peavey cs-800 broken, and being stupid

[mbates14]

I swear if I dont get this fixed im going to pull my hair out!! lol.


i have fixed this amp 3 times, and now its to the point where im stuck.

im not sure if anyone is familiar with DDT or not, but its weird.

The problem is both channels are really weak and distorted. I can turn the volume pot up by one click, the DDT light kicks in. and the sound is weak. the heatsink gets soo hot to touch, hi temp shutdown kicks in. Both channels are like this.

It was to the point where one channel was like this but the other channel was perfectly fine. I pulled EVERY COTTON PICKEN output transistor off the board, and the driver transistors and tested them. They all tested fine. ok....

put them back into the board, and I pulled the output emitter resistors and tested them, they all tested normal, except for a couple tested 4ohms and they supposed to be testing around .33ohms. I replaced those.

I ran a check on all the transistors and diodes, along wiht resistors on the driver board, and they tested fine. one transistor was shorted, and I replaced it. both channels have little to no DC offset.


I have done everything I can think of. I was curious as to where the problem lied, either in the output board, or the driver board. so one side worked, the other side didnt. I switched the driver boards, and didnt know the transistor on the bad side driver board had shorted again, and as curiousity killed the cat, it ****ed the good side up in the same way as the bad side.

So both channles are doing teh exact same.

I have the PDF of the schematics, but they are too big to post.

i dunno what to do. all output transistors test fine, no reverse leakage, or open junctions, no shorts. nothing. my supply rails are dead on.

i tested the output transistors, and on most transistors, with the DMM on diode check, it measures about .407 from base to emitter, and about .435 from base to collector on about all of them.

collector to emitter was oL, like its supposed to be.

I tested all of these out of circuit.

another thing to note that is weird unlike when it was working, you can hear the distorted sound inside the output bank transistors. they are acting like a speaker. its only doing this now. they never did this when it worked.

Im stuck

[DSP_Geek]

Originally posted by mbates14
I swear if I dont get this fixed im going to pull my hair out!! lol.


i have fixed this amp 3 times, and now its to the point where im stuck.

im not sure if anyone is familiar with DDT or not, but its weird.

The problem is both channels are really weak and distorted. I can turn the volume pot up by one click, the DDT light kicks in. and the sound is weak. the heatsink gets soo hot to touch, hi temp shutdown kicks in. Both channels are like this.

[......]

Im stuck

If this is like the CS800s I used to fix (is it already 20 years ago? sheesh), you should find a small circuit board on each output binding post. The board will contain some passives and a TO-220 triac; this is a crowbar circuit which keeps a failed output stage from blowing up speakers by shorting the outputs via the triac.

It works well, but you should replace the triac after repairing a blown output stage since the triac also invariably blows while saving the speakers. You can test that by removing the protection board and seeing if the channels start working well again. If that's where the problem lies, I recall the triacs were pretty cheap to replace. Don't have the type number, sorry.


Cheers,
Francois.

[DSP_Geek]

By the way, don't feel too bad: I was also ripping my hair out the first time I ran across one of these before someone tipped me off.

[Netlist]

Originally posted by mbates14
the heatsink gets soo hot to touch, hi temp shutdown kicks in.

Assuming it’s not the old CS800:
CR100, CR101, CR105, CR107
For CR105 you can use two 1N4006

Take your time with the plastic wire isolation and look careful at the polarity, you could easily make mistakes here.

/Hugo

[mbates14]

oops. 2? i only used one.

[mbates14]

no, this unit does not have the triac board.


ok let me explain.

this unit has the screen on the front faceplate, so you can see the last 4 output transistors. it has channel A and B left and right of that screen.

Ive also seen a couple of other versions. the power switch is all the way to the right on this one.

it only has 6 boards. Power board, both input boards, output board, and 2 driver boards. Thats it.


BTW. I replaced those bias diodes once before. but, i didnt know there was a package that has 2 in it. i only used one. i dont think that would do anything would it?

im trying hardly to explain what the sound is like,
its like trying to hook a headphone amp to a big speaker and turning it all the way up.

the sound is very very weak, and realy extremely distorted. only the bass gets through. and the DDT light is solid with or without the defeat swtich turned on.

BTW the triacs are on the output board. There is 2 of them. per channel total of 4. I dont know how to test them, and I pulled them all out of circuit and no difference in sound.


if anyone has the schematic handy, it kept blowing Q16 on the driver board, and the other driver board everything tests perfectly fine.

i was going to flip flop driver boards to indicate that the problem was the driver, or output. I forgot that q16 had shorted again, and hooked that board up to the channel A. thats why channel A has the same problem now as channel b.

[mbates14]

another thing that might help you out too, is the high temp shutdown that speeds the fan up kicked in only after about a minute when I turned the amp on.

[mbates14]

omg. I pulled out those triacs, and they are reading shorted that would bring the outptu right to ground. hmmm. I wonder why its not working. LOL

[mbates14]

hell yea dude. Them triacs were gone.

channel A is working again.

i havent tested B side yet.

[mbates14]

oh what the hell. I just pulled all 4 of those out, the amps work fine. both channels. uhhh, all that wasted time. *sigh*

anyway, im not even putting new ones in there. **** it. I rather loose speakers than the amp.