Peavey 5150 - Preamp and Poweramp seem fine, but NO SOUND

[greenridr]

Thanks, Amp Kat and John G for the input!

Unfortunately, I replaced Q10 as well as the resistors in question, and when I fired it up, 56 and 65 fried up again. Could a short in either Q9 or Q6, both before the resistors, lead to them getting hit too hard?

I'm willing to give this one or two more tries, but if I come up short, I think I'm going to just figure out a simple way to eliminate the reverb circuit.

[Enzo]

DON'T connect the reverb pan until we have zero DC on J8.

Q3 and Q10 are the only current path available to those 47 ohm resistors, but of course the other transistors can cause trouble. If you replace Q3, then replace Q10 as well. Replace them in pairs, so to speak. If the resistors burn up, that means BOTH transistors had to be conducting.

CR8, CR11 are CRITICAL here. If either one opens, then BOTH Q3, Q10 will turn on hard, and burn up the resistors, and themselves. SO with the amp off use the diode test to check each of those. But don;t just check each part, check for diode-ness from the base pin of Q3 to the anode end of CR11, and then again from the cathode end of CR8 to the base of Q10. That way you are testing the diodes through the wiring that connects them to the rest of the world.


And don't be penny wise and pound foolish. Those four transistors cost very little, just replace all four. Don't even think about if one might still be "good." Look at the layout picture in the schematic set. If your schematic lacks that page, call Peavey and ask for the 5150 COmbo schematic, it will be in it. Use that picture to make sure the transistors are installed facing the correct way.

There are only 10 resistors in the whole circuit, check each one. There are even fewer caps, any shorted? And if ANY of your transistors are not exactly the part specified in the drawing, make DAMN sure the substitute part is correct. No PNP where NPN belongs or some such.

[greenridr]

Good advice and encouragement...I undertook this project to learn more about amp circuitry, and that's what I'm doing, so why give up now! I'll set the momentary frustration aside, give each part a thorough look, and report back in a few days.

[greenridr]

OK-

So I tested the diodes, and they were both bad, so I replaced them both. I also replaced Q3 and Q10, and had extra capacitors for C38 and C39 around, so I went a head and replaced them. Resistance readings on the last resistors I put in was low, so I replaced them as well. I then checked the VDC on J8, which measured 0, so I plugged in the tank, fed the amp a signal and tested AC voltage throughout the circuit and came up with the following:

R31 - 2.4/2.4 V
Q9 (base) - 2.2 V
Q6 - 2.3
Q10 - 2.3
R58 - 2.0 V both sides
J8-J11 all read 2.4 V

Is it possible that Q4 is causing the trouble after all?

[Enzo]

Q4 is isolated from the reverb drive by a cap, so it cannot affect the DC levels in the drive amp. Q4 can ground off the signal or not.

J8-11 ALL have 2v of AC? 9 and 10 should be ground, are they? Ground shouldn't show 2VAC.

Make yourself a "signal tracer" and follow the signal audibly down through the circuit.

[greenridr]

Yes, J8-11 all show the same voltage when I have a signal running through the amp. Turned off, I checked for continuity between 9/10 and ground, and it was positive, so they are grounded, apparently. I can't figure out what is causing the voltage to show up on J9/10...

[greenridr]

Components for Audio Tracer ordered, will report back when I've had a chance to check it out!

[Enzo]

Wwhat are you using as a ground point for your meter?

[greenridr]

I was just connecting an alligator clip to the chassis...