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Thread: Peavey CS800 burning R20 after power up 5.6 ohms 5 watt resistor

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    Peavey CS800 burning R20 after power up 5.6 ohms 5 watt resistor

    Have a Peavey CS-800, old style, but I don't believe, real old. Using the Revised 10/5/04 schematic, original date 7-18-77. This is a 5 watt resistor, mounted on the output board, underneath, the PC board, that houses all of the main outputs. Only problem found, so far, was that this resistor was burned open. Have replaced it, and checked all outputs, and all check good, out of circuit. All 12 of them. Have not swapped the preamp board with the other side yet, as the problem could well be on that, but am wondering if any of you, have crossed this bridge already, and can tell me any info, that will get me headed in the right direction. Am wondering if the triac, and the other component, on the way to the output jacks, could be playing a part in this. Any knowledge would be appreciated. I have repowered it up, after replacement of the big 5 watt resistor, and it continues to heat up, and will go ahead, and burn, if I allow it to stay on. It's a symptom, but not the problem. To give you an idea of this age, there is a red rocker switch on one side, and a white High/low temp plastic light cover, on the other, and then the attenuators, like they've always had. One left, and one right. Thank you.

  2. #2
    Supporting Member Jazz P Bass's Avatar
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    The two 5 watt resistors are part of the output 'zobel' network.

    It could be that your triac borad is tripped & shorted.
    That would explain the resistor getting hot.
    You should be able to disconnect the triac board.
    It's o/k to test the amp it without it.

    If not that, I would check the capacitor part of the zobel. (C 8 0.1 uf) or simply replace it.
    Attached Files Attached Files

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    Senior Member Enzo's Avatar
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    Revised in 2004??? I was not aware they updated anything 25 years after the amp was made. Can you post the drawing you are using? We really need to use part numbers, rather than the resistor under the main board.

    Resistors do not burn up on their own, there is ALWAYS some other problem causing it. First thing to do is find the resistor on the schematic. Now what current paths through it can burn it up? Does it connect to a transistor?

    When the amp channel blows, it often kills the output triac, that is the triacs job. In 30 years doing authorized service, I never saw one of the little things next to the triac fail.

    Pick one channel to work on at a time.

    If this is the model I am thinking of, the top cover comes off exposing two finned modules side by side, yes? A very few screws, and the m,odule lifts off, and ther is enough wire slack you can lift a module up and prop it agaisnt the outside. I like to fold a newspaper in between module and chassis to prevent possible shorts. On the underside of each is a four pin Molex connector. Disconnect that and the module is removed from the circuit. All the plugs on the driver card can stay. Now work on the other one

    Yes you can trade driver cards for test purposes, but only after you know nothing is blown on the main boards.
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    Supporting Member The Dude's Avatar
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    You can check the resistance across the banana output terminals and it will usually tell you if there is a triac problem. Easier than pulling the board.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    Thank you, all. Enzo, I am using the schematic, that Jazz P Bass. has posted there. I was wondering about the triac, but didn't know for sure how to test the thing. From what each of you have instructed, I'll do the procedures, and see what I find. R20, is my culprit that is doing the burning. I'll be checking the triac, and replacing the C8 .01 cap, like Jazz P Bass, mentioned, also. It's cheap, and if the resistor has been stressed to the point of burning the first one, it's not going to hurt. This is the model, just as you described Enzo. I have two in my system, downstairs, that I've ran for years. I'll get this thing tested, again, from what you've told me, and give you a progress report. Thank you, all, guys. It is very much appreciated.

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    Supporting Member Jazz P Bass's Avatar
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    The Triac can be tested by looking for a short across the output connector.

    I would think that a shorted Triac would burn R19, not R20.

    If C8 is bad, now that would heat up R20.

    Or a high frequency oscillation.

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    Anyone have a part number, say through Digikey, or similar, for the SAC 187 triac, that is in the CS-800, Peavey's? Tried Digikey and they needed more info, than just the SAC 187. Didn't have it. Thanks guys.

