Peavet 400BH power amp problem

[Jazz P Bass]

At R66 i have 9 and 49 VDC, at R68 -9 and -49 VDC, at C43 9VDC and at
C44 -12VDC

Something is amiss with these voltages.
The indicated +9 & -9 should be +15 & -15Vdc.
Either something is seriously loading them down or both zener diodes are bad. (CR20 & 21)

[Enzo]

You say you have 49 and 9 volts on both those resistors, yet you have 9 and 12 on the two caps. Since the caps and resistors are wired together, the voltages at the right ends of the resistors ought to match the voltages across the two caps.

In any case, since both low voltage supplies are low, it MIGHT be two bad zeners, but I'd be more suspicious of a bad IC on the board. Does either IC get warm just sitting there? Remove the ICs from their sockets, now does the voltage at those two supplies rise back to 15v?

By the way, your preamp works? Fine, disconnect the cable to it from the corner of the 400BH board and set it aside. That will allow more room for you to work.

Sort out these low voltage issues, and you may find the rest of the amp falling into place.

[Streamer]

Hi Jazz,

thanks for your help , I've removed C20 & C21 and I think they are okay.
with my tester on resistance 20K neg lead on the dark ringed end I get 12.30 swop the neg and pos leads around and the meter just stays on 1.
so something is happening one way and not the other - does that make them good?

Thanks as well for the attached file , I'll leave C 20 and 21 out for the moment until I hear that you think they are okay.

cheers for now.

[Streamer]

Thanks Enzo,

when I have the okay to put C20 and C21 back in the board, I'll disconnect the pre amp, power her back up and check out the ICs.

Thanks again.

[52 Bill]

with my tester on resistance 20K neg lead on the dark ringed end I get 12.30 swop the neg and pos leads around and the meter just stays on 1.
so something is happening one way and not the other - does that make them good?

You are charging the cap with the battery in your meter. When you reverse the leads your meter is seeing the small voltage that is being stored by the cap, that is why you are getting the 1 reading. If you keep the leads attached the voltage will slowly drain off and then you should see a rising number probably back up to 12.30.

While this test does not actually test the capacitor, it does prove that they are not dead shorts and are probably ok. I'd put them back in, watching the polarity, and go on to the ic test that Enzo suggested.

[Streamer]

Thanks for that bill 52,

I have put them back in and have done as Enzo suggested

[Streamer]

Hi again Enzo,

I have put the zeners back in and plugged her in, strangely at R66 I am now getting 11 and 52 , and at R68 -14 and -50

I would say the larger IC warms up a little

with the larger IC removed R66 goes up to 15 and 51 R68 remains the same, and removing the smaller IC makes no difference

Looks to me like more progress, I'll buy a new large IC

Thanks again

[Jazz P Bass]

That 'large' ic is a TL074, IC1.
It is the output section 'driver'
Have you checked Q8?
It may have taken a hit through all of this.

[Streamer]

Hi Jazz,

Thanks for that, i've taken Q8 out and it seems okay so I have put it back in.

I have a bass head with the same power amp so I'm thinking I might borrow the IC from that, just as a test.
Thanks again.

[Streamer]

Hi Enzo,

I had Q 8 out and it seemed okay.
I have just replaced the large IC with one from a working bass head I have with the same power amp, I'd be keeping away from it but thought it just worth lifting the IC to try it, the numbers are the same.

The result was the same, no voltage change. so I put the original back in.

Then I took a step backwards, I was probing with my tester on Dc volts, and like an idiot, I managed to short between the body of C38 and the leg of CR21,
I can't believe I did it again what a fool. There was a flash but the fuse didn't blow, I tested the DCv quickly and it seems I now have,
-52 and -126 !!! VDC at R68
and 52 and 57 at VDC at R66, Iturned it off real quick. can't believe I did that.

I have to thank all you guys for your patience - I'd test a saint - sorry

[Streamer]

If at first you don't succeed - stick at it,

Hi Enzo,

I had Q 8 out and it seemed okay.
I have just replaced the large IC with one from a working bass head I have with the same power amp, I'd be keeping away from it but thought it just worth lifting the IC to try it, the numbers are the same.

The result was the same, no voltage change. so I put the original back in.

Then I took a step backwards, I was probing with my tester on Dc volts, and like an idiot, I managed to short between the body of C38 and the leg of CR21,
I can't believe I did it again what a fool. There was a flash but the fuse didn't blow, I tested the DCv quickly and it seems I now have,
-52 and -126 !!! VDC at R68
and 52 and 57 at VDC at R66, Iturned it off real quick. can't believe I did that.

I have to thank all you guys for your patience - I'd test a saint - sorry

........................................................ ........................................................ ........................................................ ............................
I'm not beaten yet, I'm going to pull out CR21, CR20 and C38 and test them,

[Jazz P Bass]

You really, really should have learned by now, that you must power down the unit before making any connections with your meter.
I know it is a PITA but that is how it is taught in trade schools.

[Streamer]

I couldn't agree more Jazz, and I can see the sense in powering it down - I thought I was just clumsy, but - heck another stupid question coming up, how do I test for DC and AC voltage with no power in the unit, am I missing something really obvious - yeah I know, a brain, but besides that.

Keep your thoughts coming, I really appreciate the time and help you guys give.

[Jazz P Bass]

What I was implying was 'turn off the unit, hook up your test probe, turn on the unit.'

[Streamer]

Hi Jazz,

That makes sense , it's so obvious when say, I need to get some crocodile clips,
thanks again Jazz, that's something else I've learnt.