Ampeg SVT-4 Pro. Had the same problem I often see in these, damaged solder connections on the rear of the main PCB. Bass heads get lots of mechanical stress because they’re always sitting on top of a box full of speakers. Solder joints on flexible and poorly-supported PCBs go intermittent, making the amp cranky/possessed/unresponsive/etc.
This amp in particular wouldn’t turn on (no blown breaker) because the solder under the IEC AC mains jack had crusted and flaked away. I reinforced it with buss wire and epoxy, and resoldered every pin on every jack along the back. This is a needlessly expensive repair, because of how time-consuming it is to get the whole board out, for such a silly problem.
Trace Elliot V-Type bass head with the entire power stage removed. The ‘Bipolar Bear’ power modules in these amps are notoriously unreliable, and this one had shorted all the outputs. In this case, the client deemed a board swap too expensive, so it’s now a preamp only. Rewired the line-out as a master output from the master volume control, which can drive a rackmount power amp. FX loop functionality is unchanged, and all the front panel controls still work the same.
I did enjoy that on the bottom side of the power module, Trace Elliot had printed into the PCB: “Make it so, Number One.”
The 5150II from earlier, after repair. Peavey sent a new power board to drop in, which significantly reduced what would have been a prohibitively time-consuming repair. Just needed to replace a few other resistors on the main and preamp boards, and the amp plays good as new.
For how much people turn up their noses at Peavey gear, you have to respect how easy they are to work on, and how easy the company is to deal with. They sell modular replacement parts to anyone (affordably!), and they’re always helpful on the phone. And I love the 5150 sound anyway. It’s probably the best sound-for-dollar of any high-gain amp. When I’m on tour sitting backstage, I can identify one being soundchecked in the other room without ever seeing it.
In case it wasn’t obvious, if you have a dead 5150 or 5150II, I’ll happily take it off your hands.
1970s Sunn Model T Super. This is a transitional model with the 2nd-gen preamp but without the red hardware. It needed to have the bass side of the James-Baxandall network rebuilt, since a previous repair attempt hadn’t been wired correctly. I rewired the master back to a standard configuration, without the dual-gang setup that makes dirty sounds tougher to get out of the 2nd-gen version. Both coupling caps were a little leaky and got replaced.
Now that the preamp is working properly, it’s still not producing the rated power. May just need new tubes. These amps are so simple that it usually doesn’t take long to narrow down the problem.
Warning: graphic imagery. Gutted 5150II. Still trying to figure out what to do with this one, since the power tube PCB is burned through in multiple places and completely shot, and there are component failures all over the amp. Basically, one initial flashover carbonized enough PCB area to permanently couple the plate voltage into the heater circuit, which blew every single tube in the amp, preamp cathode circuits, the LV relay supply, heater reference resistors, and a few sundry other bits.
I suppose the moral of the story is that you should always take an amp that “makes lightning” to a tech immediately. Don’t assume that because it turns on again that it’s ok, and don’t sell it to an unsuspecting buyer without telling them.
Harman Kardon A50K (kit version of A500). 2x25W 7355-based integrated amp with phono preamp. Bad volume and balance pots and lots and lots of bad coupling caps. Somehow, the supply caps are fine. These amps have a lot of design weaknesses — added features that increase the number of points where failures can occur. For example, the OT secondaries are routed through switching contacts on the headphone jack, which were corroded and open on this amp. I spent far too much time cleaning corroded contacts on this one. It could be reworked into something simpler and much more reliable, but that would take a lot of time and effort.
In the above picture, the amp is being tested after repair. The test ‘signal’ is Stars of the Lid’s Austin Texas Mental Hospital.