Peavey CS400 - help troubleshooting bad channel

[deanrantala]

The issue:

One channel is down. The DDT/Clipping light is stuck on

What I have done thus far:

1) Swapped the input/control boards (with the push/pull knob). Problem "sticks" with the same channel, so must not exist in the input board.

2) Completely removed the speaker panel plug - I understand there is a triac on the backside. I figured this would eliminate some variables from the equation.

3) Tested all (12) output transistors and all (4) driver transistors. Even though the other channel works fine, I figured I would do this. All tested fine.

So at this point I started measuring voltages on the driver board.

Before I started measuring voltages, I took the following steps:

-> Disconnect the input board (so none of the DDT stuff can interfere)
-> Removed one end of CR13 and CR14 to disable limiters Q7 and Q8
-> Completely removed Q1 (muting MOSFET?)

Now I begin testing...

Off the top, I find something. The problem seems to be that rather than +/- 15 volts after R21 and R42, I am getting -17 and +12.

So instantly, a picture is being painted: since I do not have a clean +/- 15VDC, nothing is balanced and U2 (diff stage) is likely holding the circuit off-biased. This is being "seen" as clipping and the DDT circuit is stepping in.

It looks like something on the positive voltage side is trying to draw more current that it should - causing a shift in excess current to the negative side. This is the only way I can account for the voltage drop AND spike. I tested R21 and R42 - both measure the correct 1k.

Perhaps U2 was screwed up and driving things the wrong way - these were known to go bad from what I hear... So I swapped U2 from the good channel. Same problem. This eliminates U2.

This is where my experience starts to dwindle off..

-> Something in the feedback network pushing this excess current back to one side?
-> Something on one side just screwed up and drawing far more current than Peavey engineers accounted for?

And while I am on the bandwagon.. the power supply scheme seems cheesy. IMO, the "proper" thing to do would be provide a +/-15 volt set of legs off the transformer and use THAT for the the driver boards rather than a couple resistors and relying of the precise current draw of the driver board components. Sorry to rant - but somewhere, I hope a Peavey engineer reads that one

Anyone have any advice before I just start replacing transistors on the driver board?

- Dean

[J M Fahey]

Start by posting the schematic, so we all refer to the same.

[deanrantala]

Schematic attached

Here we go:

PV CS400 c.pdf

Additionally, I have based a large subset of my testing thus far based on information provided in this other thread: http://music-electronics-forum.com/t28730/

A gentleman by the name "Enzo" provided some insightful details regarding the operation of this exact amp.

Perhaps I will try to ping him to peek at this thread as well.

[Enzo]

Isolate the problem. Each channel has a main power board, a smaller driver card plugged into the power board, and the volume control board. Plus the other stuff.

You swapped volume boards. Now swap driver cards. problem move with the driver card or stay with the power board?

I'd prefer to operate with the volume board connected. Without it, your input op amp is left with an unterminated input pin. If you want DDT out of the way, just pull U1 from the volume board.

Your DDT blinks? Does your pull switch to defeat the DDT affect that?

If you are concerned over the +/-15v, then check zeners CR10, CR6, which establish it.

[deanrantala]

Isolate the problem. Each channel has a main power board, a smaller driver card plugged into the power board, and the volume control board. Plus the other stuff.

You swapped volume boards. Now swap driver cards. problem move with the driver card or stay with the power board?

I'd prefer to operate with the volume board connected. Without it, your input op amp is left with an unterminated input pin. If you want DDT out of the way, just pull U1 from the volume board.

Your DDT blinks? Does your pull switch to defeat the DDT affect that?

If you are concerned over the +/-15v, then check zeners CR10, CR6, which establish it.

Already swapped driver boards, problem stays with the driver board. Volume and power boards are all fine.

I have in fact left the volume boards completely disconnected - I will re-connect it and pull U1 here in a bit.

The DDT does not blink, it just stays on solid. Here's the strange thing through - if you turn the volume knob either direction (left/right), the light goes out. As soon as the slightest of signal is applied, it instantly comes back on.

I will check CR10/6 here as well - is it not strange that both would go out at the same time?

I will post back soon - hopefully with some progress.

[Enzo]

Both bad at once? One is off a volt, the other is down 4 volts. It may not be the zener's fault at all, something in the circuit could be loading the rail down or not supplying enough voltage in the first place.

You swapped U2 on the driver cards, but did you swap U1? Simple to do.

Aside from the DDT light, what does the amp do or not do? Does it amplify? Pretty much to full power? Is the sound clean? You say the channel is "down." As in not operative? Or just keeps its DDT light on? You have provided a bunch of information but no syptom other than DDT lights. We could simply have a bad DDT circuit.

