Peavey 300 Bass amp - power section repair

[mindofone]

I've got an early 1970s Peavey 300 Series Bass amp (The Bass). The amp seemed to be working just fine, but after some spirited (i.e., loud) playing, some of the magic smoke in the power section was released.

Upon further examination, it was obvious that the two 10 ohm resistors on the power board were burned up. It would be easy enough for me to replace those resistors, but I suspect their failure was a result of something else, and I'm curious if anyone has some suggestions?

I'm attaching the 150 Watt power amp schematics with the offending resistors circled in red.





Thanks in advance for any suggestions!

[darkfenriz]

Looks like a seriously wierd failure to me. That suggest most of surrounding circuitry is toast...

[Enzo]

Of course something else burnt them up, resistors don't burn on their own.

To get a burnt resistor, excess current had to flow through them. SO look for a path for that current.

First in my mind is to look for open 0.33 ohm 5w resistors on the outputs. Those normally carry the output current, if they open, then the current seeks its path through your 10 ohms. WHich also points to maybe shorted small transistors 2N3642 and 2N3638, check them. And all that current came from somewhere to start with, which makes me think the output transistors themselves are shorted, check.

[mindofone]

Quick follow-up on this amp. Turns out someone (not me - really) installed a 20 amp fuse, and proceeded to play it at full tilt. For reasons of natural selection, one of the power transistors failed, and took out a LOT of other components. Will probably just find another amp PCB rather than desolder 2/3s of the parts on this thing.

[The Dude]

I'm not sure you'll be able to get that board. Generally speaking, when parts burn, the carbon residue makes things look worse than they are. If it were me, I'd clean it up and fix it. It will probably be cheaper even if the board were still available.

[Enzo]

Good luck finding a board for a 45 year old amp.

You probably need a new set of output transistors, and I would then replace the drivers as well. Check those large cement resistors for opens.