reinstall them.
If you acquired this amp with issues then someone may have already worked on it and if small parts have been lost then you have no idea why it hums.
*I would be researching ground path issues long before replacing parts in hope.*
I got caught once, I did not notice an isolation washer dropped off a PCB post and the hum drove me nuts for a few days.
finally found it hiding in the chassis corner. >:(
Phil.
FWIW, I think Fender isolates signal ground from the chassis in all their small SS amps.
Quote from: phatt on April 24, 2022, 09:39:20 PMA scope may not help much if it's ground hum.
I'm assuming the amp is working and passing signal except for the hum?
Well as there is no ripple on the supply and voltages are within spec then the most likely issue is one of 2 possibilities.
1/ open ground connection somewhere.
If the star ground node of main supply is not connected back to chassis correctly then it can cause hum issues. (Which is the node between C47 & C48 or Conn CP4)
You can test simply by shorting the main Common back to chassis with a wire. If the hum stops then you found the issue,, if hum increases then you just created a ground loop,, which brings us to,, 2/ Something is grounded that should Not be grounded, which would cause a ground loop hum.
Ground plane issues can be a nightmare to track down.
sometimes in design phase one has to use an alligator clip to find the best ground path before committing to a layout.
Often components like input jacks, FX loops and speaker outputs, even the PCB need to be *Isolated from chassis*
This will depend on how the circuit common was designed which is hardly ever noted on schematics.
Tiss easy to loose these little isolation washers or forget to reinstall them.
If you acquired this amp with issues then someone may have already worked on it and if small parts have been lost then you have no idea why it hums.
*I would be researching ground path issues long before replacing parts in hope.*
I got caught once, I did not notice an isolation washer dropped off a PCB post and the hum drove me nuts for a few days.
finally found it hiding in the chassis corner. >:(
Phil.
Been there. I did this on my Dean Markley amp years ago. Took forever to figure it out!
I am reading around 24ohms resistance from cp4 to chassis ground.
Quote from: FenderDeluxe112Plus on April 28, 2022, 11:45:12 AMI am reading around 24ohms resistance from cp4 to chassis ground.
What about from CP2 to chassis ground?
Cp4 24.13ohms
Cp2 24.24ohms
R4 and R5 run in parallel from circuit ground to chassis (pot brackets).
As they are each 47 ohms, 24 ohms seems the correct reading from CP4 to chassis.
It's possible there is another connection to chassis somewhere but I don't see anything on the schematic.
You could try shorting across R4 or R5 as a test and see if it affects the hum/buzz.
Thanks g1,
Have just checked r4 and r5 with multimeter and the grounding seems good and as expected.
Checking grounding of some of the electrolytics-I am getting 757 ohm resistance to chassis on pos lead of c53.
And at the other end?
Just turn the amp on and with a length of wire with alligator clips on the ends and just probe the Known ground points of the pcb and related ground points back to chassis.
**Just make darn sure you don't short power nodes back to ground.**
Use the schematic to verify test points and ground nodes.
If hum increases then you have created a ground loop.
If it reduces hum then you know there is a ground missing somewhere.
BTW the speaker Neg terminal is NOT Ground. This is a current feedback system and spk NEG is lifted from circuit common via that big 10 Watt resistor, R76.
You can visually work out the ground path but often not obvious.
Hence the wire probe can help define the problem.
Maybe post some circuit pictures and we might be able to help more?
Phil.
Quote from: phatt on April 29, 2022, 02:23:11 AMJust turn the amp on and with a length of wire with alligator clips on the ends and just probe the Known ground points of the pcb and related ground points back to chassis.
**Just make darn sure you don't short power nodes back to ground.**
Use the schematic to verify test points and ground nodes.
If hum increases then you have created a ground loop.
If it reduces hum then you know there is a ground missing somewhere.
BTW the speaker Neg terminal is NOT Ground. This is a current feedback system and spk NEG is lifted from circuit common via that big 10 Watt resistor, R76.
You can visually work out the ground path but often not obvious.
Hence the wire probe can help define the problem.
Maybe post some circuit pictures and we might be able to help more?
Phil.
I'm with Phil on this one. I actually have an old test lead from a dead multimeter than I connected an alligator clip to for this purpose - works great.
