Dating fender champ 12

24-Nov-2014 22:51 by 9 Comments

Dating fender champ 12 - updating action replay

Hi Jeff, I have a Fender Champ 12 from the late ’80s that I’m curious about.The input jack is broken, and it has never been re-tubed or probably even serviced since it was new.

I primarily use it as a practice amp at home, typically at lower volumes, and I’d love to get tones similar to a Fender Tonemaster. Thanks, Leonard Hi Leonard, When a customer brought his Champ 12 in for service years ago, a couple of this amp’s unique “innovations” had me scratching my head.There are aspects of the electronic design I’ve not seen in any other Fender amp.The Champ 12 I repaired had multiple problems: Its output was low and distorted, and it wouldn’t switch channels.Before pulling a schematic on the unit, I took a few minutes to troubleshoot it.Sure enough, the output looked very bad on the oscilloscope, and there was no low voltage for the switching circuitry.When I started tracing the wiring associated with the switching circuitry, it unexpectedly led me to the output stage!

I figured this can’t be right—someone must have already gotten their inexperienced, grubby little paws into the amp and moved some wires. The amp had been designed to tap voltage off of the cathode circuit in the cathode-biased output stage as a low-voltage source for the switching circuitry. I checked the resistors in the cathode circuit, and then replaced one that was way out of tolerance.

That was it—the switching circuitry now had voltage and the output of the amp was great.

Okay, I won’t forget that the next time a Champ 12 comes in with the same set of problems.

But let’s move on to your amp and see if we can address some of your concerns.

The first thing I’d suggest is to re-tube the amp, especially if that has never been done before. Also, be sure to have the cathode resistors in the output stage checked, as we already know what kinds of problems they can cause.

I’d also recommend having the broken input jack replaced.