Peavey CS400 one chanel down

[Nyqusit5]

I am looking for help repairing this old CS400 (79 model). I got the schematic from the forum and I have now one chanel working. The other (chanel A) is giving me trouble... Its SAC187 output Triac was blown, and the chanel's DDT is always on. I didn't find any faulty ouptut transistor (I have checked them out of the amp and switched it with a couple of good ones and had the same problem). I have checked pretty much all of the diodes and transistors of the driver board and they all seem fine (same for the opamps). Chanel A voltages are not exactly equal to chanel B's but they are in the same ballpark and seem logical. I have also check for dry joints and faulty molex connectors.

With the DDT switched off, I can trace a clean signal up to the U2 opamp's input, then it goes funny. I have replaced Q2 but no change...

So I must have forgotten or missed something because I just can't get this chanel to work... Any suggestions? Please?

[Enzo]

You have removed the SAC187 so it doesn;t interfere? When those fail I always leave them out until everything else is fixed, then install a new one.

All the CS800 troubleshooting in recent threads will apply to this amp also.


Can you specifically state the problem? Turn the DDT off and apply a signal. No output? Distorted output? Weak but clear output, weak but distorted output? Strong but distorted output? CLips? SYmmetrical clip or assymetric?

Isolate the problem. The two channels are the same, so if one channel works, swap the driver board from the other channel into it. Does that other driver board work in the good channel? Or does it hold the problem?


Did you replace U2 or Q2? If Q2, why not U2?

SIgnal OK at U1-6,7? Remove U2, signal OK at pin 5 of the socket?

[Nyqusit5]

Hi Enzo, thanks for your help.
Yes, I removed the SAC187 for tests. With the DDT switched off (I removed U1 from the input board to make sure it is), there is a very distorted low output, and with the DDT active, I only get some signal on transcients. The signal is clean at U1's output (driver board, pin 6-7) and at U2's input (pin 5). The signal starts to distort right after U2 which I have also replaced (no change): the distortion is pretty severe and erratic at that point. I have isolated the problem to the driver board of that particular channel (I did swap them) but I can't find the faulty component, even by comparing the two driver boards.

It must be a faulty diode or transistor but I just can't seem to identify it. I have also read all of the latest CS800 or 400 threads, but that didn't help me for that particular amp. I think I am going to replace each one sequentially as a desperate measure... Unless these last details give you a clue?

[J M Fahey]

Dear nyqusit, please post or link to the schematic so others can see what you are talking about.
Thanks.
PS: and don't shotgun if at all possible; bad transistors and other defects can often be found without *that* much desoldering through voltage measurements.
Good luck.

[Enzo]

And please don't focus on "bad part" exclusively. A broken connection is just as harmful as bad parts in most cases. Like in your car, if you are getting no gas to the cylinders, it COULD be a bad gas pump, but a broken gas line to the pump would be just as problematic. You may have bad parts or you may not.


Start at the start. Your problem is on the driver card? OK, is there good +15 and -15 on the power pins of the ICs?

Q7 Q8 are limiters, unsolder one end of CR13, CR14 and lift them from the circuit. That removes the limiters from the circuit. ANy help? Really, do this step.

Mute JFET Q1 is between U1 and U2, it could be dragging things down, remove it. ANy help?

U2 drives the amp with the input signal, but it also is your differential amplifier stage. It samples the output and tries to correct any distortions. Now if the input signal wants to go a certain way, and the output is unable to go that direction for whatever reason, U2 will try as hard as it can to drive the output that way, so the resulting output from U2 will be grossly distorted tryiong to "correct" the distortion caused by the failure.


Kinda like when a car skids off the road on some ice, the driver will be steering harder and harder back toward the road even though the car no longer can go that way. yeah. Kinda like that. Sorta. Morgan Fairchild.


Point being that possibly gross distortion you see there is due to the inability of the amp to function elsewhere.


And the schematic is full of DC test voltages. Not all are to ground, some are from point A to point B - pay attention. See how your stack up.

[Enzo]

And just because... I am currently finishing up a good repair thread on the old CS800 over in the power amp forum at Peavey. You might find it useful. CS400 and CS800 are pretty much the same thing but for power supply and some more output transistors.

[Nyqusit5]

Thanks again for your help.

Dear J M Fahey, thanks for your wise words. The reason I am resorting to replacing the transistors is because I have gone through all the other methods of finding the problem. Don't worry, it is not my first repair and I am not a hack Like you, I try to find the cause of the problems before replacing any and all components.