You were concerned the diiy stage was skewing the output, but more likely it will spends its last breath trying to center offsets. Is ther actually any DC offset on the output bus? You mentioned taking voltage readings, but stopped at the zener voltages. They may be off and needing attention, but don;t stop there, we need to know what else is wrong.

You mentioned the two 10 ohm resistors, R21 R42, did you measure them to see if they were still 10 ohms each? Note that they are not where to measure the 16v rails. You have 60v main rails, from there we drop through those 10 ohm guys, but then also through 1k 5w resistors to the zeners. I expect the 10 ohm guys to have pretty much 60v on both ends. If not, 'splain to me. For that matter are the 1k 5w guys OK?

[deanrantala]

The 1k, 5w resisters are good - triple checked em. I have checked the raw 60v rails as well - and all is ok. I have even moved each driver board to different power board "sides" - the problem follows the driver board.

Regarding U1 - yes, I have swapped that one as well. I also tried swapping the 2 zeners a couple hours ago: still no dice.

As far as symptoms go, well.. Not only is the DDT light on, but the channel itself puts out no audio. Now, I have NOT tried testing for amplification since I disabled the muting and safety stuff. But I have tested a few other things...

I **am** getting DC voltage to the output - here's the really strange thing. Measuring from chassis ground to EACH of the two output lines running from the power board, I get -3 volts. That's right! -3 volts on BOTH output lines! I get the same behaviour when the bad driver board is switched to the other power board - so this IS something the driver board is causing. Measuring from + to - on the output lines yields normal (8mv or so). The DC voltage on speaker out is only from chassis to speaker out and it is happening on BOTH output lines for that channel.

How can there even be -3 volts on BOTH lines? I would imagine one line is + and the other is - 3 volts.

I can hook a crap speaker up to the thing later and see what I get. I do have an osciliscope but never really dig it out for much.

[Enzo]

There is no DC on the output itself, but there is 3vDC from chassis to the output? OK, is there 3vDC from chassis to the output "ground" terminal - the black one? Um, do we have the volume, input, and speaker output boards all connected? Those conections may carry ground to portiuons of the board. Or when you say both wires you mean the output and output grond wires?


Alternatively, power down, and measure resistance from black output post to chassis. Should see continuity, if not, that may be part of this.

Hmmm, your +/-16v was 17 and 12? I wonder if that relation stays the same if you use the black post as meter ground instead of chassis?

I don't know what all you have wrong, but it does seem you have some sort of ground problem or missing ground connection.

1k OK, I believe you, but how about those 10 ohms? Probably OK< but check. They are right in the corners of the card, by the 8 pins.

The schematic offers a number of DC voltages on the driver card. Use a ground on that board for your meter. May I suggest one of the shields on the cables that go to the volume board. STick your black probe down one of those holes, unless we have cleared up the ground/chassis problem. Of course some readings do not involve ground. But those DCs will tell us a lot about the circuit.

In the driver portion of the drawing, lower right, is a connection going to the power board, connecting their grounds together. There is a 4.7 ohm in series with the connection on the power board side, R18. Make sure R18 hasn't opened. or the connections to that pin on the card.

Idle thought, I wonder if your 3v ground offset could be involved in the 15v rail measureing 3-4 volts off. What voltage appears ACROSS each of those two zeners - NOT to ground, but across the part itself.

SO that 4.7 ohm R18 deal - the ground connection on the four pin molex runs through R18 to pin 5 of the driver card. Look at the layout drawing. From pin 5 of 8 follow the ground tracery around the sides of the card to the ground pin on the four wire Molex pins. That ground trace runs all the way around the card except for a small gap on one side to prevent loops. Look closely at that ground trace, in particular where it gets narrow, like next to the Molex, and also over by one of the 1k 5W. For reasons we can discuss later, that trace may be burnt open at a narrow point, like a fuse. Has this happened here? When it happens it can be hard to spot.

Sorry for the wandering stream of conciousness. Look for that missing ground or burnt open ground first.

[deanrantala]

Ah ha!!

I may be closer to finding the issue...

From the ground ON THE BOARD, I am indeed getting +/- 15v! I also get perfect 30vdc across both legs.

Measuring from ground on driver board to ground on chassis, I get my mysterious 3vdc.

Keep in mind: the problem follows the driver board - I have switched driver boards to different power boards - so the short obviously cannot be in the power board....

...or could it..

Time to do some more testing - I think we are getting very close now.

[Enzo]

Did we look for that break in the ground trace around the outer edge of the card?


Power board? The other channel is OK, right? Does its card work OK on the suspect power board or not? You could have both a bad driver card AND a problem on a power board.