Thanks for the help,
Jumping the node at cp4 to chassis ground produces same buzz. With neg lead of c53 shorted to chassis getting same buzz. I double checked Enzo's suggestion of shorting q7 at r59 but still buzzing.
I just plugged guitar in to check signal is passing through amp to speaker. Input 1 produces very distorted sound on clean channel. Input 2 is less distorted.
Clip of the sound attached...
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Hello Phil,
Here are some pics of the wiring from the Transformer and power inlet area of the circuit.
The dodgy(!) looking joints on the board have since been reworked and checked with a mm.
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The pot brackets in these amps are used to carry ground connections. I've circled one example in your photo below. I would just resolder them all.
Suggest you deal with the obvious audio problems first (ch.1 distorted), maybe you will get lucky and fix the hum while you are at it.
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Oh dear, Reset,, that noise is not ground hum/buzz that sounds more like a blown poweramp. :o
I asked if the amp is working and passing signal except for some hum/buzz,, if so you may have a ground issue.
Obviously this is NOT the case.
In your fist post you said you replaced the power transistors but there might still be a problem in the power amp.
My best guess is the opamp at the input of power amp stage might be stuffed.
Check the DC test points on U1. (pin7&4)
Phil.
Thanks,
Tp16 (U1 pin 7) and Tp17 (U1 pin 4) give the expected voltages of +25.7 vdc and -25.7 vdc.
Could the U1 still be causing problems?
Yes it's possible when the power Tr's blew it also effected U1.
U1 is the power stage input, those 4 Pwr Tr's boost the current needed to drive the speaker so both U1 and Pwr tr's could have failed.
The other 2 tr's (Q8 & Q9 I think) are part of the over current protection.
Now assuming none of the passives or Q8&Q9 are at fault then replacing U1 Should fix the problem.
Hopefully others more qualified will chime in,, if I've missed something.
As you have noted that plugging into the Pwr Amp input still sounds distorted then the Power stage is most likely at fault.
If you want to check if the preamp is working normally then take the Preamp output to another amp (if you have one) if that produces the same Buzz then the problem maybe further up stream in the signal path.
Phil.
As real MC1436 are hard to find, and I think the circuit is picky about the exact part, post your DC voltages for U1, pins 2,3, and 6.
Thanks, will do. I do have an Mc1436 in my parts box from a supplier that I have bought good components from in the past. Can I test it in any way to check if it would be suitable?
Quote from: FenderDeluxe112Plus on May 03, 2022, 03:57:37 PMThanks, will do. I do have an Mc1436 in my parts box from a supplier that I have bought good components from in the past. Can I test it in any way to check if it would be suitable?
Throw in a socket on the board and then try it. If it works, you can leave it in the socket, if not, take it out :-)
Hi all,
I have finally got the amp out to work on again.I have installed an ic socket as suggested by Joe. Unfortunately the solder pad for pin 8 has lifted. Am I right in thinking this is not a problem as it is not on diagram?
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For the MC1436, pin 8 is a no-connect (N.C.) so you can ignore a connection there.
However, be careful. Often, the supply pins are omitted from the main schematic page(s) because layout techs usually put all the op-amp power supply connections and all of their bypass caps in one area, usually separated from the other pages. This makes the schematic cleaner and easier to read, but can be easily mis-interpreted.
- Will
With the new op amp in U1 the amp seems to be drawing more current through the 100w bulb. Not a huge amount, but the bulb is glowing slightly more. This is with the speaker still disconnected. What is a sensible way to proceed next? Does this suggest power amp diodes or q7 and 8 breaking down under load?
Should I connect the speaker at this stage? I would prefer not to blow the entire power amp again :)You cannot view this attachment.
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This is the bulb previously.
I did check all of the new solder joints with a mm for continuity to other components.
Check for DC on the speaker output and DC Voltages around the preamp. Something must have changed.
As mentioned yes check for DC on output but tiss a fair bet that now that the circuit is correct it maybe well be just drawing more idle power.
I would suggest, try a speaker (with bulb still connected)
If the bulb glows very bright at idle (no signal passing) then there is still an issue.
Phil.
Thank you both, will check for dc later.
Was hoping it might be the case that amp is now running correctly and just drawing more idle current!
