SVT-4PRO, just a whisper!

[Sowhat]

I have an Ampeg SVT-4PRO in with the complaint that it don't make any sound. I checked a few important preliminary things like the breaker, rail voltages and DC on the output… well, I got at least a half a big rail so there's that but everything else looked O.K. I pulled it apart… no small job, and flipped the main board over and sure enough the rectifier leads were cracked. Once I got that all cleaned and fixed up I reassembled it… once again no small job and now I have both sides of the big rails and I can see a few millivolts of properly shaped signal at the outputs with non clipping input to the preamp. So, it's working pretty much kind of but signal is not getting through very well.

I did trace through the tube section and that all looked good but I have not done much signal tracing beyond that mainly due to the crammed in design, it's hard to get probes in and even see some interesting areas. It would be nice if I had a board layout for these boards, I have a nice schematic and that helps a lot but locating test point 6 or like that would make things so much easier. Anyone got something like this? Or does someone have any likely suspects for low audio throughput in this amp.

[g1]

Layout should be in one of these: http://music-electronics-forum.com/s...l=1#post128013

[Sowhat]

Layout should be in one of these: http://music-electronics-forum.com/s...l=1#post128013

Actually 2 of these, and thanks for these layouts, now I can do some strategic planning!

[Sowhat]

Strategic planning done and finished, actually not much to it, it was V2. Replace that and it came right to life. So it was no negative big rail because of cracked rectifier solders on the mainboard and a faulty 12AX7A at the V2 position. Hey, G1, thanks for pointing me to those layouts.

[nevetslab]

That bridge rectifier's solder leads on the PCB is a common ailment on that amp. I've always been fortunate not to have the amp go down in flames from that, with it being a 'simple' fix, once it's been gutted, and you've marked all the primary & secondary leads to re-assemble it.

[Sowhat]

and you've marked all the primary & secondary leads to re-assemble it.

Yeah, I made my little wiring cheat sheet before disassembly but that yellow wire, the one that plugs in right in the middle of those 4 big caps can be a doozy to get off and especially back on. Not the greatest design for servicing.