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    Supporting Member The Dude's Avatar
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    BTA16-700BWRG STMicroelectronics | Discrete Semiconductor Products | DigiKey

    The SAC187 has been discontinued for quite some time. This is a suitable substitute.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    Old Timer Leo_Gnardo's Avatar
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    It's been a long time since I had to deal with a shorted "crowbar" triac in a Peavey. If memory serves me, there's an associated 'diac', looks like a diode, that should also be replaced at the same time as it usually perishes along with the triac. Maybe this is only on CS800/400's but worth a look while you're doing the repair.

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    Senior Member Enzo's Avatar
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    I have repaired many blown CS and other series amps from Peavey, and I have never once had to replace the diac. Worth checking, of course.

    The triac is just a triac, you should be able to find a data sheet on it, even if the part is gone. It is a 200v 12A TO220 triac. The thing you need to look for other than those is that it fires on all four quadrants. PV shows 2N6346A as the generic. I forget what I stock, some common Moto types.
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    Thank you, Dude. It's looking like I have an old one. This has the silver power supply cans, even says "made in USA," on the big caps. No big triac, riveted to the chassis, like I've seen in the past. After looking at Google images, and seen the little board, that the triac you are describing, as the schematic shows; they are not even in this one. This is the same amp, I was inquiring about in the past, as I've been recovering from back surgery, and got the ok, to start lifting this thing, again. This does have the white, two speed fan button cover on the front. Am sure it's the ones, they speak of prior to 1982. No docs, on that one. I am going to replace the R20, 5 watt 5.6 ohm resistor, as it checks all of 2 ohms out of circuit. Shows extreme heat burn circles, in the center of it, and like you recommended previously, to check/replace C8, the .1uf, cap, in series with the resistor, in parallel, with the triac/diac that the newer schematic displays. Thank you, all, for the knowledge, and I'm going to try, and nail down the older schematic. Any further ideas, I'd definitely appreciate any, and all. Thanks.
    Last edited by RogerT; 03-16-2017 at 04:34 AM. Reason: spelling

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    Supporting Member The Dude's Avatar
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    I don't recall ever seeing a CS800 without the triacs, but it wouldn't be the first time I'm wrong. If it's a '77 model, like you said in post #1, the triacs will be mounted on the little circuit boards that the banana jacks for the speaker outs mount to. That's why, as suggested, you can check across the speaker terminals to see if they are shorted. It saves you having to remove that board, which is a bit of a PITA. FWIW, I've never had to replace one of those diacs either, and I've probably repaired literally hundreds of these things.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    Senior Member Enzo's Avatar
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    The earlier A and B series CS800s did not have the big triac for the power switch, that started with the C series. But right from teh start they all had the crowbar triacs.

    The SAC187 is a TO220, the three legged little tab transistors with the mounting hole in the tab. Looks same as say a 7815 voltage regulator. The traics are not on the board with the power transistors, they are right on the boards with the binding posts on the rear panel.

    The much newer CS800X, the one with the large screen vent on the front, does have the triacs on the power transistor board, right next to the relays for each channel, but from your description, you don't have that more recent version.

    The oldest CS800 I am aware of is the A series. If we don't have it here, a call to customer service at Peavey will have it in your email.

    But look at the speaker binding posts. Inside the amp where the wires connect to the posts, is ther not a SMALL circuit board, no larger than a business card, mounted on the posts? Is ther not a couple parts that look like transistors?


    SAC187:
    f_f65cf27324e748cca4b93a14b62ae3df.jpg
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    You are very correct. They have been robbed, off of this amplifier. Even found the evidence, of where the old screws glue/adhesive, or corrosion, was, that held them in place. Like Enzo, said in the article, I read, there is much to confirm, before bolting those bad boys, back on. Found a thread that Enzo, discussed in great detail with one, who had no business working on the three 800's, he had acquired. Learned a bunch. I think I know why this guy, got this amp given to him. One side is down, and the biggie was the burning 5.6 ohm five watter, I mentioned. That cap, below it, had to be faulty, and found a defective resistor up in the B+ line, R21, which shows on the schematic, to be 10 ohms. It was fried. Fixed that, prior to coming here. I'm going to go the extra mile, and swap output boards, and then drive boards, to confirm, I've got everything. Just knew, I'd better get those triacs looked at, as I was sure, they'd been tripped, and I'm sure they were. This joker, just got this thing running, at least one side, and didn't explain to the guy, that he could have his own pyrotechnics going on, if it failed again, sending flaming speaker coils, across the room. Much appreciate the advise, and the time. Thank you. I'll give you a heads up, if I can get this conquered. First, is a call to Peavey to even see if the triac board assy's are even still available. I think, I know what that answer will be. Not found a thing on the net, in the aftermarket variety. If this thing were to go down, while my bud's speakers are hooked up, at rehearsal, he's not going to like what happens. Nothing like the 4rth of July, in the practice room. ha. Peace, Dude, and thank you.

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    Senior Member Enzo's Avatar
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    The 5.6 ohm with cap in series across your output is your zobel network, look that up. If it burnt, it is more likely the amp went into high frequency oscillation at high levels, rather than the series cap was bad. The cap could be, but in my experience, it almost never is.

    I highly doubt Peavey has any 40 year old board assemblies, but there are so very few parts, you could easily make a couple with perf board.

    From your description you have an A series CS800, but the 10 ohm resistor is on the C version driver board. So you need to determine if the expected driver card is in the amp, and get the A series schematic from PV. Actually I think the file covers A and B, and B series has part numbers, while A series is almost identicle but has only part values.

    WHichever card that is, if the 10 ohm resistor burnt there is more wrong, resistors do not burn up by themselves. Check for a shorted cap or shorted semiconductor on the left side of that resistor.
    Education is what you're left with after you have forgotten what you have learned.

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    I believe you are exactly right, on that Enzo. I know you're right, about something burnt that resistor. Was going to do the pre board swap, and confirm, if the one board, was still faulty, as I do have one channel, that is working correctly. I also read, a very long explanation to a guy, and you have the patience of Jobe. Learned a ton on what you were explaining. This thing has had the triac board assy's removed, and the guy is lucky he didn't have voice coils as part of an unwanted pyrotechnics show. They are discontinued through PV, and I'm just going to build my own, and install them, after I get both channels, sound. Anyone have a digikey number for that SBS 14, which is Q2, on the triac board. Have most of the other parts rounded up. Dude gave me the number on the SAC 187, and I'm putting together an order for digikey, today. Gone into the pedal modding hobby. Boss pedals mostly. Bought the Wampler book, and learning a bunch. Am cursed with loving to build stuff, if one could call it that. Being one who can fix things, there's never a shortage of things to do. Thanks so much, guys. Would be a real treat to work beside you, for a week, or two. The magic, that one could learn. I got the Devry tech degree, and then went into the fix it business, years ago. I wish I understood circuit analysis, like some of you guys to. You're great to learn from. Thanks again.

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    Senior Member Enzo's Avatar
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    Did you ask Peavey if they still have the SBS14 in stock?
    Education is what you're left with after you have forgotten what you have learned.

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    Yes, I did ask Peavey. They were real good about it all. They have some of the nicest people down there. I found the SBS 14, on the net, finally last night, as Peavey had discontinued, it along with many of these little parts. New part number, that I found is MBS-4992=8volt switch.(SBS-14) Maybe, it will help others, that might be doing the same thing, I'm going to do. Found a PCB board, that I can get for under $5 bucks. Going to die grind it in half, and build me own triac boards, with components, and I'll have my circuit protection, for my speakers, and maybe, have all of $15 bucks, in the boards, and components. I make things, when I need to. Digikey is going to get the order, today. Am almost positive, the real problem, that started it all, on this amp, is a high freq. oscillation, like several of you, have mentioned. Whether it's there, or not, there's a handful of components that could have failed, off that line, and caused the burning resistor, on the driver board. I'll find the little buggar, one way or another. Klondike Kat, always gets his mouse. Even it the mouse trap, might have an obsolete part, here, and there. Ha.. Never say die. Thanks guys.

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