Some very slight fluctuations. And has increased since opamp change.You cannot view this attachment.
Out of interest what would be considered acceptable dc?
Less than a hundred millivolts DC at output is fine. You have about 50mV so no problem.
On the bulb limiter with power on, connect the speaker.
Some amps won't start up right with load connected when on the bulb, so that's why I said connect speaker while power on.
Just fired the amp up- still making the same noise. :(
Bulb did not brighten any further with speaker connected.
Sorry to hear that. :(
Ok I'm gonna take a stab in the dark and asking better minds to look at this.
Look at J1,J2 & J5 as they are all interconnected to the low voltage rails via R1 & R49 as well as CR32, C54 & C55.
Now IF??? one of those switching circuits inside J1,J2 or J5 was faulty then it could cause a big DC offset. yes/no?
I've never liked those encapsulated plastic sockets as they are rather fragile inside.
My thoughts,,If the input side of C33 was lifted that would completely isolate the power amp input from any interference.
If that resolves the Buzz then I'd hunting down the offending socket,, or just replace all 3.
Phil.
So you now have no significant DC on the output, and the bulb does not get any brighter with the load connected. That is good so far.
Connect your guitar to the 'power amp in' jack. Do you get the same distorted sound?
Can confirm same sound with guitar plugged into power amp in, but sound is slightly quieter.
Confirm that CR14 cathode (or CR13 anode) is still 0VDC with no load connected.
If so, connect load and recheck.
Also check DC volts at pin6 of U1.
-0.036vdc at cr14 cathode and cr13 anode without load.
00.22vdc on pin 6 of u1 without load
-0.042 cr14 cathode with load
-0.044 cr13 anode with load
0.016vdc u1 pin6 with load
Good so far.
How about DC voltages for E,B,&C of Q1,Q10,Q2,Q11
Also Q8 and Q9
Q1
B01.22
C39.22
E-00.02
Q10
B01.22
C39.20
E-00.02
Q11
B-01.19
C-39.47
E-00.05
Q2
B-01.19
C-39.44
E-00.05
All without speaker load
Q8
E-00.03
B-00.09
C 02.92
Q9
E-00.04
B 00.02
C-02.59
Double check those collector voltages for Q8 and Q9.
Where could they be getting a voltage higher than the base voltages of the outputs?
(ex. Q1 base is 1.2V, Q8 collector gets voltage from that line)
Thanks g1. Have just double checked the dc voltages.
Q1 base 01.23 vdc
Q8 collector 03.29 vdc
Q2 base -01.20 vdc
Q9 collector -02.74 vdc
According to the schematic, that is impossible.
So something funny, or we don't have a schematic match.
Check resistance from CR39 anode to Q1 base. And resistance from CR39 cathode to Q8 collector.
Same for CR40 to Q2 and Q9.
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Continuity between cr39 anode and Q1 base.
Continuity between Q8 collector and cr39 cathode.
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Test point pairs marked in red and green.
Quote from: FenderDeluxe112Plus on May 28, 2022, 02:07:53 PMContinuity between cr39 anode and Q1 base.
Continuity between Q8 collector and cr39 cathode.
Please specify reading in ohms. Continuity is a somewhat vague term that changes depending on model of meter. If relying on a meter beep for continuity, it can range anywhere from zero to 200 ohms.
Apologies .0002 ohms for both readings.
That's good. Now measure DC voltage at CR39 anode and cathode.
With com clipped to chassis.
C39 cathodeYou cannot view this attachment.
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Cr39 anodeYou cannot view this attachment.
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That is really strange. There is no where for that extra couple volts on Q8 collector to come from.
If it were a solder bridge, it's unlikely the same exact thing would show up on the negative side (Q9).
So I would guess it's either an oscillation that the meter is interpreting as DC, or it's just a ghost reading and irrelevant.
Do Q8 and Q9 check out ok with diode check in circuit?
Diode check Q8
Pos to collector com to base 1
Com to collector pos to base .6489
Pos to base com to emitter .6536
Com to emitter pos to base 1.0033
Pos to collector com to emitter 1
Pos to emitter com to collector 1.0390
Q9
Pos to collector Com to base .6760
Com to collector poss to base 1
Com to emitter pos to base 1.0018
Pos to emitter com to base.6780
Pos to emitter com to collector 1
Pos to collector com to emitter .8232
Quote from: FenderDeluxe112Plus on May 28, 2022, 04:53:24 PMQ9
Pos to collector com to emitter .8232
That seems a bit suspect. Have Q8 and Q9 been replaced?
I would remove them and re-check them out of circuit.
Also, have CR5 thru CR12 and CR39 thru CR42 been checked in circuit? Just looking for proper .4 to .7V readings forward biased, and no shorts.
Thanks g1,
Cr9-12 and cr39-42 read between 0.6 and 0.7 with a reading of 1 with leads reversed.
Cr5 -8 are within the heatsink so I can't read them at the moment - I did pull them to test when I changed the output transistors and they were good.
Agree about q9. I have a few 2n4403s so I will replace it next time I pull the board.
Cr13 and 14 have a forward voltage of 0.5 which seems ok.
Reversed they read 1.9020 and 1.8064
Does that check out ok?
Quote from: FenderDeluxe112Plus on May 29, 2022, 03:18:32 AMCr13 and 14 have a forward voltage of 0.5 which seems ok.
Reversed they read 1.9020 and 1.8064
Does that check out ok?
Yes that seems right.
In post #41 sound file, is that the sound it makes with no signal, or is that with a tone at the input?
Yes, that is the sound at switch on with nothing plugged in.
So that sounds like oscillation to me.
Try jumpering CR3 anode (or CR4 cathode) to ground and check if the oscillation sound is still there.
With a jumper to ground at cr3 anode the buzz is there but quieter.
Similar to plugging guitar into power amp in.
Quote from: FenderDeluxe112Plus on May 30, 2022, 07:31:21 AMWith a jumper to ground at cr3 anode the buzz is there but quieter.
Better if we can put some numbers on it. Measure AC volts across the speaker terminals with and without the jumper.
1.48vac with jumper to ground.
3.90vac no jumper
These are the readings that it settled at. Both readings started higher then dropped.
Initial ac voltages
1.72vac with jumper
4.5vac no jumper
That is a substantial difference.
I searched for "100Hz squarewave" on youtube and it sounds pretty much the same as your soundfile in post #41.
Are you on 50Hz 240V power?
Can you run the 'preamp out' to another amp and see if it has the same sound?
Yes, am on 50hz 230v mains in the U.K.
I looked up 100hz tones on youtube and thought it sounded close.
I don't have another amp here to test it at the moment. I might be able to dig out a sampler to test it though.
If you have a stereo receiver, you could even run the preamp out of the amp to an AUX input on th receiver for a listen.
Have got the 100hz tone coming from the preamp out.
So there is excess 100Hz hum in both the preamp and the power amp. That pretty much narrows it down to the power supply.
Recheck again the DC and AC volts on the power supply rails.
Tp1 30.31 vac
Tp2 30.31 vac
Tp3 30.12vdc 00.04vac
Tp4 - 30.20vdc 02.45 vac
Tp5 16.40 vdc 00.02vac
Tp6 -16.42 vdc 00.01vac
Just double checked ac on collectors of both tip147s
Q11 and Q2
2.34vac 2.35vac respectively.
So it seems the problem is on the -48V rail. I'm not sure why it didn't show up when you measured the AC volts there before.
Check that the solder is good and there are no broken pads at C48. Sometimes the pads break near the cap and you can't see it because the cap is glued down. Measure resistance from the (-)lead of the cap to Q2 collector just to be sure.
If no solder or pad/trace issues, replace C48.
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Thanks g1,
I guess either I misread or recorded the ac voltages incorrectly, or something changed after I replaced parts.
I reflowed the solder on the main filter caps and checked resistance to all connections when I originally pulled the board so will assume c48 is bad. Will order new filter caps!
Thanks again
Hi all,
I finally got hold of the new filter caps and had time to install them. I also replaced q9 while the board was out.
Long story short - I still have the same or similar buzz happening.
After cap replacement I checked for dc on speaker 0.023vdc
Tp1 30.05vac
Tp2 30.00vac
Tp3 39.02vdc
Tp4 -39.03vdc
Ac voltages
Tp3 0.066vac
Tp4 0.209vac
I also checked for broken pads/cracked solder and all checked out ok.
