Peavey Musician Mark III 400G (400B/G Board)

[shakenbreak]

My dad has had this amp for a long time, since I was little and it just sat around because it stopped working. A few months ago I wanted to bring it back to working condition. I started off by replacing all the transistors with new ones (he said that's what went wrong with it before.) Then I noticed it didn't have a fuse in it or fuse holder cap and I tried finding the right fuse for it but couldn't come up with anything solid enough and forgot about it for awhile. Anyway, I ordered some 3AG 6amp (that's what the backplate says) 250v fast blow and slow blow fuses and a new fuse holder. I took the old fuse holder out and wired in the new one, put in a fast blow fuse and it blew it upon power up. I then put in a slow blow fuse and it powered up fine but it smoked a resistor or diode on the board. Looking at the schematic it looks to be CR9 is the one that bit the dust. I'm pretty new to wiring and technical stuff so any help at all to get this back up and running would be appreciated.

[Enzo]

Does CR9 LOOK burnt? Or did you try to measure it and it seemed shorted? See that it has a 33 ohm resistor in parallel, so in circuit it will never chek like a regular diode. It is a possible failure, yes, but pretty unusual if so.

[shakenbreak]

Does CR9 LOOK burnt? Or did you try to measure it and it seemed shorted? See that it has a 33 ohm resistor in parallel, so in circuit it will never chek like a regular diode. It is a possible failure, yes, but pretty unusual if so.

It is burnt I saw it light up like a christmas tree. Looking around in the amp further I noticed something important might've been unplugged from the board, the 3 pin connector that is in between C33 and R70. It most likely came unplugged when I pulled the backplate out to do the fuse holder and I never noticed it until after it burnt up the diode.

[Enzo]

That is the transformer wires to the low volt supply, I see no relation to CR9.

CR9 is hard to burn up. the current to do so must come from somewher and go somewhere.

Does your amp have the output transformer as shown on the schematic? If so unplug it. Just connect your load directly. BUT, no load at all until the amp is stable and not making DC>

Um just a thought. CR9 is between the two driver transistors Q2, Q12. Well off to the side a bit too. Right near it is a two wire molex connector. The wires SHOULD go to a single diode on the output transistor heat sink. Does it? It may be possible to connect the various molex connectors to the wrong places. Otherwise I am having a hard time seeing a current path to burn up CR9.

[shakenbreak]

That is the transformer wires to the low volt supply, I see no relation to CR9.

CR9 is hard to burn up. the current to do so must come from somewher and go somewhere.

Does your amp have the output transformer as shown on the schematic? If so unplug it. Just connect your load directly. BUT, no load at all until the amp is stable and not making DC>

Um just a thought. CR9 is between the two driver transistors Q2, Q12. Well off to the side a bit too. Right near it is a two wire molex connector. The wires SHOULD go to a single diode on the output transistor heat sink. Does it? It may be possible to connect the various molex connectors to the wrong places. Otherwise I am having a hard time seeing a current path to burn up CR9.

Yes it has the output transformer, I'll attach some pictures for you to double check (I know the amp is dirty inside I'm gonna clean it before I try anything else.) The two wire molex connector is soldered to the right side transistor board.

[Enzo]

Disconnect the output transformer during repair. If the amp makes DC offset, that transformer acts as a dead short across the output.

Note the burnt resistor has a parallel diode. Is that shorted as well? Also verify that NEITHER end of the resistor is shorted to ground.

[shakenbreak]

I made a mistake, after a little cleaning and plugging in the low volt transformer wires back in and unplugging the output transformer I tried powering it on to the 8ohm setting and it seemed okay and I made the mistake of powering it on with a load. R61 burnt up as soon as I did that. Not sure if it's helpful to figuring out the problem since it did or not helpful since I messed up

[Enzo]

We all make mistakes, god knows I made my share over 65 years of electronics.

leave the output transformer off until we are done. Do not connect a load until we know the amp works AND is not putting DC on the output.

Please tell me you DO have the two three-wire plugs to the output transistors connected.

Verify all of those 8 transistors are not shorted. Be aware there is a 1 ohm resistance from base to emitter on six of them.

If R61 burnt up, Q12 is likely shorted, and quite possibly Q2. R61 is a little 47 ohm job. Check R26. Note also in between is 4.7 ohm R44. On many of these boards, the parallel L1, which looks like a resistor anyway, is already replaced by another 4.7 ohm resistor.

Make a "light bulb limiter" and use it.

