Seeking some advice for repairing a Peavey Mark III bass head

[Ceredox]

Hey there, this is my first post and I hope I poste in the right section. I have a 1978 peavey Mark III bass head that I'm trying to repair for my girlfriend's father. It blows a fuse as soon as you turn it on everytime. I don't have a schematic for this... Where could I find one? I checked the transformer for shorts and everything seemed to check out. I also checked the output transistors for shorts and found none. I don't have much info on the transistors though... Does anyone know what the values should be on them? And should they be unsoldered to be checked? I'm an electrical engineering student and know my basic theory very well but I'm a but weak on transistors. Any advice would be greatly appreciated. Thanks!

[Jazz P Bass]

You might want to check the power supply diodes for shorted condition.
Also, if you feel inclined, use the Search function (upper right corner)
Enter in "Peavey Mark III".
There may be previous posts that have usefull advise.

[52 Bill]

In general shorted output transistors are the first thing that usually cause this failure mode, followed by shorted power supply diodes, and then any number of other shorted components. You should probably look up the instructions for a light bulb current limiter and build one. It will save you from replacing the fuse while you are troubleshooting the amp. Also, don't connect the speaker to the amp until you have the problems staightened out.

The most common failure mode of the output transistors is a short from collector to emitter. Because the output transistors are wired in parallel, it will be difficult to test them in circuit. I believe that on that series of amps, the outputs are TO-3 cased transistors that have the collector connection made through the case. If you remove the two mounting screws, you will remove the collector from the circuit and you can then test the transistor for E-C shorts. Use the diode test function on your meter and take a reading from the case to both of the two pins, which are the base and the emitter connections. In any case, there should not be a zero or near zero reading. Test all of the outputs this way and see if you find any shorted ones.

If you search for transistor testing directions, you will find instructions for testing any transistor, but while these are in circuit there are resistors and other parts that will throw your readings off. So if you find what seems like a bad transistor you will need to remove it from the circuit and retest it before you can be sure that it is bad.

If there are no bad output transistors, then find the power supply diode bridge and test each one of the four diodes with your meter. The meter will charge or discharge the filter caps as you test the diodes, so hold your leads on each diode for a long enough time to get a real reading. Again, you should not find any near zero or zero readings across any of the diodes.

Once you've made these tests, let us know what you find out.

[Enzo]

One other thing that will blow fuses is if the circuit through that dual diode is open. Then both sides turn on hard. That is the two-wire connector with wires running to the little diode under the clamp in the middle of the heat sink.

Ceredox, welcome to the forum. I am assuming the power amp board in your amp is the 400BH. That number would be on the main power amp circuit board, and probably in large letters on the rear panel of the amp too. Good to let us know for sure which board you have.

[Ceredox]

Hey, sorry for the delayed response. I haven't touched the thing since my first post. I was actually curious about the collector being the case on the output transistors because I didn't see a lead for one. I've only tested one so far but I'm going to remove the rest and test them tonight. If not, Where are the power supply diodes usually located? I'd love to tell you what board I have but I'm at work and don't have access to it at the moment. From what I've heard on this forum and what I'm seeing. it sounds like an output transistor. I really appreciate the guidance.

[Ceredox]

Well I removed all of the transistors and tested them and I came up with one that had the same resistance reverse biased as it does forward biased. Since its supposed to be infinity, I'm assuming that this would be my short. Also, I had another that had a lower than average reading from B-E. Not a gigantic difference but I was wondering if I should replace it too. Where can these be ordered and should I get the original replacements?

Thanks

[Jazz P Bass]

Peavey is where I would go for the parts.
877-732-8391
As to the iffy transistor, if you do not have "diode check" on your meter & it appears suspect, simply replace it.