Thanks,
Andy
Checked the ac on speaker 3.59vac
It drops to 1.32 wiith a jumper from cr3 anode to ground.
Just ran the preamp out into another amp. The buzz is there but it is quieter now and does not sound like the previous 100hz hum.
With the main filter caps replaced, could the next suspects be diodes breaking down under load in the power supply?
Yes, maybe check them.
Phil
I am reading 30.00vac at r82. Does this suggest the rectifiers are faulty? You cannot view this attachment.
No.
Look at your schematic. R82 connects directly to CP5, which the schematic says has 31.9va on it. Close enough.
Thanks Enzo,
Where would be the next logical points to test?
I get the impression you are not clear on diode testing, If so then
You check diodes with power OFF and lift one end of the diode.
Meter set to diode test.
If you get a reading both ways it's dud.
To save writing it all,, go here;
https://www.fluke.com/en-us/learn/blog/digital-multimeters/how-to-test-diodes
HTH,Phil.
Thanks Phil,
This is the method I have been using so far, I thought there might be other ways to test in circuit.
Cr22,24 and 25 all have forward voltage of 0.5v or slightly above and read infinitely with leads reversed.
Cr23 slightly lower hovering around 0.47v and reads infinitely with leads reversed.
Yep diodes look fine,, moving on,,,,
You mentioned in the first post that you replaced u4 which fixed the channel switching.
well the switching circuit power comes from the main AC at CP5. As u4 was dead there maybe other damaged parts causing ac to bleed through to the audio path.
Switching circuits always confuse me and Others here will likely know more but if it was me I would lift the top end of R82 which would remove power to the whole switching circuit and see if the buzz goes away.
If it does kill the buzz then I'd be checking for dead parts around U4. maybe one of those diodes are fried.
your amp won't work per normal but it may help to isolate where the buzz is actually coming from.
maybe check C44 ,C51,C52
Phil.
Diodes around u4 18-21 tested good.
C44 51 and 52 tested good.
I haven't yet lifted r82 but will do so.
With regard to ac in signal path - should I expect to read similar vac measurements between cp1 and ground and cp2 and ground when nothing is plugged into the headphone jack?
Measuring CP1&2 at this point is not going to tell you where the issue is.
Divide and Conquer is how you find the problem.
So go lift R82 FIRST
If no change then you move on to the next step.
So lift the top end of R82 as that will isolate the WHOLE switching system.
(top end as drwan in schematic is the AC input from CP5 which powers the switching)
Then power up
If no signal passes through the preamp then try inserting guitar into poweramp as it is separate from the preamp and switching so it should still pass signal.
If you can get a clean signal with no buzz then it's a good bet the switching circuit is bleeding AC back into the circuit somewhere.
If that kills the hum/buzz then you at least know the switching circuit is likely at fault. I'm assuming it could be any place but seems obvious that Q3,4,5 or 6 are possible suspect points for AC to bleed through.
If they are working then maybe C44, C45 or C46 might be bad.
As I said better minds may know more.
Phil.
The diodes may be producing some RF induced hum. Try connecting 0.01 uF capacitors across each of the diodes to see if it reduces the hum.
Regards.
Buzz still present with r82 lifted.
Ok,, Now you had some trouble reading diodes before so maybe go back and recheck the main diodes in the power supply,
that's CR22,23,24,25.
It is the most obvious point of hum and you only need one to fail and induce hum through the whole circuit.
If you are not sure just replace them anyway as they are not expensive parts.
Phil.
Thanks Phil,
I will take another look at cr22-25.
Do the readings for those diodes in post #112 look off?
With the buzz still present with r82 lifted does it narrow the fault down to the power supply?
Yes The power supply is the most likely suspect but you said it checked ok so we went looking for other issues.
Maybe your meter is not working well,,they do get old and can give suspect readings especially cheaper ones.
PSU is 4 diodes and those 2 main filter caps.
Phil.
New main filter caps and psu diodes
Cr22- 25 replaced.
Buzz still there.
Tps 1-6 check out as per schematic
But excess ac still present on tp4
Ac on tp3 0.068
Ac on tp4 0.235
Ac across speaker leads 4.5vac
Then the most likely place is the power amp itself. You said in the first post you replaced the 4 power transistors.
