Peavey Mark III 400 b/g no output

[Enzo]

Does the fuse still want to blow with no speaker attached? If fuses blow without a load, then the outputs on opposing sides are probably either shorted or turning on at the same time.

DO not connect a speaker load until we are sure there is no DC on the output.

And unless you have a variac and ammeter and are familiar with their use, look up "light bulb limiter," and make one and use it.

[UnluckyFett]

i found a SJ-6357 that the collector is shorted to the emitter. ive contacted peavey for pricing and availability on 4 replacements. it was on the board that the foil was broken on. i have the stuff to build a light bulb limiter but i havent had the time to build it, october was crazy for me.

[Enzo]

6357 is the old style part number. Today the number is 70484140, which is generically an MJ15003.

[UnluckyFett]

i got the transistors today and i checked the 10w resistors and the one below the bad transistor reads open. is it common for a bad transistor to take out the resistor on the emitter?

[Jazz P Bass]

That resistor is called a "ballast" resistor.
It helps current sharing between multiple, parallel output devices.
So, yeah, if the transistor desided to try to pass gobs of current, as it was failing, it can very well take out the resistor.
Plus, it is such a low value, it does not "resist" much when things go south.
I= V/R.

[Enzo]

That is why we check them. Common? COUldn;t say a percentage, not the majority of cases, but it happens enough. Yes, a shorted transistor can take out a resistor.

Not only those power resistors, but the little 47 ohm resistors associated with the 5332/5331 stage too.

[UnluckyFett]

2 problems first i found that i broke the diode that was clipped to the back when i lifted the board. its a Motorola and it says 38862. second the 47 ohm resistor off the 5332 (R61) isnt on the board. component wise that section of the board is different than the schem



first question why was the diode clipped to the back panel also whats a suitable replacement? second question are the components supposed to be different in this model?

[Enzo]

Clipped???

There should not be any parts on the solder side of the board. And if someone put something under there, I wouldn;t expect it to be in clips.

I don't see how the amp could function without R61. WHat else is different from the schematic other than the missing resistor? And does the part underneath seem to be connected to the same part of the circuit?

[Enzo]

Or by back do you mean the power transistor board? The dual diode on that has the pair of wires up to a Molex connector on the main board. The replacement is to just call Peavey and order the exact part. It is on the heat sink to help the amp track the temperature of the power transistors.

[UnluckyFett]

ok so that answers one question. but what about R61 or lack there of? i posted the picture in my last post where theres the empty spot where it should be and judging by the foil there was never one there. from left to right it should be R44 L1 R61 C26 bout it looks like R44 R? Empty C26.

[Enzo]

"L1" is probably replaced with a 4.7 ohm resistor. That is a common substitution, and compare this model to the 400BH where the substitution is made in the schematic. SO that takes care of R?. And yes, I see the space for R61, as I said a couple posts ago, I don't see how the amp could properly function without it.

[UnluckyFett]

Whats the worst that could happen if i put a 47 ohm resistor in?

[Jazz P Bass]

You want to install a "47 ohm resistor" where? R61?
Yeah, it has to be there or the driver circuit is not complete.

[UnluckyFett]

SUCCESS! After i got the dual diode replacement from peavey i replaced the .33ohm ballast and put a 47ohm resistor in R61 and replaced all the SJ-6357 on the transistor board that had the bad one and of course replaced the dual diode i turned the amp on. All appeared stable. i hooked it up to a old junk persay 8ohm speaker and all went well so i put it back on the normal cab and it works perfectly! Thanks for all the help Enzo and Jazz P Bass I couldnt have done it without you guys. Im shocked at how good this amp sounds and feels for being a solid state. Its not my 69 Spec Superbass but its definitely one of the best solid state amps ive played.