Random Amp Repair Tales

Univox U65RN Front CLoserHere are some of the amps I worked on that had easy fixes. Gives you an idea of some of the easier stuff.

Orange Crush 20B Amp Front: Loose speaker connection.

This is a cheap bass amp. The original owner said there was no sound. Easy enough, once disassembled, it was obvious that the speaker connection came off the PCB. These quick connects are often loose and a quick clamp with a wrench can tighten them down. These quick connects are used in the Valve Juniors and Champs too.

Univox U65RN Front: No speaker, scratchy pots, intermittent jack.

Made in Japan, this is actually a decent sounding solid state amp with decent reverb. The reverb tank is pretty small and it's located inside the chassis, so you won't see it from the outside. Amp didn't need much besides a good cleaning and a "new" speaker. I put an old Stromberg Carlson ceramic speaker (similar to a Jensen C12R) in there and that did the job nicely. The input jack was corroded and the connection to the chassis wasn't solid. After disassembling and cleaning with some DeOxit, it was fine. Same with pots, I just hit it with some DeOxit.

Fender Champ - Front: Wrong resistor installed at the factory!

New amp from the store, someone returned it saying there was little of no output. Well, they were right, very low volume and distorted. I checked the supplies, they were fine, so I checked the bias voltages. The voltage on the cathode of the 6V6 was 30 or 40 volts and all over the place. The resistor that was installed at the factory was some oddball value 7 or 8 times the normal and on top of that, it had failed and become intermittent early in life. I replaced the resistor and the amp was perfect. How do they install the wrong resistor? and don't they test at the factory? maybe they just tested for output and the resistor had failed later. Bizarre.

Fender Super Champ Reverb

This was also a music store amp, no output. This is a hybrid amp with 1 12AX7 and push-pull 6V6 output and a DSP/opamp signal conditioning preamp. Power supply and bias voltages checked out OK. I shorted out the DSP and the amp worked perfect, actually sounded pretty nice without all that digital stuff dulling the sound. I also noticed that the LEDs which were driven by the DSP were also not responding to the controls (specifically the channel switching). Once I started poking around the DSP section (which has it's own PCB) it was obvious that is was a loose connection. Turns out, the Flash memory feeding the DSP wasn't sitting in the socket right. I pulled it out, tightened it up, DeOxit and restarted (duh, it's a computer). All was fixed.