Peavey CS800 oscillation

[Mathes]

Hello, I have a Peavey CS800 (rev B schematic) that had no output on channel B. The output tested shorted and C2, .1uF on the triac board was shorted. I replaced the cap and have full power output now but I also have an oscillation at around 250kHz when no load is connected. The oscillation stops when I plug in a load or when I unplug the input cable that goes from the board with the input jacks to the small board with the compressor LED and volume control on it.

I have swapped the small boards with the volume control and also the boards with the connectors that are mounted on the main power module boards with no change. The oscillation only happens with no load. While it was oscillating the heatsink gets very hot and I measured about 400mV across one of the .33 ohm emitter resistors.

Any suggestions would be greatly appreciated.

[Jazz P Bass]

Is C2 part of the zobel network?
If it is, the R part of the network may be open circuit.
Can you please post the schematic?

[Mathes]

Is C2 part of the zobel network?
If it is, the R part of the network may be open circuit.
Can you please post the schematic?

C2 is on the triac board across the output.
I checked the 5.6 ohm resistor that is part of the zobel network
(the one not in parallel with the inductor) and it is ok.

CS800B .pdf

[Jazz P Bass]

I cannot download the file.
It appears that you are linking to a previous post.
And that file is part of the 'lost files'.
If you have the schematic on your hard drive, please upload it from there.
(How to Upload)
Reply to this post
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Click on Manage Attachments
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Click on Browse (to your file location & click on that file)
Click on Upload
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[Mathes]

I cannot download the file.
It appears that you are linking to a previous post.
And that file is part of the 'lost files'.
If you have the schematic on your hard drive, please upload it from there.
(How to Upload)
Reply to this post
Click the Go Advanced Tab
Click on Manage Attachments
New Window
Click on Browse (to your file location & click on that file)
Click on Upload
Close Window
Click on Submit Post

I did upload the file from my hard drive, will try again.

[Enzo]

Not your fault, your file is the same file as was previously uploaded, and our system has a glitch. We need to resave the file to change its size a little, then repost.


You are using the B schematic, but is your amp actually a B? The B had only one IC on each driver card and the volume control boards are small with NO ICs and just two small transistors. If you have a couple ICs on the volume control boards or two ICs on the driver cards, that sounds more like a C version.

[Jazz P Bass]

Here is the schematic for the B.
I would try changing out the Zobel C8.

[Mathes]

Here is the schematic for the B.
I would try changing out the Zobel C8.

Turns out 5.6 ohm zobel resistor R20 was open, I thought I checked that but I was on the wrong part, I also replaced C8 because it was cracked from the heat. Also, Enzo is right, it is a rev C not B.
Thanks for the replies!

[Jazz P Bass]

, it is a rev C not B.!

The question still remains, 'what caused the Zobel RC to fail.'
I would resolder that power amp (both while you are at it.)
Heck, as old as that is I would probably (no, I would) resolder everything.
Just because.