Enzo, I have also checked the connections and possible dry joints. I am going to try your suggestions on CR13, CR14 and Q1. I am not very knowledgeable on solid-state power amp design but I had the feeling that U2 was doing what you describe. I was not able to draw conclusions from that, so thank you for pointing me in the right direction!
And another thank you for posting the schematic for me!

[Steve Conner]

if I remember right, U1 on the input board is the gain control for the DDT circuit. It's some transconductance op-amp like a CA3080, and those are known to fail. Have you tried bypassing it (unplugging the "input" cable might do that) and feeding a signal straight to pin 3 of U1 on the power board?

Or maybe you could swap the chip with the one from the other channel?

[Nyqusit5]

Yes, I actually swapped U1 which in fact is a CA3094. I had that very same idea but no luck.

[Enzo]

There are two ICs on the input board. I refer to it as the volume board, myself.

The 87478 or 3094 is the DDT compressor, and for service, we just yank the thing and do without. Once all is repaired we can stick it back in. The other IC is a dual op amp, and it takes a signal from the amp and makes DC levels to control the 87478 and the DDT LED. We can yank it too. Thus stripped, the signal path remains through the board, from input jacks on to amp circuits. Couple resistors and caps is all.

[Nyqusit5]

So... I lifted CR13 and CR14, but no luck. I did the same with Q1 as suggested and still couldn't et a clean output (no change). The +/-15V supply is good also. I checked all interconnections and everything seems fine (and coherent with what's on channel B - the one that's working).

But, I have discovered something that could give us (you?) a clue. If I apply a signal to channel B's input, with volume controls on 0 (U1 is removed from channel A's input board, the mode switch is in stereo mode), I can get that signal on U2b's output (pin 7) in a slightly fuzzy form, while I have no input signal on pins 5 and 6. Turning channel B's sensitivity control affects its amplitude but never makes it disappear. If I shunt pins 5 and 6, the signal disappear (I guess one would expect it!) although the output on pin 7 is a bit unstable (just like when I don't apply a signal to channel B's input).
It seems that there is a strange crosstalk at that stage (U2). I have replaced Q1, C5, C3, U2, Q2 and CR7 with no change. I have checked again that no connector had been misconnected (almost impossible because they are bundled and cut to length)...
Do you have any other suggestions as to what I should look for?

I have never spent that much time on any amplifier...!

[Enzo]

Uh, just for science, since channel B works, apply a signal to the channel B input jacks and set the volume up somewhere, and scope the signal at the channel B driver card U1-6,7, then U2-7 and finally U2 input pin 5. Now you should know what to want to see on channel A.

I am a little concerned that you get signal at the input of channel A, U2-5. I don't usually expect to see signal visible at op amp input pins unless the IC is bad or not working. But there it is on your bad channel. SO for a baseline, see what is normal by lookign at channel B.

You said earlier that you know the channel A driver board is bad because you swapped it into channel B and it didn't work there? Fair enough. But did you sub the working channel B driver into the channel A power module to see if the rest of channel A worked? I don;t recall seeing you did that. If the channel A power module doesn't work, we may find that hiding the driver card problems as well.

I hate to dwell, but, that +/-15v? It needs to be present at both IC power pins - its not enough to just verify the supplies exist. And we are talking about the 15v supplies generated right on the driver card, not the zeners on the small square board on the floor of the amp.

I would tend not to worry about the crosstalk. it could well be some clue, but at this point it doesn;t suggest much to me.

How about all those test voltages on the schematic, how are those coming?

[Nyqusit5]

Hi Enzo, thanks for your help.

I have done all the comparisons with channel B. Actually, I usually have both on to check what I need to see on channel A. The visible signal at the opamp inputs can be seen when U2 is removed. I did that to check that it was coming to the opamp input in a clean form. Also, I did sub both driver cards but I didn't use ONLY channel B's driver board in channel A. I'll try that.

The supply is there at the opamps' pins, I always check the supply where it is used, not where it is generated. That is actually the first thing I always do. The test voltages from the schematic seem to be in the right ballpark (I also compare them to channel B's), apart from right after U2's output where they are a bit unstable.

[Nyqusit5]

Channel A works perfectly with channel B's driver board so there is no doubt about the channel A driver board being the problem (channel B with channel A's DB exhibits the same symptoms as channel A with DB A).

[Jazz P Bass]

Good to know.
Thanks.
Sure narrows down the problem.