Solid State Guitar Amp Forum | DIY Guitar Amplifiers
Solid State Amplifiers => Amplifier Discussion => Topic started by: japBLONK on November 24, 2014, 09:04:38 PM
Hey guys,
I recently inherited a Legend A30 with some problems in the output section. It looks like repairs were attempted, some capacitors replaced etc. I've tried my best to troubleshoot the output section, but to no success since the whole situation is complicated because of the previous attempted repairs.
The output looks correct on the scope when there is no speaker load attached. As soon as a load is attached (speaker or dummy), the output quickly drops to be nearly silent. The only thing that comes out of the amplifier is highly gated sounding, and on the scope appears to be just the very tops of the positive peaks (but inverted) of the signal going into the power section.
Attached is the relevant part of the schematic (full schematic here: http://music-electronics-forum.com/attachments/24638d1376247077-30watt1_1.jpg ) and I've also shown a bunch of test points and the results of the scope measurements I made at each of those test points, both with the load attached and with the load disconnected. I also wrote up all of the transistors in this section and what they've been replaced by. The schematic is hard to read, so there's a question-mark next to a few. Also note that Q12 (D41ES ?) was replaced with an A706 and the pin different pinout looks to be correctly compensated for.
I hope the diagram helps, let me know if there's anything you'd like me to test.
Thanks again,
Welcome japBLONK, and thanks for the very complete information about your fault.
I was often called in on industrial problems after just about everybody from the managing director to the tea lady had had a go, and yes, it can really complicate things.
It's late here in Oz ATM so I'll have a look in the morning and see what I can offer.
Did you check those two emitter resistors on the output, R66 and R51? They should be 0.33 Ohm. What is the bias current with no signal?
I'm happy to say I got this going again! i ended up desoldering all of the transistors and testing them. Q16, the MPSA20 was toast. I put a 2N3904 in there and the amp fired up!
:dbtu: <3) :dbtu: