Hey Enzo and Roly,

Wow, thank you Enzo and Roly for your time again. I feel like I should be paying you guys! I went over all your equations and tried them on my own and I understand them. A few more pennies dropping for sure. You can look at this stuff for months and years on your own and not have a clue what it is you're really seeing. So thanks for the virtual apprenticeship :tu:

Thanks also for the info about the Darlington Transistors with the built-in driver transistors, I wouldn't have known that.

QuoteThe voltage across the emitter resistor is a proxy for the current passing through it and the transistor.  The voltage to ground is effectively the output voltage, whichever side of the emitter resistor you measure it.

When you say a proxy do you mean I can treat as a gauge for  current flow? With multimeter check for high voltage, therefore high current, low voltage low current?



What is the purpose of D4,D5, what is their function in the current limiting circuit? To take only the pos/neg half of the signal? D5--neg. half, D6, pos. half? Both turning on at.7v?


What is the purpose of TR9, R73,R74, TR4, TR7, C49, C47? (I might as well work backwards to fully understand this power amp). Thanks again.

Also, a buddy is bringing over an old tube radio he wants me to fix for him. Can anyone suggest a good forum to ask questions?

Enzo thanks so much for the response. Below I quote several times from a post on another forum that didn't involve myself. This person gave a fairly thorough analysis of this amp to someone trying to figure it out. When I was referring to a voltage divider I thought I knew what I was talking about as in this same post he referred to R74 and R73 as a voltage divider and set about making calculations 1800 + 680/680 X .7=2.55 volts. He did this to calculate Collector Emitter voltage across TR9


"For TR8 the resistors R116, 117, 118 and transistor TR12 and D5 make a current limiter.
For TR10 the resistors R96, 97, 115 and transistor TR11 and D4 make a current limitter."

So, below, is where my intial confusion came because I couldn't measure the voltage he refers to: 1.92V
"The calculations for the current are similar to the calculations at TR9.
It will result in 1.92 volts accross R118 , this will give a limit of 1.92 / 0.33 = 5.82 A
For R96 it will result in 2.14 Volts , this will give a limit of 2.14 / 0.33 = 6.48 A."

I somewhat understand what you're saying about  pulling down the signal on TR8 if the voltage gets to high. By signal are you referring to current, therefore this config. acts as a current limiter? 

One day the penny will drop, but it sure is taking awhile!

So this amp is working and using a voltage divider equation there should be 1.92 volts across R118 (should there?). But when I measure with my voltmeter I get .007vdc. I put a 1khz signal through the amp then I played my guitar and measured the voltage across the resistor and measured it referenced to ground but nowhere near this 1.92v. What am I missing here? I think I'm missing the obvious but what?

R117=820ohms, R116=470ohms

820+470/470=2.74X.7=1.92V

QuoteWith guitar amps, Fenders the component boards are so wired down you have no hope of swing one up, but other amps, I'm thinking Overreem, all the connections to the boards come off one end - a few screws and presto, you can work on the board hot, standing on end.

We had two rack amps in the PA, a New Zealand Perreaux, and an American CueTek.  The most significant difference between them was that the Perreaux had a removable lid and bottom, leaving the amp totally accessible, while the CueTek was poured into a U-channel case and you couldn't get at anything.

A lot of gear is designed to be manufactured.  It is not designed to be disassembled.  At all.  Ever.  Much less repaired.

A large part of my life has been figuring out how to get assemblies open, particularly stuff that wasn't intended to be opened again.

Thanks Roly, your years of experience pretty much says it all. Especially "designed to be manufactured not repaired". I'll soon discover what I can and can't do with amps, and hopefully I'll have some stories to pass down. :tu:

With this amp and others is there a way to have the circuit board out of the amp with the appropriate wires still connected so that I can test components with the amp in it's "on" state? Is it a matter of making sure the pots that need to be grounded are grounded?

Great help and answers. Thanks guys!

This amp is dirty! What do people recommend for tolex/faceplate?

Thanks. So obvious now. Amp is working as it should. I've never used outboard effects so I've learned something new.

 Probably a simple fix but when I turn the effects knob (eff)  to the right the volume cuts out, starting at about 12 o'clock and then goes completey dead when turned all the way to the right. I used Deoxit and got rid of all the scratchiness but same problem. I removed the pot and measured 22K as it should be. As I turn the pot to the right it's as if the signal is being shorted to ground and then at the far right: dead, volume completely cuts out.

 Thanks for the detailed answer JM!

QuoteThat's how I'd do it - but why?  Why would you want to attack the power amp module while trying to trace a source of hum/noise injection? (I've lost track...)

Sorry Roly,  I'd gone on to another question as I was hoping for a reply to my reply regarding my novice attempts to apply Enzo's hum/noise injection statement: " "But you can get an absolute handle on it by actually measuring the hum and noise on the output with the gain, master, and EQ controls all set for 12 o'clock.  This will give you a representative peak-to-peak value which you can compare to the full output peak-to-peak (the +/-ve supply rails, in this case I think +/-60V, and work out in dB just how far down your hum and noise is on full output.  You may discover that it's within a reasonable spec. dB = 20 * log10(V1/V2)"  Okay, this really interests me but not quite  sure how to proceed. Do you mean V1 is the 12 o'clock voltage and V2 is the voltage with the gain and master full-on?  With the settings at 12 o'clock, and the speaker output attached to a 4 ohm dummy load, and nothing plugged into the amp, I get a value of 13 mv (using my voltmeter) across the dummy load. With the gain and master at full volume I read a voltage of 70 mv.  Am I on the right track before I start play with the log calculation?  Thanks for all your help.

