Continuing my taking-of-a-break from guitar projects, I have this 1978 Fender Vibro Champ on the bench. Isn't it cute? I've wanted one of these for a long time, but I refuse to pay the crazy asking prices. I managed to get this one at a reasonable price - it needs a small bit of work but it will be easy for Amplifier Boy to get it back to snuff.
The intensity pot is a rare bird - it's a 25K reverse linear taper. This is the only Fender amp that used that value and taper pot. (Most other Fenders use a 50K reverse linear here). I'm pretty sure Mouser sells a modern replacement, but I have a plan to resurrect the old pot.
Wish me luck.
The amp does work, but it's down on volume and punch. I'm going to do the usual service - replace the electrolytics, and fix the pots. I also may play around with a mod or two.
First, take off the upper back panel. Then unscrew the four machine screws that hold the chassis in place from the top. They also attach to a chrome chassis strap.
I didn't show this part, but you'll want to put your free hand under the chassis where the screw is - there is a nut there that will drop off as the screw is undone.
The amp chassis will remain seated in the cabinet - there are blocks that hold it in place.
If your straps are corroded or rusty, you can clean them up - or get exact replacements from Fender.
How brilliant is that? Did I say Mr. Fender was a genius? We take this for granted today, but this was a new idea in the early 60s.
This process is the same for any brown, black, or silver Fender amp made from about 1963 to the early 1980s. And, of course, modern reissues.
Be aware that with the big amps (Super, Twin, etc.) the chassis are quite wide, the transformers are huge and the chassis weights a lot. As the furniture movers say, "lift with your legs!"
What I don't like to see, however, is the sloppy wiring Fenders of this period are notorious for. This isn't too bad, since the amp is so simple. I've worked on other 70's amps that were awful. The sloppy wiring affects tone and can cause oscillation, hum and other issues. Some folks will rewire these amps to clean them up.
Turns out three of the four tubes - the preamp tube, the tremolo tube (aka 'vibrato') and the rectifier are all original! Very unusual. The preamp and the trem are Fender-labelled 7025s. Pretty rare to see them.
Next up, we'll change the filter and cathode bypass caps.