Test Flight, part 2

After reading lots and lots of stuff about amp repair and troubleshooting, I was fairly certain the issue of volume was due to lack of gain. Either there was too much negative feedback killing my signal, or one of the gain stages wasn't properly connected, or possibly one of the components was shorted/defective.

First, I got the multimeter out and checked continuity on every single connection of the amp. Everything was fine. Next, I began testing all the resistors. If a resister was defective or shorted out/fried, I would be able to see that using the resistance meter (ohms). Most of them checked out almost exactly what their values were supposed to be, which is quite amazing considering most manufacturing tolerances allow for up to +/- 20%. There was one section of the board that I kept coming back to, however. Something just didn't seem right about it. The resistor wasn't measuring correct. I assumed the component was bad, so I removed the original one and replaced it with a fresh one. I tested the amp again, and no change.

One of the FAQ pages I was researching was talking about a feedback wire Fender usually installs on the vibrato tube. It goes to a 2200 ohm resistor and then ground. I didn't remember seeing one, so I examined the schematic for it. It wasn't there, but there was a 2700 ohm resistor. The very same one I had just replaced. Then I saw my embarrassing mistake: On the schematic/layout, there was a dotted line connecting that resistor to one of the coupling capacitors. Usually, a dotted line refers to a wire that is under the circuit board. I checked my board, and I had indeed forgotten that wire! How I missed it during one of the triple and quadruple checks I do not know, but nonetheless I had missed it. 

The dotted line, the one that completes the "Y" shape (and looks remarkably like a Flux Capacitor, thank you Doc Brown!), represents the connection I accidentally omitted.

With all the jumpers already soldered in place, it made no sense to disconnect the turret board and solder this missing wire to the bottom. I simply connected a bare wire between the two turrets.

Although it's not the prettiest soldering job I've ever done, it should hold. My main concern was just to get a wire in there so that I could test the amp again.

With that connection finally made, I plugged everything in and tested it again. Right away I could tell it was fixed. The hum is a little more noticeable now, but still not overly bad at all. My giant 100 watt Fender Quad Reverb is a pretty quiet amp (at idle), and it has more hum than this. I played a few notes with a guitar and WOW! I had lots of volume. It is now every bit as loud as the Vibro Champ I emulated. In fact, there might be a little too much, as now if I turn it all the way up I can hear some unpleasant distortion, probably from the speaker overloading. It is like a harsh clippy sound, not a smooth, desirable overdriven sound. As this test speaker isn't what I plan on keeping in the finished cabinet, I'll just deal with that later.

I tested the tremolo again, and while the sound no longer cuts out completely when you turn up the depth knob, it now gradually dampens the overall volume. Still no tremolo effect.

At least I solved the riddle of the missing gain. I'll do some more research and try to get the tremolo fixed soon.