Peavey CS400 repair struggle

[mailk]

Hello everyone!

I've read several threads on this and other forums about the CS400/800. Enzo's posts were a big help to this point, but i'm stuck now.

A received an old CS400 last week. It's the 2 power board and 2 driver board type with triacs on the output.
It was blowing fuses. I took it apart and on the B channel there were 2 transistors blown.
On each power board the 3 0.1u 250V capacitors were blown with the near by 5W 5.6ohm resistor.
There were a few little resistors that blackened too.
I replaced everything and put it back together.
I took out U1 from the input board.

I disconnected the B channel to get one channel working, so i'm talking about channel A from now on.
As i power it up, my current limiting bulb shines bright.
No DC and no shorts on the output terminals.

I connected an old speaker and a signal from a phone.
On power up there is no output, but when i disconnect the speaker and connect it back, i can hear the music from the speaker, but its like 1V peak to peak and i can hear a loud humm.
I measure around 5V on CR6 on the driver board. I replaced it, but the problem is the same.

My measurements on the driver board ic-s:


I would appreciate any help, because this thing drives me crazy...

Excuse my English, i'm not a native English speaker.

[The Dude]

A few things in no particular order:

1) Your negative 15V supply looks slightly low. Possibly poor filtering. Check the line for ripple.
2) Did you check the triac(s)? Often when a channel blows it takes out the triac. That's what it's for- to prevent DC from reaching the speaker. You can unsolder them for testing. Just make sure you check for DC before connecting a speaker. Without the triac, there is the possibility for DC.
3) Are you using a variac? With the triac installed, if the rails don't come up evenly, there is DC at the output and the amp will draw heavy current via the triac until things stabilize.
4) With the triac(s) installed, you probably won't measure DC on the output, but that doesn't mean there's not a problem. That's why it may be best to temporarily unsolder it.

P.S. Welcome to the place!

[mailk]

A few things in no particular order:

1) Your negative 15V supply looks slightly low. Possibly poor filtering. Check the line for ripple.
2) Did you check the triac(s)? Often when a channel blows it takes out the triac. That's what it's for- to prevent DC from reaching the speaker. You can unsolder them for testing. Just make sure you check for DC before connecting a speaker. Without the triac, there is the possibility for DC.
3) Are you using a variac? With the triac installed, if the rails don't come up evenly, there is DC at the output and the amp will draw heavy current via the triac until things stabilize.
4) With the triac(s) installed, you probably won't measure DC on the output, but that doesn't mean there's not a problem. That's why it may be best to temporarily unsolder it.

P.S. Welcome to the place!

Dear The Dude,
I don't have a variac unfortunately.
I unsoldered the triacs. Still no DC on the output without them.
My measurements on the triacs:

What did you mean when you wrote "Check the line for ripple"?

Thanks.

[Enzo]

The ICs run on +15 and -15, your +14 is close enough, but that -11 looks suspicious. One reason a 15v supply line might measure low is that it is not being filtered well, in other words high level of ripple. Ripple is the ragged waveform of rectified DC with no filter cap. So check that -15 to see if it has a lot of AC ripple on it. And that would cause hum certainly.

Pin 8 of the 458 and pin 7 of the 5534 are wired together, or should be. SO if you have 14v on the 4558, it should be also on the pin 7 of the 5534. You have 4v written there. is it really 4v, or was that supposed to be +14?

Note that pin 4 of the 5534 gets its _15 through a dual diode CR5 on some schematics. If there is a problem with CR5, it might explain the lack of -15 at the IC pin 4. Newer boards eliminated that diode, and pin 4 of the 5534 is wired directly to the other IC, and the connector pin on the edge of the board.


If you get 5v on CR6, how can you have 11v on pin 4 of the 4558? They are wired together.

Where are you grounding your meter for these readings?

[mailk]

Dear Enzo,

Sorry, i must have messed up my measurements.

With no triacs, input or output connected, i have +14,8V and -14,23V on U2 and 14,3V on CR6, on the driver board. (same on U1, because they are connected directly as u said)
Seems good.
I don't have a CR5, the schematic shows it but i cant find it on the board. 4th pin of U1 goes straight to the connector -.

If i power up the amplifier with the speaker connected, i have 5V DC on the output, when i disconnect the speaker and reconnect it the DC disappears and noise appears (i can change the volume of the noise with the volume control knob).

The power transistors must be good, i took it apart several times now and measured them every time. Also i changed the bulk of the parts on the power board. I changed the 5,6R 5W resistors, capacitors, little resistors.
The big resistors measure fine.

UPDATE:
As i mentioned in my first post i'm working on channel A at the moment. So i completely disconnected channel B and its input board too.
If i reconnect channel B's input board but not the board itself, i get the messed up measurements on channel A's driver board U2 +14V, -11V. If i disconnect the B input board, it goes back to +14,8 and -14,3.

[The Dude]

Something on the B channel is loading down the -15V supply. There are several versions of the CS400. Can you post the schematic of the one you are working on?

[Enzo]

I thought I mentioned that CR5 was on the original schematic, but was eliminated from later versions.

[mailk]

I thought I mentioned that CR5 was on the original schematic, but was eliminated from later versions.

Yes. I just verified what you wrote.

This is the schematic i'm using:
Peavey CS-400 (C-Series, 7-79) Schematic.pdf

[The Dude]

Another thought: Make sure the amp is in stereo mode and not bridge mode while testing.

[mailk]

Another thought: Make sure the amp is in stereo mode and not bridge mode while testing.

I made sure it's in stereo.

[mailk]

I spent half of the day trying to figure this out, here is where i am now:

I put back the triacs into the output board. The output terminals aren't shorted together.
Whenever i power the amplifier with a speaker connected, i get 5V DC on the output, if i disconnect it and immediately connect it back the DC disappears.

It seams that channel A's driver board is faulty, I measure around -11V on CR6. Channel B is fine voltage wise. (This is only true when the board powered up alone.)
But the 5V DC thing is present no matter which channel is connected up.

If i power up the two boards together, the CR6 voltage falls to around -5V on both boards.

[Enzo]

Mark them so you know, and swap the two driver boards. Do the issues move with the boards, or does some of the problem stay on the channel even with the new card?

[mailk]

Yes, the problem moved with the driver board. But the 5V DC was present on both channels.
I decided to change all the electrolytic caps on the faulty board. It wasn't much of a change.

I disconnected the faulty channel an tried out the good channel without the current limiting bulb. I got a good +-15V on the opamp, and there was no DC on the output. Great.
After this, i connected only the "faulty" channel without the bulb and fired it up again, and surprise, there was no DC on the output either, and i measured a stable +-15V there too.

So its now back in perfect working order.
I've tested it over the weekend in a small party, and it's working great.

It seems after i replaced the bad parts on the power boards, it was working okey, but the current limiting bulb messed up my voltages and tricked me. :s

The only thing bugging me now that it has a really loud pop when i turn it on or off. It can't be normal.
Any idea why?

[Jazz P Bass]

Unplug the amp from the wall outlet with the power switch still On.

If it does not 'pop', then you have a switch issue.

[Enzo]

Oh yes it can be normal.