Peavy CS-400 Blowing resistors

[dewme5]

I've read through a few threads, and I've not had much luck so far.

I was given a CS-400. Serial 1A-822928 81A I was buying it, until it smoked on start up.

No inputs, or outputs were connected. Just simply plugged into the wall, and released the magic smoke.

When I pulled it apart, the channel A board had a blown 120 ohm resistor on the large blue board. I used channel B to determine the rating. I contacted someone local, and was told to replace all the transistors. So, I put in their recommendation of MJ15024G 16A 250V I replaced the resistor. On start up, channel B popped the same resistor, and about 1/2 a second later, channel A popped the same resistor again.

I've disconnect the power plugs from the channel boards, and the amp powers up just fine.

I measured across the channel outputs on the back, and the reading starts at about 5Mohm, and climbs to infinity over about 3 seconds using a Fluke 87.

I'm guessing there is something simple that I'm missing. Can anyone help?

[The Dude]

Were/Are any of the output transistors you replaced shorted? For future reference, randomly replacing parts (shot gunning) is not helpful in diagnosing a problem. You need to find out what is wrong and replace those parts. If you still have the original transistors, can you check them and see if any of them are indeed shorted. That will at least get us going in the right direction. Your reading across the output indicates that MOST LIKELY the output triacs are good. Most of the time output transistors short, the triac also shorts to protect the speakers from getting DC. For this reason, I am doubtful that you had a shorted output transistor from the get go.

The schematic can be found here:

PEAVEY cs400 Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics

Also, before you replace the resistors again, make sure to clean away all of the residue. It is conductive.

[dewme5]

The shot gunning was from the local place. When I called, they said replace these items. Agreed, it should have gone differently.

However, since replacing the transistors, I've had the exact same problem, so I'm $60 poorer, but in the same position. So, I've just checked all transistors and all measure consistently now.

At this point, it may be obvious, but I don't know what I'm doing. It's a learning experience. I do understand quite a bit of theory, and I can build simple circuits, but the specifics are sometimes beyond me. That being said, I would like to get the amp working, but if I can't, such is life. Just wanted to be honest and upfront.

I have the schematics pulled up, and started to dive into measuring the driver board. But I've stopped. The power board, attached to the Driver board, is the one with the blown resistor. So, since both power boards had the same problem, I'm thinking the problem is outside of the board. Something in the Power supply, or the output board. Any suggestion on what to check there?

[The Dude]

$60 lesson learned. It's done, so let's move on.

Can you determine what resistors are burning from the schematic? Knowing the circuit board designation number would help. Try to find the resistor on the schematic and report where it is. Something attached to that resistor is obviously drawing excessive current, so we need to find out what it is. Resistors do not burn by themselves for no reason- much like fuses in that respect. If you have a burnt fuse or resistor, it is an indication that something else is drawing too much current (likely shorted). We need to figure out what that "something else" is.

[The Dude]

Posting for easier reference:

[T9c]

4 of those output xistrs are NPN and the other 4 are PNP, so if you replaced all 8 with NPN's, the 15024's, there's a problem. You need to get the complimentary PNP xistrs and install them in the right sockets. Sorry, can't remember off hand what the 73180's cross to, but it's in the Peavey Semi-Conductor Cross Reference guide.

edit: hmm, actually the 2 drivers are yet different than the 3 outputs on each rail, so look those up in the guide and replace with their cross too.

[The Dude]

4 of those output xistrs are NPN and the other 4 are PNP, so if you replaced all 8 with NPN's, the 15024's, there's a problem. You need to get the complimentary PNP xistrs and install them in the right sockets. Sorry, can't remember off hand what the 73180's cross to, but it's in the Peavey Semi-Conductor Cross Reference guide.

edit: hmm, actually the 2 drivers are yet different than the 3 outputs on each rail, so look those up in the guide and replace with their cross too.

Good catch T9c! I hadn't even thought of that. If the original transistors check good, I would put them back in.

[The Dude]

peavey cross reference.pdf

[dewme5]

Power board 22278, driver board 111778

The blown resistor is between pin 2 and 3, on the 8 pin header, counting from the left.



These connect on the driver board. Pin 2 leads to the resistor my finger points to, and pin 3 leads to the center leg of the 6019 also pictured.



I know that doesn't answer your questions, but that's as far as I've gotten.

[dewme5]

I do see the problem with the PN. When I asked previously, I was told to replace them all with that same PN. Ugh.

[The Dude]

Apologies. Just looked at the earlier schematic link I posted. I believe it's for the wrong version of CS400. I can't find the one for the older series (the right one). Hopefully someone here has it and will put it up.

[Jazz P Bass]

See if this one is closer to what you have.
It is from 1979, and all output transistors are NPN type.
There are board layouts to help maych things up.

[52 Bill]

Please build yourself a light bulb limiter. You might be able to save your new parts the next time.

[Enzo]

Lets be sure we are all on the same page. The newer CS400, the one with the large square vent grate on the front, may use both NPN and PNP power transistors, but the older version uses all NPN.

At this point, I would pick ONE channel to work on, disconnect the four wire power connector from the other channel so it is not in the way of our measurements.

Now on the lone bad channel, just pull all eight big transistors and test them out of circuit. Look at the schematic and at the power board (the board with the 8 large transistors) there are a number of large power resistors. Check each with a meter looking for opens. Note sic are 0.33 ohms and two are 5.6 ohm. There is a 100 ohm and a 22 ohm for each four transistors, check them for opens. I suspect the two 47 ohms are OK.

Unfortunately, you probably have a problem on the driver card too. That is the smaller board that plugs onto the underside of the power board. Oh, you might get lucky, but that burnt 100 ohm resistor makes me suspect a predriver.

Once that channel is 100%, only THEN, unplug the now good channel, and reconnect that other channel. Then find out what may be wrong with it.

[R.G.]

Yep, what Enzo and 52Bill said.

As a bit of additional information, bipolar output amplifiers often have a chain-of-destruction failure mode where a failing driver or output causes other failures. I personally think that you really need to check the drivers, pre-drivers, and outputs all before doing another fire-up (in the worst sense of that term!). Also, put the thing on a light bulb limiter for the fire-up so you have a chance of finding the problem before setting off another destruction chain.