Peavey 400 B/G Module - Supply Voltages Low

[g1]

Don't let the looks fool you, it's a capacitor. If the schematic in the first post is the correct one for your amp, then C26 is 180pF.
Any 180pF cap should work, it doesn't have to be one that looks like a resistor.

[52 Bill]

Bro TomCarlos, I have the same board, same area burned. replaced all the resisitors in that area, knocked out pre's, replaced those. also q12 replaced. one prob so far. C26 is burned. little mustard yellow resistor looking fellow, what is it and where do i get it???? the C26 thing is throwing me. C= cap , but sure looks resistor to me. in either case, i cant find one anywhere. hope someone can help on this one. Even sent email to Peavey parts dept, they gave me a "Dunno"

The schematic show it as a 180 pf cap. You should be able to find one at any of the major suppliers. It can be a simple ceramic disc type if you can't find the tubular style version.

[Jazz P Bass]

Is the cap open or did it simply take on some of the smoke?

[TomCarlos]

Alta ... The others beat me to it. As mentioned, it's a Cap ... 180pf. I have worked on a few of these boards over the years and I'm sure that I have used different looking caps on all jobs. BUT... the value was the same and never had a problem replacing this cap with a generic part.

[altazepsta]

Alta ... The others beat me to it. As mentioned, it's a Cap ... 180pf. I have worked on a few of these boards over the years and I'm sure that I have used different looking caps on all jobs. BUT... the value was the same and never had a problem replacing this cap with a generic part.

Thanks all, finally got back to check on this. emailed Peavey and ordering several other parts as well. they dont use those types anymore either.

[slipnote]

Sorry to revive an old thread, but all the relevant info is here so ... Working on an XR600B with the same 400 B/G board in it. R61 is burned beyond recognition. Here's where it gets interesting: There is no L1 inductor next to it. Instead, there is a 4.7 ohm resistor in parallel with the identical 4.7 resistor R44. By the looks of the solder side, there is no question this is original from the factory. Is this a factory error? Not sure what to do ... any thoughts appreciated!

[52 Bill]

There is no L1 inductor next to it. Instead, there is a 4.7 ohm resistor in parallel with the identical 4.7 resistor R44. By the looks of the solder side, there is no question this is original from the factory. Is this a factory error? Not sure what to do ... any thoughts appreciated!

Factory error or factory change?

If you are certain that the amp came this way from the factory and you are certain that it was once working this way, stop looking for reasons why and just fix the amp.

Companies make changes all of the time and not all of them are documented.

[g1]

Looking at the second picture in post #1, those burnt parts also all look like resistors. Isn't L1 one of those?
If they found out and inductor was not required and a resistor would do the job, that would probably be a significant cost saving.

[slipnote]

Yep, I know what you're saying. Not sure of the history of this thing - belongs to a friend who picked it up somewhere in non-working condition, but certainly reasonable to assume that it was working at one point. The thing is, I'm just not smart enough to understand the consequences of replacing an inductor with a resistor. Will replace the burnt resistor and check for shorted transistors on that side of the circuit.

[slipnote]

g1, yes L1 is the second burnt part from the left. Does look like a resistor, but it is possible to get molded body color-banded inductors, so hard to say.

[Enzo]

Finding that small inductor replaced by a 4.7 ohm resistor is common in these amps, don't think further about it. it is not an error at all. Look at the slightly later and almost identical 400BH and the change is incorporated into the schematic.

If R61 is burnt then we can about guarantee that Q12 and/or some of the output devices on the lower side are bad. Resistors never burn up on their own, it is always from some other failure drawing too much current through them.

[slipnote]

Thank you Enzo.

I've replaced R61, also replaced the 4 large power supply filters caps - one had already been changed out (poorly). I brought it up slowly on the variac and everything is holding - no smoke, no excessive current draw. However, output is weak and distorted through the power amp (mixer section tests good). Main supply voltages are out of whack at +39 volts and -62 volts so yes, something else is definitely wrong.

[Enzo]

There is a vertical string up through the 47 ohm R61, then Q12, then the pair of 4.7 and on up to V+. Check ALL the parts through there. Check all the lower power transistors and important to check all the resistors associated with them.

[slipnote]

Problem solved. After checking all the transistors, went back to the power supply and discovered a small break in the ground trace for C34/C35 - no wonder the bias voltages were nutty. Also found hFE readings on the outputs ranging from 12 to 65 so changed them out for new. Sounds great now, thanks to all who contributed.

[Enzo]

Glad you figured it out. Welcome, by the way.

Just a suggestion, in the future, you will usually get better response starting a new thread for an amp problem rather than tacking o at the end of a dead thread.