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Thread: Peavey CS800 keeps blowing same power transistor

  1. #1
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    Peavey CS800 keeps blowing same power transistor

    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.

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    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.

  3. #3
    Supporting Member mozz's Avatar
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    If there are driver transistors for the outputs, you need to check those too.

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    Supporting Member The Dude's Avatar
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    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.
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    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.

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    Senior Member Enzo's Avatar
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    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.
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    Supporting Member Jazz P Bass's Avatar
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    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
    Last edited by Jazz P Bass; 11-23-2017 at 10:29 PM.

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    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?

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    Senior Member Enzo's Avatar
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    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.
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    Old Timer J M Fahey's Avatar
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    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 itīs 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 itīs 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, itīs 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"
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    Quote Originally Posted by Enzo View Post
    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.
    Last edited by glebert; 11-23-2017 at 06:50 PM.

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    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?

  13. #13
    Supporting Member Jazz P Bass's Avatar
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    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.
    Last edited by Jazz P Bass; 11-23-2017 at 10:28 PM. Reason: spe,,ing

  14. #14
    Supporting Member Jazz P Bass's Avatar
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    Quote Originally Posted by veganxxx View Post
    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.

  15. #15
    Senior Member Enzo's Avatar
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    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.
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    Thanks for everyone's input. I've ordered MJ6451's to replace my mismatched transistors

    I checked the Q10 insulator for metal/cracks and didn't see any.

    I will check the binding posts

    I think I previously stated this, but I did get the amp to run and produce signal through a speaker with the shorted transistor taken out.
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    Senior Member Enzo's Avatar
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    Yep, which is why I worried there might be something shorting the case of Q10 to ground.

    So there is a test we can do. Bolt a transistor into Q10 position. No power. No load connected. Measure resistance to ground from its case. Should NOT be continuity.
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    Ok I have some results-

    1. Both sets of banana jack posts measure about 4K ohms with the amp off nothing plugged in.

    2. There was no continuity between the body of the transistor and the heat sink.

    3. I put in all new transistors and put everything back together. Back on the light bulb limiter it is dim. The one thing I notice is a 4 second pulse. The clipping light on the "good" channel lights and then every 4 seconds it goes out and the light bulb gets bright for one second and then dims. It also did this previously when I had the "bad" channel unhooked all together.

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    Ok so I plugged it in to real power since I had a dim bulb and it immediately blew a fuse. This time it took out 3 power transformers. Once I remove them and put it back on the light bulb it's dim again. If there was a short causing the power transformers to blow why wouldn't that light the bulb bright?

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    Old Timer J M Fahey's Avatar
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    Then *maybe* there was not a short but one transistor was overbiased one way or the other and when fully connected passed too much current and exploded.

    Which, just thinking aloud, might be caused by a shorted *driver* transistor which lets all the current in the World into those power transistor bases: you do not measure a _case to ground_ short but when you apply full power rails happens what I said above.

    IF so, you should have applied mains power through a bulb limiter, "just in case".
    Oh well, these things happen all the time, going slow and steady is the way to go.

    As Enzo said, these amps **are** very repairable, just may take some time.

    So to check whether my above suspicion applies or not, turn amp on again, thriogh a bulb limiter and no load applied, and measure Vbe at the last 3 blown transistor sockets or PCB pads.
    * IF you measure anything above 0.6 or 0.65V there, they are overbiased ... let alone if you measure full volts there, then check backwards (drivers cough cough) *why* such a high Vbe voltage appears where it should not.
    If drivers fine, continued backwards, maybe *they* are overbiased. .... why?
    * IF you measure , say, between 0.6V and 0.5V (or even slightly less, Peavey often underbiases power transistors *on purpose* to increase reliability) , unless you miswired those power transistors (say inverted Base and Emitter wiring or put PNPs where NPNs should live) , then those transistors were fakes or poor quality leftovers from the bottom of the barrel, sorry.

    And so on, thatīs the way.
    Juan Manuel Fahey

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    Not that I’m too concerned at the moment, but is there any reason why the good channel produces a pulsing on the light bulb limiter?

