Peavey "Musician" Blowing Speakers

[deluxe reverb92]

Hey everybody! I haven't been on here in a long time, but I have another problem I hope you all can help me out with.

My buddy gave me his Peavey Musician amp to look at. Its stamped 1972....I'm pretty sure its a bass amp. Anyway, I plugged it into a speaker, it made a loud humming noise, and blew the speaker. I plugged my meter in, and its passing 40V through the speaker jack!

I checked the output transistors, and they aren't shorted, however the wires that are connected to the emitters and collectors are, and when the amp is on, there's 40v on both sides of the transistors. Before I start checking all the components, anybody know where I can start?

Thanks in advance!

[deluxe reverb92]

Also, there's 5 wires coming out of the transformer. Yellow (wired to ground) two red and two black. The two red are shorted to the yellow one. I don't know if that helps at all haha thanks!

[R.G.]

I plugged it into a speaker, it made a loud humming noise, and blew the speaker. I plugged my meter in, and its passing 40V through the speaker jack!

I checked the output transistors, and they aren't shorted, however the wires that are connected to the emitters and collectors are, and when the amp is on, there's 40v on both sides of the transistors. Before I start checking all the components, anybody know where I can start?

1. Get a schematic. Find out whether it has a bipolar power supply or a single-ended power supply and an output capacitor. This will make a difference. The output capacitor, if it's there, is at risk of being shorted.
2. I'm assuming that since you say "transistors are not shorted" that you tested them out of the circuit. If they test good out of circuit, something in the circuit is telling them to be fully on, all the time. This is often a bias string problem. Can't tell much without a schematic. This last could be as simple as a broken wire or open bias diode.
3. Solid state amps don't need speaker loads, and will not die if a speaker is not connected. Quit frying speakers. Run it unloaded until you get it fixed enough to not put DC on the speaker.
4. Make a light bulb limiter and use it during debug and test.

[Jazz P Bass]

Schematic for power amp.

[R.G.]

That helps.

1. Pull out Q4,5,6,14,15,16. Leave the output unloaded.
2. Short the base of Q2 to the base of Q12 with a wire temporarily.
3. Test it with no load. What's DC volts on the output?