11 June 2014

1964 Fender Blackface Super Reverb Repair

The latest bum amp in my workshop was a Fender Blackface Super Reverb, ca. 1964, which is quite possibly the heaviest amp I have ever carried. The Normal channel was fine, but the Vibrato channel had an intermittent volume drop, just enough to be noticeable.
Fig. 1. The Beast.

Fig. 2. This is why The Beast weighs 70 to 80 pounds.

A visual inspection revealed a missing solder joint on the bias capacitor (see Fig. 3 below), which was not the cause of the problem, but certainly worth soldering.

Fig. 3. A conspicuous absence of solder.
I didn't see anything else untoward, so I did the chopstick test (with the amp on, poking each component with a chopstick). Still nothing. New tubes did not solve the problem either, so I did another chopstick test to no avail. I recorded all the voltages, but nothing jumped out as weird.

I started testing continuity among all the components and I found a cold solder joint on the board going to pin 1 on V5. I resoldered it, and the problem was gone. I took voltage readings again, and THEN the first measurement looked weird, even though it didn't jump out at me at the time. I guess I was too busy not electrocuting myself to notice the first time.

Both problems appear to have been the result of a recent recap job (I didn't do it) that was mostly good, but with a couple of iffy solder joints.