QuoteDo we have a baseline measurement of hum+noise at the output with the input of the power amp shorted (Main 0/10) ?

Please see my attempts in the above pgh.

So from the pic you can see the heat sink and two power transistors at the front of the pcb, but there are also two at the back, plus others. From the circuit board  there are holes so that you can use a screwdriver to remove the transistors. Guess the only way to remove a defective transistor here is to remove the heat sink from the back of the chassis then unsolder all the power transistors/transistors so that the pcb can be removed so that I could  change the defective transistor (there isn't one but just wondering). Am I missing something here? Is there an easier way? Should I consider long thin pliers to snip the transistor leads and unsolder the leads? Or would it be best to unscrew the pcb from the heat sink and unscrew all the transistors and re-apply thermal paste to all of them and screw them, including the new one down? Lot of work to change one output transistor.

What works best for you techs out there? What would be the most efficient way?

Also, the red caps. To get a true reading in both the small pre-amp caps and the small power amp caps it looks like I'd have to remove the boards so that I could unsolder the leads and check them with a cap meter? Is there an easier way?

Thank-you!

QuoteLook just above the posting box while you write.  Just below the FONT SIZE box is a small symbol that looks like a thought balloon like in a newspaper cartoon.   THAT is the quote button.  If you click on it first, you find the starting and closing things for your quote.  Just copy and paste the quote between them.

Thanks Enzo! (Still hoping for a response to the first part of my post re:hum/noise calculation :cheesy:


'.

Enzo said: "Before just throwing new caps at it, you might check to see if the cap is really the source of the hum.  If only one of the caps measured funny, then presumably the two supplies will have differing amounts of ripple.  Do they?"

On the 250 volt supply I probed at points from the rectifier to the tube and there was no AC ripple. But when I checked the 15 V supply I found ripple on the neg side of the 6800uf cap and on the pos. side of the other, approx. 40 mv. But when I probed R1, R2, and the diodes/zener diodes there was no ripple...all tests done with a 50mv/1khz signal injected. Thanks Enzo!

Great, thanks. Excellent info.

I'm going to try and drill down here so I understand everything that is being said.

"The other supplies to check are the +/-15V for the preamps (across the 1N4737A zener pairs) the hum here should be quite low"  Here, using my scope  I found some ripple but very low, approx 20mv. Even with a 50 ma 1khz signal injected the ripple wasn't much more. Acceptable?

"With the master at 5/10 but the gain at 0/10 you will add the hum/noise contribution of the preamp back end, and with the gain at 5/10 you will add and get and idea of the hum/noise contribution of the preamp front end." Tried this but no audible difference. A good thing, right?

"But you can get an absolute handle on it by actually measuring the hum and noise on the output with the gain, master, and EQ controls all set for 12 o'clock.  This will give you a representative peak-to-peak value which you can compare to the full output peak-to-peak (the +/-ve supply rails, in this case I think +/-60V, and work out in dB just how far down your hum and noise is on full output.  You may discover that it's within a reasonable spec. dB = 20 * log10(V1/V2)" 
Okay, this really interests me but not quite  sure how to proceed. Do you mean V1 is the 12 o'clock voltage and V2 is the voltage with the gain and master full-on?  With the settings at 12 o'clock, and the speaker output attached to a 4 ohm dummy load, and nothing plugged into the amp, I get a value of 13 mv (using my voltmeter). With the gain and master at full volume I read a voltage of 70 mv.  Am I on the right track before I start play with the log calculation?  Thanks for all your help.

Having listened to and tried to apply everything so far I'd say it's fair to assume that this amp doesn't have a lot of hum, and, at first, a lot of it was coming from my bass plugged into the amp. The controls are dirty so I'll apply some deoxit. Otherwise, at this point, I'd say this amp is doing well.


P.S. How do I insert quotes on this forum?   Also, how can I save all these posts as they are great?

Thanks.

A few things: by shorted you mean attaching a 1/4" jack but with out cable attached? (You're right about the noise being worse with the guitar plugged in. Without anything plugged in the amp, the hum is much better I just want to know that I can reduce the hum as much as possible. Again, increasing either the bass/treble increases the hum considerably. I don't have a new amp to compare it to so I'm not sure how much is too much).


I took the ground lead of the scope and attached it to chassis ground and the scope lead to the positive lead of the cap, then did the same with the other cap. However, now that I look at the schem. I see that the pos of one cap goes to the +ve rail and the neg side of the other cap goes to -ve, so my inclination is to check the -ve as well which I didn't do before. Is 40mv ripple acceptable? Also shouldn't I really be checking at R1/R2 as the caps are supposed to reduce ripple and the result would show at R1/R2? Thanks everyone!

(I have assumed that the caps are wired in correctly but I have will check that out later when I'm home.)