Whatever caused them to fail may well have taken out other parts of the power stage.
We have been over the power amp before but maybe there is a need to look deeper into that. You will need better minds to establish just what has failed.
Phil
Thanks for your help Phil- I will have another look at the power amp- will pull the heatsink diodes cr6-8, plus cr10/12 and check the electrolytic caps.
If anyone has any ideas about how to track this fault down, I would be glad to hear them. Thanks, Andy
The most likely fail of the power amp was a short circuit or running very low Z speakers at high volume for long periods. Well you could try replacing Q8 & Q9 as they may have blown when the power units failed.
They are just there as over current protection but that setup is not fool proof so now they maybe actually causing the problem. They are not part of the Audio pathway only protection from shorts and over current.
The active audio parts of the power amp are only the opamp front end and the 4 power darlingtons which deliver the grunt to drive the speaker.
Don't hold me to it but I think that the amp might work with Q8&9 removed.
Try at your own risk. :-X I'm kinda busy at the moment but I'll try to draw up a signal path overlay on the Pwramp section might help you understand the circuit. Phil.
The Blue and Orange traces are the main audio path from U1 output.
Blue side carries positive wave while Orange is Negative wave.
CR5 to CR8 are the bias diodes for the output Tr's.
C39 & C40 are the Bootstrap setup (Google these things to learn more)
R76 is the ground lift to speaker Neg terminal, Current Feedback is then sent through R77 & C35 and returned to pin 2 of U1
The circuit uses both voltage Fback as well as Current Fback.
R65 sets Voltage FB while the combination of R76, R77 & C35 deliver the Current FB and meet at Pin2 of U1.
Remember these amp designs are Current Feedback via R76 so Speaker neg is ground lifted. *So don't ever ground the speaker neg back to chassis Com*. :-X
I've deleted most of the current Protection parts to help you see the circuit in it's basic form,, *Which is just a current Boosted opamp*
Of course without the over current protection the Output Tr's would instantly blow if the speaker wires were shorted out.
(No need to ask how I know that to be true,, :'( )
So if all the parts in my Edited drawing check out ok then I'd suspect the protection setup has malfunctioned when the Pwr tr's blew.
So that would be Q8 & Q9 as well as those associated diodes an Caps etc.
Now I may have missed something so don't take this as completely correct.
hope it helps, Phil.You cannot view this attachment.
Thanks a lot for this - will start working through this over the next few weeks.
Great explanation Phil, helped me understand it better.
Thanks.
Michael
Glad it helps,, I'm not qualified enough to understand some of the more complex power amp designs but yes once you remove the current protection it makes more sense.
I've designed a few plain and simple power stages and had great success just by installing a PTC on the output. In this case one PTC would replace 18 extra parts and as likely in this case the Protect circuit may well be adding to the problem.
If a poly switch fails the amp stops working and you only need to replace one part.
PTC is a Poly Thermal Capacitor, Often called a poly switch because they switch to high resistance when they reach the set threshold. They do this switch faster than a power transistor can fail, effectively doing the same job as the rather complex current limit in the power amp.
BTW Tassie I buy them at Jaycar if you want to source some. ;)
Link,,, https://www.jaycar.com.au/rxe185-ptc-fuses-speaker-protection/p/RN3468?pos=2&queryId=4192c822761e620ef506c2f53a24ce71
Re my drawing;
I'm still miffed about R69 & R66. ???
I think R66 is added Fback from output but R69 is really weird as they connect to
C37 & C38 which are filter caps for the opamp supply which I would assume would normally go to ground but they are summed at R65 which goes back to main output.
While R66 takes it back to the junction of R62 & C35.
Maybe better minds might like to comment on that subject.
Phil.
Hello,
I am looking at replacing the power transistor bias diodes cr5-8.
Can anyone advise me whether I can substitute the Byv26d diodes for Byv26e in this application?
The Byv2de is easier for me to get hold of and a lot cheaper.
Datasheet attachedYou cannot view this attachment.
Thanks
The only difference between the diodes is the breakdown Voltage. In this amp, the diodes are not being operated anywhere close to the maximum rating so substitution is OK.