Peavey XR-1200 / CS-400 Blowing Fuses and Power transistors

[Bobby.Blaze]

I have this issue with my Peavey XR-1200 powered mixer, which has the CS-400 power amp:

Got it free from a friend, used to work fine, brought to my house; worked ok then after 5 minutes of operation blew fuse/ power transistor on B channel power board; replaced transistor and tried again; still blew fuse, this time blew same transistor again and transistor on opposite rail too.

I have swapped out driver boards with A channel which holds fuse when no power is going to B, so I know both driver boards are OK. I have triple checked the resistors and caps and those dual diodes on B channel power board and found no bad components (?)
So I resoldered the leads from the problem transistor's socket and tried again and its still blowing that same transistor and fuse (!)
The heatsink also seems to be getting extremely hot prior to the fuse blowing after about 5 minutes of operation. Not sure if that is indicative of another problem?

I've read the other posts on this subject in these forums and I'm still missing something.
If anyone has some advice/ or suggestions, I'd really appreciate it, 'cause I'm stumped.

[52 Bill]

I'm not sure, but does this amp have the speaker protection crowbar circuit on it? Take a resistance reading across the B channel output terminals, does it read as a short? If it does, look for a shorted SCR on the board that is wired across the speaker output. It is part of the protection circuit in some of these amps.

[Bobby.Blaze]

This is a very old version of the CS-400 from 1978. Don't think it has crowbar.

The circle on the photo indicates the transistor socket that keeps blowing.
The diode in the circle ( I think it's a zener?, can't see the label) had the same reading as the good A channel's when idle. But I'm not exactly sure about it, since all the resistors read appropriately and nothing else is in immediate contact with the transistor in question.

There is another diode that broke off where the two wires from the driver board molex connector come in...labelled z3886-2. Not sure when that happened, or what it's function was.
It was mounted off of the board on the heatsink, (see drawn symbol on photo)

[bkahuna]

There is another diode that broke off where the two wires from the driver board molex connector come in...labelled z3886-2. Not sure when that happened, or what it's function was.
It was mounted off of the board on the heatsink, (see drawn symbol on photo)[ATTACH=CONFIG]10510[/ATTACH]

That would be your problem. That's a dual diode (two diode junctions in series inside one package) that's used to adjust the bias as the amp heats up. Without it, things get hot and blow up (as you've already discovered )

[Bobby.Blaze]

I didn't realize that was the dual diode. After digging around in some schematics for the cs800 I found out that the part # is not z3886-2, it's sz-13886-2 which is a special selection from the MZ-2361 for voltage drop characteristics. vibroworld has sz-13886-2 for around 3 bucks each. Anyone know of a suitable (cheaper) substitution from Mouser?
Thanks

[olddawg]

I didn't realize that was the dual diode. After digging around in some schematics for the cs800 I found out that the part # is not z3886-2, it's sz-13886-2 which is a special selection from the MZ-2361 for voltage drop characteristics. vibroworld has sz-13886-2 for around 3 bucks each. Anyone know of a suitable (cheaper) substitution from Mouser?
Thanks

Don't know if it would work in this case but in a lot of old power amps with stacked diodes, I used just to solder rectifier diodes in series. The schematic symbol will usually tell you how mant to use, or you can measure the combined junction resistance of a good one and figure it out. Seriously though, if you can get an exact replacement for $3 buy a couple. Why bother? BTW check your bias and run the amp up on a variac if you can. Check for oscillation and thermal runaway.

[Enzo]

Or just call the parts department at Peavey and tell them you need the dual diode for their power amps. 13886 should be sufficient description, but the real part number is 70413886. Why screw around, get the real part.

Your XR1200 has the CS400 C series powr amp, and by the schematic it does include the output crowbar. It would be mounted right next to the speaker jacks.

Peavey will send you the schematics if you ask for them. Here is the CS400. Working with some other schematic like the CS800 only works if you are awate of where the circuits differ.

[bkahuna]

Don't know if the CS400 works this way, but I just remembered that I once powered up one of their smaller amps (old XR600 or something) without the dual diode connected and got all kinds of pretty sparks instantaneously at power on. It took out the whole drive section. You might check the other dual diode and Q7-Q10 on the drive card.

[Bobby.Blaze]

Wow, thanks for the schematic, I could only find more modern schemos for cs400, this is what I needed.
Although, the crowbar protection circuit must have been added after the 78 model, because this unit does not have it - just goes right from power board output to speaker jack - nothing in between. But thanks to your schematic, I can put one in.
I did ask peavey, though, haven't heard from them yet.
U guys Rock!
now, to find that diode...

[Enzo]

Well OK, my support site says the XR1200 was introduced in 1979, if ther are some earlier ones, so be it. If this amp has banana posts on the rear, aren't they mounted on a small pc board? In any case, a quick read of resistance across the speaker output tells the tale, if it is not a dead shoprt across, then if ther waqs triac it was not bad anyway.

yes, if the dual diode is open, or its wires disconnected, then the bias string is open, and both polarities of output transistors turn on full at the same time. Max current flows and fuses blow, not to mention output transistors.

[Bobby.Blaze]

No banana plugs on this one, just two parallel 1/4 in. jacks for each channel.

After realizing that vibroworld.com is "temporarily closed" since 2008 , I decided to try
OldDawg's method of using two rectifier diodes in series.
You can see in this picture that I used a little piece of an old circuit that just happened to have two 1n4007's in series, and hacked it up a bit and soldered it in.

The reading is near identical to the original dual diode.

However, the unit still blew the fuse and took two more power transistors with it.
On revisiting an earlier photo I took after the first time it blew a transistor it is clear that the loose lead that appeared later on the dual diode was still intact, and in fact the damaged 13886 still reads good with dmm.

Anyway, took it all apart again, and still can't find any bad components on either driver or power boards...
What else might be happening here...???

[Enzo]

The parts department at Peavey has not been closed since 2008, and they will happily fill your parts order.

A couple regular diodes on a board may check as a similar voltage drop, but will you have them on the heat sink thermally tracking the temperature? That is why the dual diode is mounted there. Make darn sure the wires running back to the driver board are correct in polarity.

Look up "light bulb limiter" and make one and use it.

Are all those small resistors intact on the big board?

May I assume when we are firing this up that we have mounted the board back to the heat sink with insulators under the transistors?

If fuses and transistors are blowing, then it is a pretty safe bet your two polarities are turning on hard at the same time. That is often more likely due to an open somewhere than a shorted part.

How is the solder to all the male pins in a row where the driver card plugs on?

ANy time a transistor blows or a fuse blows, you can no longer assume parts that checked good previously are still OK. SO you need to run down the large popwer resistors looking for opens, and recheck the transistors for shorts. Each time.

[Bobby.Blaze]

I will get the right part eventually, but I wanted to see if the dual diode was actually the problem... but the temp fix should work well enough for testing purposes without the fuse blowing instantly, which is what happened this most recent time.

I recall seeing that mixed power transistors could cause problems, do you think
this could all be happening because of mixed power transistors?

all insulators were in place
driver and power boards were all attached properly to grounded heatsink.
pins from driver to power board check out ok
diodes check out ok
not sure how to read that inductor HQ-813 though.
all resistors and caps on power board check out ok
I will look into the light bulb limiter and use it when i get some more fuses

[Enzo]

Mixing output transistors can hurt thermal tracking. This becomes an issue at high powr levels. It won;t pop fuses at power up.


For power up purposes, yes, a couple diodes is OK. If you short dead across the thing, all that happens is crossover distortion increases. However, if it opens, your amp turns on hard. SO it is critical that it be wired in correct polarity. If it is backwards, it might as well be open.

The inductor is a coil of wire. It is open or it is OK. Tug on it. If either end comes free, it is broken. It is paralle to 50 ohm R16, so measure R16 in-circuit. If you get 50 ohms, then the coil is oppen. If you get a short circuit, then the coil is intact across the resistor. When those coils break off, then the full power of the amp flows through the resistor to the speaker load, and generally burns up the resistor. I doubt your coil has a problem.

[Bobby.Blaze]

Well, I got some more 8a slo blo fuses, and I've just verified a couple of my earlier impressions:

When B channel power board is removed from circuit, A channel power board will hold
fuse with either driver board

B channel speaker output is not shorted to ground

Voltage supply to B channel board as read from disconnected molex is identical to A channel supply.

Dual diode fix on B board is in correct polarity

All resistors, caps, and diodes on power board's values check out.

Still looking for bad solder joints but have found none so far...