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Thread: Peavey Mark III Series 260 C

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    Peavey Mark III Series 260 C

    I was just given this amp by my drummer. It was stored in an attic with no climate control for years. When I turn it on, it howls. None of the controls have any effect. The sound is not high pitched. I disconnected the front control panel section and the results are the same. Any idea what might be going on? (I'm not an electronics techies but have some minor skills.) Thanks in advance.

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    Lifetime Member Enzo's Avatar
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    I don't know what howls means. I assume it makes a loud noise all by itself. Is the noise a loud hum? Hum being the sort of noise you get if you touch the end of your guitar cord. Of is it more of a WOOOOO sound? Or is it a high pitched SCREEEEECH? It is capable of making noises, but the nature of the noise suggests different problems.

    If hum is the problem, then we look for either DC on the output jack or we look for loss of filter cap function. And that latter would be either a cracked soler connection to a main filter cap, or the cap itself has failed.

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    Hum being the sort of noise you get if you touch the end of your guitar cord
    Yes. That's it.

    DC on the output jack or we look for loss of filter cap function. And that latter would be either a cracked soler connection to a main filter cap, or the cap itself has failed
    How might I go about identifying these? I have no schematic. I know enough to possibly be guided through a repair though.

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    Lifetime Member Enzo's Avatar
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    You look at your speaker cones. When you turn on the amp, do they move one direction and stay there? If so, turn it off immediately, that is DC on the speaker. Or you can disconnect the speaker and measure for DC voltage across the output jack.

    If there is not DC on the speaker output, then you probably have lost a main filter cap - one of the two large cylinders side by side in there. It is possible one has simplpy cracked fre of its solder, check for that. Otherwise we probably need to replace it.

    You can get any PV schematic from customer service at Peavey. But here, does this match your power amp?
    Attached Files Attached Files

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    Thanks Enzo. This is a huge help. If there is DC on th speaker output - what kind of voltage range are we looking at and what might be the remedy?

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    Lifetime Member Enzo's Avatar
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    If the amp is blown, you would see something like 20-40 volts there. ANything over a quarter volt is a problem. But this won;t be subtle. You'll find 40v there or not, most likely.

    Rather than go through a technical discussion over its remedy, let us find out if that is the situation first. if you have a bad filter cap, we needn't waste time on bad transistor talk.

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    Thanks again. I'm in the middle of selling a house andgetting divorced, so it may be a little while before I can check these.

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    39.7VDC reading taken on the speaker output pins......................

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    Lifetime Member Enzo's Avatar
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    OK< blown power amp.

    First check all the large power transistors for shorted. Check the two small transistors on the indiovidual finned heat sinks, the 5331 and 5332. There is also a small 47 ohm resistor associated with each of those. Check for open resistor.

    There are a number of large 10 watt rectuangular cement power resistors. Check all for opens.

    Look at the schematic. There are DC voltages at points all over the powr amp. Check them. They should all be reasonably close. Let's say within 10%.

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    check all the large power transistors for shorted
    Are these the 67376's

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    Lifetime Member Enzo's Avatar
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    According to the schematic, though various other types work there.

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    Enzo,
    I removed the base screw on all four power transistors. One of them read a short from B-E and C-E. I guess the mystery is solved.
    I found a previous post where you said you buy MJ15003 power transistors from Allied to replace failed 67376's. As you stated earlier, there are four 67376 power transistors in this amp. Two are 67376-7943-H's and two are 67376-8029-H's. The failed component is a 67376-7943-H.

    Do I need to replace all four transistors or just the two identical units?

    Is the MJ15003 a suitable replacement for both the 7943's and the 8029's?

    Couldn't find the MJ15003 at Allied but it turned up on here on Mouser http://www.mouser.com/Search/Refine....eyword=MJ15003 with a slightly different part number. Do you think it will work as a replacement?

    Thanks again for all your help. You're a lifesaver.

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    Last edited by howarda; 03-19-2010 at 10:33 PM.

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    Quote Originally Posted by howarda View Post
    Is the MJ15003 a suitable replacement for both the 7943's and the 8029's?
    Those number are date codes 43rd week of 1979 and 29th week of 1980.

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    Lifetime Member Enzo's Avatar
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    What Bill said. the part is a 67376, those other numbers are just when they were made.

    They are all four the same part because this amp is quasi-complementary. But two of them run the positive side and two run the negative side. Two on the left and two on the right, I forget which is which. You want the two one a side to be the same type, not a mix. You don;t have to have the positive ones the same as the negative ones, but you do want the two positive ones to be identical.

    SO if you have a couple good ones still, put them both on one side, and put the replacements on the other. Ignore those date codes.

    MJ15003 and MJ15003G are the same part. The G stands for Green, and all it means is that the part was made in compliance with the new environmental regulations for lead content - it has no lead in it. YOu may mix and match MJ15003 with or without the G.

    Yes, looks like Allied is out of stock. Please, never buy the NTE versions of transistors, wherever you shop.

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    Thanks Bill and especially Enzo. I'll give you a full report once the repairs are made.


    never buy the NTE versions of transistors
    What's that about?

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    Last edited by howarda; 03-20-2010 at 06:31 PM.

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    NTE is a brand of general replacement semiconductors (NTE - NuTone Electronics). They have a large catalog, and you look up your transistor, and it tells you which one of theirs you can use in its place.

    Their part will probably be close to the original, but it is not the original. If it were just one transistor all by itself, I wouldn;t be so concerned. But your amp here is exactly where it matters. When there are two MJ15003 side by side, or two 67376, they have to share current equally, you can;t have one of them hogging the current while its partner idles. The parts also need to track thermally to prevent runaway and proper bias.

    I mentioned earlier that you want two the same on either side, no mixing. The NTE parts would not be the same.


    Not only that, but the NTE parts usually cost three to ten times more than real parts.

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    Thanks for the heads up.

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    Quote Originally Posted by howarda View Post
    Quote:
    never buy the NTE versions of transistors

    What's that about?
    Well...lets put it like this. you go to the tire store and ask for 4 tires size 225/70/15 the guy hands you 4 tires that while are the correct size, are made by 4 different manufacturers, one is a whitewall, one is a truck tire, and 2 are snow tires. yet they all say NTE 225/70/15 on them...get the picture?

    NTE parts are generic. one part number crosses to MANY other part numbers with wildly varying specs. you never really know what you are getting. so for a pinball machine NTE parts are great. but for an audio amp....forget it.


    Zc

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    I see

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    Replaced the two power transistors and still have the loud hum. All 4 power transistors became extremely hot. Now they are not even warm. 40VDC reading at speaker output. I did note that with the compressor turned on the loud hum is reduced greatly.

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    Last edited by howarda; 03-26-2010 at 05:52 PM.

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    Lifetime Member Enzo's Avatar
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    Please DO NOT connect a speaker until you get rid of the DC on the output.

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    I checked the two power transistors that I replaced. They read good out of the board.

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    Lifetime Member Enzo's Avatar
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    The amp is more than two power transistors, it is a whole circuit. GO back to post #9. Did you check ALL those things?

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    I did check all that previously. I guess I need to recheck now. Is the only true way to check the transistors, to remove them from the circuit board (or at least the base)?

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    Lifetime Member Enzo's Avatar
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    You check them initially in circuit with the diode test function of your meter. Note from the schematic if there are low resisyances between any two elements. If they test funny, then remove them and retest out of circuit.

    ANy time you blow a fuse, all the semiconductors become suspects again. You always need to verify the outputs are OK, an the drivers, and also important is making sure the various resistors havve not gone open.

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    Will do.

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    Sorry to revive and hijack an old topic, but I have the exact same amp with the exact same problem.

    EVERY semiconductor checks good, with the exception of U2... I can't get a reading with the diode check going from the output to the +/- inputs... not sure if that's the way it's supposed to be or not, but regardless, when I take it out of the circuit, the 40 VDC goes away, but I still get the hum when I do a quick check with the speaker (and by quick, I mean power on for less than a second). Either way, how would this send 40 VDC across the circuit when it's only getting 15 VDC?

    Something else I've noticed is that when I turn off the compressor/limiter, the 40 VDC goes away, but the hum is still there.

    It's definitely not the preamp; no 40 VDC coming out of that jack, and I can plug the preamp output into another amp, and it works perfectly fine like there's nothing wrong, and I've also disconnected the front panel and powered up the power amp, but still got 40 VDC out the speaker connectors.

    Another thing I've found is that when I kill power to the amp, no matter how I have it hooked up or configured, the output jumps to 40 VDC, then trickles down to 1 volt after 60 seconds of being off; this tells me that the huge caps are still working I guess.

    Any insight?

    EDIT: I forgot to mention... at points when I don't get the 40 VDC anymore, I instead get 19 VAC, but I still get the noise !?!?!?! I'm so confused.

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    Lifetime Member Enzo's Avatar
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    Hi turbo, welcome to the forum.

    At this point I don't remember what the eact same problem might have been, and I don;t want to read through the whole history to find out. MAy I suggest you just start a new thread for your amp. Most of us are not going to check in at the end of an old long thread to fin d that someone has started a new project at the end of it. And threads are cheap to start...

    And I have to admit - and I am an old guy, so I am sometimes slow on things - I am usure what you are writing about, when I see sentences that refer to "it" a lot. Let's use part numbers for clarity CR2, Q10, R19, etc.

    As near as I can tell, you have DC on the output at least some of the time. DC on the output results in loud hum because the severe strain on the power supply exposes its ripple. DO not connect a load to the amp until the DC is resolved.

    You amp has 42v for both V+ and V-. check them both. The voltage is not critical, it could be anything from 40-45v or even farther off. What matters is both polarities are about the same DC and clean. Now flip your meter to AC volts and remeasure. We are checking for ripple. If yo9u get under a volt or so, that is fine. If one measures 20VAC, then the filter cap is not working - either it has worn out or hte solder has failed that conects it.

    Either way, how would this send 40 VDC across the circuit when it's only getting 15 VDC?
    What does that mean exactly? This is a solid state amp, and as such is DC coupled throughout. The whole point of this amp is to control when the +42 and -42 power rails are connected to the output bus. Even the small parts at the input stages are involved in that process.


    Pulling U2 makes the amp work? Or just removes the DC?

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    Hate to really revive this one from the dead, but did you ever find a solution for Howard's issue? I had the same thing going on (260c combo), and replacing the two large electrolytics fixed the issue. Having another issue (distortion) though, does somebody have the schematic for the 260c they could send?

    Thanks!
    John

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    Lifetime Member Enzo's Avatar
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    peavey does, contact customer service at Peavey. I know of two versions, the '78 and the '81. They are VERY similar. Just ask for both.

    Howard likely solved his problem, but didn't come back to share that.

    We'd be glad to help you too, but please start a new thread for yours.

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