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    I couldn't get measurements with the pulsing light bulb, the readings would jump with the light flashing. I unplugged the "good" channel and got a consistent dim bulb. Here are the readings from the bad board-

    Rail 1 (no blown transistors)
    Base- .138VDC
    Emitter- 0VDC
    Collector- 31.49VDC

    Rail 2 (3 removed blown transistors)
    Base- -17VDC
    Emitter- -31.6VDC
    Collector- 0VDC

  23. #23
    Supporting Member Jazz P Bass's Avatar
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    Yikes!

    Rail 2 base is way too high.

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    Is that indicating the driver is letting through too much voltage? Or does the problem lie elsewhere and what could be the cause/what should I test next?


    Quote Originally Posted by Jazz P Bass View Post
    Yikes!

    Rail 2 base is way too high.

  25. #25
    Supporting Member Jazz P Bass's Avatar
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    Being a direct coupled amp, yes, the problem is at or before the drivers.

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    With the driver removed the 17VDC is gone. Put in a known good driver and the 17VDC is back. I'm guessing whatever feeds the driver is giving to much voltage. What should I check next, i.e. what produces that voltage?

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    Q6, neg. pre-driver
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    Not sure what terminals are what, I couldn't find a data sheet for that part number, but on the + and - Q5/Q6 I get similar readings on each side-

    30VDC
    30VDC
    1.3VDC

    -30VDC
    -30VDC
    -1.3VDC

  29. #29
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    Driver is back in?
    How can you have -17V at the driver but not at the pre-driver? Blown trace or bad solder?
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    Unless I'm reading it wrong, according to the schematic the base of the power transistors are connected to the collector of the driver transistor. I'm getting -30VDC coming into the base of the negative driver from the negative predriver, which also has the same reading. Isn't that what the predriver is supposed to give?

  31. #31
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    You didn't answer whether you have a driver installed or not?
    Your bad voltages are not showing up when the driver is removed, so it needs to be installed to take measurements.
    If you do have it installed, somehow we skipped Q10 measurements anyway.
    Please re-do Q6 & Q10 measurements with good driver installed.
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    Here are more accurate results. Everything is plugged in except for the shorted power transistors-

    Q2 (power board)
    B- -30.79VDC
    E- -17.06VDC
    C- .01VDC

    Q10 (driver board)
    B- -1.365VDC
    E- -.796VDC
    C- -30.55VDC

    Q6 (driver board) I think I got the legs correct based upon the schematic, couldn't find a datasheet to verify
    B- -20.97VDC
    E- -30.31VDC
    C- -1.353VDC

  33. #33
    g1
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    Yes you must verify according to the schematic. Q6 base connects to R38, Q6 emitter connects to R39.
    Q6 is an NPN transistor. If it is good, the base can only be maximum .7V more positive than the emitter. Your readings say the base is 10V more positive than the emitter. Either the transistor is bad or you are making an error.

    Q2, your emitter should be tied to negative rail by R7, which is only 5.6ohms. Yet you measure around 15V across R7. So either R7 is open or Q2 has a problem.
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    Ok, so first, I mistyped and it isn't 20VDC, it's 30VDC so they are actually a similar number.

    Second, I put a meter on R7. It isn't open but.... it does read over 1 megaohms not 5.6 ohms. I had tested all these resistors before, but I probably saw the reading as around 1 ohm and didn't notice the "M" on my meter. Since it gave a reading that seemed low I thought that's what it should read in circuit and wasn't open. Big whoops! I ordered some 5.6ohm 5 watt resistors and will report back when I swap the bad one out.

    Thanks for the help so far!

  35. #35
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    Sometimes when we loosely use the term 'open' we just mean high resistance relative to what should be.
    So even if the meter doesn't say "OL", 1M for a 5.6R resistor can be considered 'open'.
    By the same token, we might call a low resistance a 'short' even if it is not measuring near zero. For example we might call a cap 'shorted' even if it measures 53 ohms or some low resistance.
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