[billmorris52]

Guys: I also have a broken Peavey Mark III with the 400 B/G output boards.
A) I found a shorted output device Q3 on my schematic which is the driver for the other 3 in 1 channel.
B) I found a shorted Q2 5331 pre drive transistor and also 2 of the 47 ohm resistors were burned up with "craters" left under them on the PWB.These were R26 and R61 on my schematic revised 3/14/83
originally drawn 2/20/79 P/N 81505010 Foil traces are still OK in damaged areas but fiberglass is just thinner there now.
C) Found that L1 100mh on schematic in parallel with R44 ( 4.7ohm resistor ) at Q12's ( 5332 PNP ) emitter is NOT an inductor at all but rather a 2nd 4.7 ohm resistor in parallel with R44. What's up with that ?
E) Found the lead for the dual diode CR14 is broken.Can 2 other diodes be used instead ? soldered neatly with short leads together as described in another thread using 2 1N 4007 diodes ? Or do I need to deal with Peavey ? So far Peavey has been terrible with customer service. Over 2 weeks now just to get the diodes PN and price from them. I had transistors from Antique Electronics Supply in 3 days regular ground postal rate, and I still can't figure out how to place an order with Peavey. All my calls go to the same persons messaging who never calls me back and my emails come back unsigned with MINIMAL info given to my questions This is like picking the meat off a dead skunk roadkill as far as their customer service has treated me so far. If at all possible I do not wish to deal with them any further.
D) found rear aluminum heat sink plate bent probably due to line cord wrap up on the outside pushing against it. I have re-bent it almost perfectly straight. Looks like Peavey could have done a better job supporting it in the wooden cab head so this would not happen. I will probably add some wood blocks to inside of head cab to support this heat-sink plate better for the future. This probably was the failure mode holding the bad transistor slightly away from the aluminum back which is the heat sink.
Can anyone post the specs for the original output transistors used in this amp Q3 thru Q6 and Q13 thru Q16 ( Peavey PN 99009016 ) since I can not find a cross or spec for them anywhere in my cross ref's ?
Problem here is it is difficult for me to replace them with other devices I have on hand without the spec's since cross ref's usually lead you to a more robust device than the one you are replacing. Nothing wrong with that but can sometimes be like putting a Maserati engine in a VW for no good reason other than it does the same job but better until you get the bill for it and have to pay for it.
I have replaced the dead 5331 pre-driver transistor Q2 with a part MJE 243 the cross number posted in another thread that I got from Antique Electronics Repair website. I have checked also all the surrounding diodes and also Q12 ( 5332 ) and Q9 ( 5331) on my Tektronix 575 curve tracer - and they both seem OK including breakdown voltages, and family of curves ( Gain ). This is the first time I have ever seen a PNP in - rail circuit like this survive when the NPN in the + rail fails. Usually seems like the PNP's always would fail first and usually leave the NPN intact - OH Well live and learn ! So I guess I need to get the dual diode from Peavey since I have no cross for it either. I have literally thousands of used semi's in stock stripped from multitudes of electronics gear I have scrapped out over the last 30 years. I would bring it all home from recycle centers and pull off components that looked good. I use my TEK 575 curve tracer to test all the semi's and also reform caps using current and voltage limiting with the curve tracer. I actually have 2 of them and I am considering posting one for sale as I am now trying to downsize at almost 70 years young. One thing I do not see discussed at all is matching the output transistors for gain so that one or 2 do not current hog when driven in parallel. Is there circuitry in these Peavey amps that controls this? Am I missing something when looking at the schematics for these Peavey amps ? How much difference between the output devices in current gain can be acceptable to get good life out of these semi's in these amps once rebuilt ? I am also trying to conquer a CS-800 amp with similar issues. 1 channel OK the other no output. I think that might be in the SCR circuitry for the protection circuit. I have enjoyed reading up on everyone else's similar issues here, and on other threads in other sites. If you can make any suggestions to me I would greatly appreciate them.
Mr. Bill If you like the weather today in New England wait a minute and it will change. ( usually for the worst )

[Enzo]

Hi Bill, welcome.

Please start a new thread for your amp rather than add on to a thread about a different amp.


C. The tiny inductor was replaced in production at some point. You will find the 400BH has the change already in it. No big del, just an engineering change.

D. I suppose Peavey could have done a "better job", but considering this amp is over 40 years old, I won't fault them. Peavey amps have held up to the road for decades.

GO to the Peavey web site and onto their forum. Under guitar amps, their rep Roger Crimm posted a thread "Need Parts?" it tells the story of what is going on at Peavey.

The schematic calls out all eight power transistors as type 6357 Actually that is short for SJ6357. That is a house number. If you search the web and even this forum, you should find the Peavey Semiconductor Cross-reference. It will tell you the 6357 is an MJ15003 under the skin.

Current gain matters at high loads, and unless your curve tracer can handle that... Each transistor has a ballast resistor. The job of those resistors is to make sure the transistors all share current well.

Also if you download the cross reference, it discusses mixing output transistors. IN a word...DON'T. When they are the same type, the ballast resistors are sufficient to make them share. But if you mix an RCA and a Moto part, they can be totally different inside and not mix well at all On each of those four-transistor sub-boards, you want them all the same type. SO if you want to replace one with 2SD424, go ahead, but then you'd need to replace the other three on that side with 2SD424. SO I appreciate scrounging for parts. but please don't line up a 6357, an 2SD424, and a NTE130 or something.