Peavey CS800 keeps blowing same power transistor

[veganxxx]

I have a Peavey CS800 (late 70's early 80's).

I actually got two. One worked and one blew a fuse when I plugged it it.

I plugged it in a light bulb limiter and discovered I got channel A to run fine and channel B caused a bright light- dead short somewhere.

I took the driver board off to test components and discovered two blown power transistors and one blown .33ohm 10Watt resistor

After putting in the new components I put it on the light bulb limited and it was dim- no shorts

I turned it on and the fuse blew. Put it back on the light bulb and it was bright. I retested the power transistors and discovered one was blown.

I took it out, put it on the variac and light was dim. Then I ran it with the power transistor taken out and it didn't blow fuses and made a signal.

I ordered another power transistor, installed and it was dim on the variac. Plugged into straight power and the new transistor blew.

So something is blowing just the number 10 power resistor and main fuse. I can't figure out what that is since the transistors are seemingly hooked in parallel.

Any suggestions? I'm a relative electronics novice but I can read schematics, understand explanations and how to test components. I just don't have a full grasp on electronic theory.

[glebert]

Do you have a schematic that matches your unit? It is always nice to post that along with your question so that we can follow along easily, and also tell the component number of the failing parts. The schematic I found says Series C, which implies there is an A and a B as well.

There are a lot of power transistors in CS800 in parallel banks, if you replace a couple at a time (even same manufacturer and model) you could have considerable mismatch on the transistor parameters, which may lead to others failing down the road. Also, are you getting transistors from major dealer or from something like Ebay? Lots of fake power transistors on ebay.

[mozz]

If there are driver transistors for the outputs, you need to check those too.

[The Dude]

Also check the triac on the output board. It could be leaky or shorted. It happens often when outputs short on the CS models. It's a TO220 package SAC187 right on the little board that the output connections are on. If it's bad, your output is shorted and the amp will blow again. You can check it by measuring continuity at the banana jacks, with the amp off of course. It should be wide open. If you get any sort of resistance there, it's likely the triac is bad.

[veganxxx]

http://mudpods.com/Peavey_CS-800_files/CS-800.pdf is the schematic. Not sure if I have the C series, but everything I've seen translates to my boards.

On the printed board diagrams at the bottom show Q10 as the transistor that is blowing.

I did get one transistor from ebay, maybe it was bad? I could try taking transistors from the good side for testing purposes.

Also I have been using NTE388 to replace SK3947

When I get back from thanksgiving I'll check the continuity on the output posts. I did previously pull that triac board and test the components and they tested good. But fuses and a transistor have blown since then so it's worth checking again.

[Enzo]

SK3947??? That sounds like the old RCA replacement line - an NTE competitor. And NTE388???

The only reason to ever use NTE is if it is an emergency and they are locally available. Since you are ordering parts, order the real parts. The ywill better match your circiut and be a lot cheaper as well.

For the 10 outputs, use MJ15024. That is the real replacement for the original SJ6343 or MJ6451. The SK39347 was not the original part.

There are eleven 0.33 ohm 10w resistors and four 5.6 ohm 5w resistors. Make sure none are open. Note the 5.6 in parallel with the inductor will measure shorted. And the two 100 ohm base resistors for the drivers.

Dude mentioned the triac. SImple. Look at the binding posts on the back. Measure resistance from red to black. If it is shorted, then your triac is bad. If not shorted, triac is likely OK.

Note there is a Q10 on both the driver card and the power output board. Be careful to specify which.

You must first be certain ther are no defects on the power board, as they can blow up your driver card.

Note driver card Q10 relies on two resistors on the power board, R2, R3. Plus its own R37. Make sure none are open.

Power board Q10, carefully inspect the insulator under it, a hole in it or fleck of metal cutting through could let the collector short to the heat sink, and that could easily blow up that transistor.


ALWAYS operate with no load on the output until we know the amp is stable.

[Jazz P Bass]

Wait a sec.

Exactly which transistors are in this thing?

You should NOT mix transistor types.
As noted, one may draw more current, hog it and die.

The part that you need is MJE15024.
The Peavey # is SJ-6343or SJ-6451.

Here is a readable schematic: CS800C.pdf

[glebert]

Dumb question (or playing devil's advocate if that makes me sound smarter): what if he just pulls out a pair of transistors (assuming the triac isn't blown)? Couldn't tell from the OP if these amps are ones that he has had a long time or just recently acquired, but buying $60 worth of transistors to fix 1/2 of an amp that can be bought for $150 may start to be a bit burdensome. Obviously it couldn't source as much current, and would have less output power, but would it be a stability problem?

[Enzo]

Which would you rather pay for a used CS800? $60 or $150?

One need not replace the whole set of transistors if just one or two are bad. The amp may work if we leave out the problem part, but what guarantee do we have that the underlying problem won;t spread? best to just find the problem and fix it. I can fix CS800s all day long, they should run forever. They are surely repairable.

[J M Fahey]

Not on this specific amplifier but in general.to the question "why would I spend $100 to repair something which can be bought (used) for $200?" my standard answer is: sometimes it´s a solution, sometimes it is not, depending a lot on replacement current state and history.

If replacement is flawless, perfect, but often it is not.
Sometimes it works, but, say, somebody messed with the PCB and it´s full of lifted and poorly repaired tracks, or filter caps ooze, or fan squeaks and stalls more often than not, or it was kept in a damp place and is rusty or pots are scratchy, switches and jacks worn and make poor contact ... or ... or ...

So on a well kept piece if, say, replacing power transistors or a couple big caps or a fan can restore it to "quite good" state, it´s a good investment.
While buying something untested/unseen from "somewhere else" may be like buying a used car from "Honest Joe, best used cars in the entire State"

[glebert]

Which would you rather pay for a used CS800? $60 or $150?

One need not replace the whole set of transistors if just one or two are bad. The amp may work if we leave out the problem part, but what guarantee do we have that the underlying problem won;t spread? best to just find the problem and fix it. I can fix CS800s all day long, they should run forever. They are surely repairable.

I will choose option C, a 300 watt amp for free, if that option is viable. As far as replacing transistors, your above post (#6) very highly implied that he should swap all the transistors for MJ15024s.

It's funny, because when I was writing my question, I was thinking "Juan and Enzo are going to crap all over this." I think you guys are great, but you have a certain mindset that not everyone shares, at least in certain situations.

[veganxxx]

I was referring to Q10 as the power transistor not the one on the driver board.
I previously found one of the 10w resistors open and have replaced it and double checked the rest

I would like to fix this amp the right way.

Here are the transistors for the bad board currently-

SJ6343 x 1
SJ6343B x 4
70483180 W09108 X 1
SK3947 x 1
SK3947 274 CL 9251 X 1
NTE 388 (that I put in) x 1
Blank spot that blows x 1

The good channel board has these-

70483180 W09108 X 2
SJ6343B x 3
SK3947 x 4
SK3947 274 CL 9251 X 1

should I replace those too?

[Jazz P Bass]

Post #6 did not recommend replacing 'all 10' of the outputs.

Enzo was stating that you should not mix different brands (especially throwing in an NTE).

IF the originals (original SJ6343 or MJ6451) are in the amp & static test good, I would test the amp with just them installed (not at full power obviously) to see how they fair.

As the OP has not specified which transistors are in the amp, it is not known how many need to be replaced.

[Jazz P Bass]

Here are the transistors for the bad board currently-

SJ6343 x 1
SJ6343B x 4
70483180 W09108 X 1
SK3947 x 1
SK3947 274 CL 9251 X 1
NTE 388 (that I put in) x 1
Blank spot that blows x 1

The good channel board has these-

70483180 W09108 X 2
SJ6343B x 3
SK3947 x 4
SK3947 274 CL 9251 X 1

should I replace those too?

The Peavey #'s are SJ634(B) & 70483180.

[Enzo]

glebert, I am sorry if I don't connect with you, but it is a forum, a place for opinions, and you are free to ignore mine. You didn't offer me option C at the time.

there are specific reasons not to mix output devices. I mentioned the proper part is MJ15024. he has some of them, and some SK series, and NTE all stirred together in a stew. Odd transistors may have similar voltage and current specs, but they are made by different processes, and will not reliable SHARE current nor track thermally. That can result in either the odd transistor not turning on with the others, and so not carrying its share of the load. or in other cases hogging the current. Think of it like your car. Imagine all four tires are different. Maybe different diameters, maybe different widths, maybe a radial and a cross ply. Etc. The car would still drive, but would not be as stable on the road.

I don't care so much whether you use MJ15024 or NTE388, but that you use all the same in the amp.