I got it to work for about 30 seconds at normal volume and then it would drop off without even being touched. It did this repeatedly.
This is what old capacitors can do. Same thing my Super Reverb did when I first got it. I replaced every cap in it because I knew they were almost 40 years old and probably dried out, it started working right when I replaced the last couple of caps. I replaced everything but the ceramic disk caps. I was replacing the "orange drop" capacitors when it started to work right, so this is not limited to electrolytics.
If this unit is very old, try replacing every capacitor in it if possible, especially electrolytics. Electrolytics dry out after a few years, (usually 20 years or so actually) and can't work properly. Some of the other types can grow weak after a few years, especially if the unit sits unused for a while.
Replacements must be as close as possible to the came microfarad value, voltage can be higher but not lower. (A 10 volt 1uF cap must be 1uF but a 16V could be used as a replacement. A 6v would not work.) If the uF value is not possible get as close as you can. This is common, many older values have been changed. 47uF, for example is no longer made, 50uF is the common replacement. The voltage rating is the maximum voltage that cap can handle, not a mandatory voltage value. A 16 volt cap will work great on a 12 volt circuit. But a 6 volt would not handle it. Same for higher voltages. modern 600 volt caps will work fine as replacements for older 450 volt caps. The 450 volt versions are no longer available.
Avoid NOS caps. (New Old Stock) after 30 years or longer they are probably dried out or weak.
I think the reason caps cause the volume drop is the way they work. