Peavey Mark III 1978 Sereis 400GH

[BassAmp]

I have a Peavey Mark III 1978 Sereis 400GH Bass Amp that is blowing fuses. I am going to check the output transistors for shorts as a starting point, but am curious about the electrolytic capacitors. I would imagine given their age, that they are dried up by now. Is it normal practice to replace them? If so, is there a site somewhere that sells all of the needed caps in a kit?

[Enzo]

Fix it first, then worry about upgrading the caps.


Those large blue 5000uf 55v caps have not been available for years. What I recommend in their place, and what Peavey ships when you try to order the blue ones, is a common 4700uf 63v cap. They won't fit the old holes, so at least one new hole will have to be carefully drilled in the pc board for each. But if the old ones are working, I'd leave them.

[BassAmp]

Thanks Enzo, I'll hold off on the caps for now and worry about the fuse blowing first. I wasn't sure if the caps could cause the fuse to blow since I haven't looked at a schematic yet, but figured they'd be dried up by now.

[Jazz P Bass]

To add, any 'shorted' component will challenge the fuse.

The first thing I would suggest that you check is the power supply diodes.

If they are good, then the next suspect is the output transistors.

[BassAmp]

I checked the 8 output transistors and found one that is shorted. Are these available locally at a Radio Shack type store, or is online my only option? I hate waiting for parts to arrive Also, are there any other parts that may have been damaged by this shorted transistor that I should check before placing an order?

[Jazz P Bass]

What number is on the transistor?

If it is '6357', then that crosses to a MJ 15003.
Radio Shack will not have them.

[TomCarlos]

I have worked on these Peavey amps... asking the same question regarding the output transistors. Besides having the recommendation of the MJ15003, it was recommended that we not mix and match... you replace them all. I know, extra expense. But... if you are doing the work yourself you save on labor PLUS you will probably be getting a much better transistor. Besides, this is a good reason to lift the transistors and apply some new thermal "goop."