Solid State Bass Guitar Amp not Working

Thread Starter

steve.pierce7

Joined Jan 6, 2016
5
Good afternoon folks. I have a 60s National Valco solid state bass head that has 3 diodes that cannot be identified by the best repair guy in the area. I have absolutely no idea what the diodes are, the repair guy said they run with the output, and on the diodes are a 5, a T, and what looks like maybe an L or a 7. I was just wondering if anyone can help me out before I toss the thing for good. I've attached a couple of pictures of the diodes, and the bass amp circuit board, with arrows pointing to the locations of where the diodes should be.
20160106_162837.jpg 20160106_163453.jpg 20160106_162825.jpg 20160106_162818.jpg 20160106_163447.jpg
 

Lestraveled

Joined May 19, 2014
1,946
OK, they look like 1N4148 diodes, which are rated at 1/4 amp. I suspect that other parts are bad that caused these diodes to fail. Have you tested (ohm meter) the D525 transistors??
 

Lestraveled

Joined May 19, 2014
1,946
Looking at the your pictures, the D525 transistors have been replaced before. Their true part number is really 2SD525. They might be bad.
 

Thread Starter

steve.pierce7

Joined Jan 6, 2016
5
the repair guy had changed some of the bad parts, and said everything besides that was in good working order, I'll make sure he does a test once the diodes come in. Thank you so much for giving me a possible solution sir!!!!
 

dl324

Joined Mar 30, 2015
10,921
Unfortunately the schematic was so old the ink had chipped off virtually all it, so it was of no use to the guy I took it to.
Any repair person would be able to trace the circuit and make a schematic. Are you able to do it for the circuitry around the diodes?
 
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