Fender Twin Amp Has No Reverb

[g1]

Alternate theory: there was a bad or intermittent connection at the ribbon cable to the reverb assign switch (shaky solder joint or broken wire). When you were scraping the flux from the ribbon cable the pressure applied "fixed" the bad connection. The problem may or may not come back again. I would suggest resoldering the ribbon connector and assign switch and make sure none of the wires in the ribbon cable are broken.