Peavet 400BH power amp problem

[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]

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.

[Streamer]

I'm putting an order in for some parts, I think possibly C37 and C38 have been leaking at some time. The big looking resistors R65 and R67 may have also leaked. I had been looking more for things that looked burnt but am now thinking this possible leakage is of real importance especially from the capacitors.

How important is it to replace the resisitors and do they need to be exactly the same - will any 1k 5w and 2k 5w resistors be okay.

I am having trouble finding the transistor no's for Q1, Q7, Q8, Q17, I thought it might be worth getting these while I am on it.
I have found a very helpful supplyer of electronic parts, but obviously Peavey part no's don't help him.

I'm sorry if these seem really dull questions

Thanks again for any advice

[J M Fahey]

Search this Forum for the page with the equivalence of Peavey numbers > Commercial numbers.

[Enzo]

It is a sticky even:

Peavey transistor cross reference

[Streamer]

Search this Forum for the page with the equivalence of Peavey numbers > Commercial numbers.

Thanks for that , I'll have a look

[Streamer]

I'm back,
I've been in the UK for a while, now I'm back in France with some parts. I have replaced some caps but am still short of an IC and another capacitor, am returning to the Uk next week so hopefully will come back with the parts I want. When I have fitted them I will let you know how I get on.

[Streamer]

I've been in the UK for a while, now I'm back in France with some parts. I have replaced some caps but am still short of an IC and another capacitor, am returning to the Uk next week so hopefully will come back with the parts I want. When I have fitted them I will let you know how I get on.[/QUOTE]


Well, I've put the new IC in and the capacitor, switched her on she has a nice hum, tested the speaker socket -54VDC, so I didn't plug a speaker in. So that wasn't so good. With the pre amp disconnected and even the glass fuses removed I still get -54VDC at the speaker socket.

She's not blowing fuses and I'm pretty sure I fitted the capacitors etc the correct way round.

It's probably best if I don't think - I'll just follow instructions (careful now) if anybody is has any,

thank you,

[Jazz P Bass]

The minus sign on the output voltage is a clue.
Hook the amp up to a lamp limiter.
Follow Peavey's voltage test points.(they will be a bit low because of the lamp limiter)
You will find something way out of wack.
Please keep this in mind.
The amplifier is a class A/B design.
Meaning: no output devices should be on (Class B) (well maybe a little ie:Class A/B) at idle (no signal)
Look at the base voltages & you will see some that are on too much (the negative voltage on the output says the lower rail is on)

[Streamer]

Thank you Jazz, I've made a limiter using a 40watt bulb, using the limiter the larger readings are down by 20 volts, and it has stopped R65, R66 and R67 getting very hot.

R48 has -55VDC and -55VDC on it, R56 & R57 have -54 and -55 on them, Q7 has -54 on all three legs, C18 has -55 and -55 all taken yesterday before the limiter, taken just now with the limiter C18 has -35 and -35VDC.

I've got to pack up now it's 1.30 in the morning here.
I did have one idea, I just wondered if you could possibly mark on the schematic where i could get the most useful readings without unsoldering anything,
perhaps that's not possible - it was just an idea, gotta go my brain cell is aching, thanks again

[Jazz P Bass]

Check Q12.
I'll bet it is shorted out.

[Streamer]

Hi Jazz,

You were spot on, I lifted Q12, tested it and it was shorted, fair play well done. I've replaced it
The voltage at the speaker socket has now dropped from -54V to 6MV .
I still haven't plugged a speaker into it as I wasn't sure if 6MV was safe, it certainly doesn't sound much - but it's not zero so I thought it best to play safe.
R65 is still getting pretty hot without the lamp limiter - too hot to hold your finger on, I don't know if this is normal or not.

With the 40w lamp limiter in i have these Voltage readings, I don't know if they are relevant or not,
R65 37 and .3, R67 -37 & 0, C35 -0 and 37, VR1 15.2 3.7 20, VR2 -.2 -20 -15
C34 -8 & -8, Q12 12 -37 5.7

The limiter knocks about 20 off the higher readings
Thanks again

[Jazz P Bass]

6mv's dc offset is pretty darn good!
I don't quite get why R65 is hot.
Of coarse it is 'dumping voltage'.
And it is rated at 5 watts.
So it will get hot.
I do see that the dropping resistors are different for the + and the - rails.

[Streamer]

Thanks Jazz,

I connected up to the pre amp plugged her in and switched her on, she hums but not that much, plugged a speaker in and a bass to try her,
and I would say just the pre amp is working. But she didn't blow any fuses or the speaker, R65, R67, R68, and R66 are all really hot, they even smell hot.
So I turned her off. I took the small IC out just to see if it made any difference but it didn't.
Any ideas

[Jazz P Bass]

No way I can repair this over the Internet.
You have to take some voltage measurements.
Look on the schematic exactly where the +15 & -15 connect to the output stage.
Something is wrong that both are getting hot.
Use the schematic & the voltages that Peavey so thoughtfully provided.
If you find something that does not agree with the schematic voltages, try to think through why it is that way.
Wrap your head around the fact that it is a class B amplifier.
The output transistor bases should sit at around 500 mv's.
If they are not, find out why.

[Streamer]

Thank you Jazz, you probably won't believe this but up until this moment I couldn't see how the different parts of the schematic joined together,
but then reading your reply and looking at it again it suddenly became obvious.

I have been trying to get an understanding of capacitors, resistors, diodes, IC's and zener diodes and have even gone back to check up on ohms law.
I shall spend some time with the schematic, my tester and my brain cell into gear.

Thanks again Jazz, I really do appreciate your time and effort.

[Jazz P Bass]

As far as reading the schematic goes, here is a simplified explanation of the transistors.
Again, basically.
An NPN (emitter marking N ever P oints i N) transistor set up as a switch is considered off if the base voltage is low.
Raising the base voltage will turn it on.
Set up as a Class A amplifier the base will be biased on to pass an ac signal.
The PNP (emitter marking P oints I n P ermanently) as a switch is considered off when the base is a high voltage.
Lowering the base voltage will turn it on.

[Streamer]

Thanks for the extra info Jazz, cheers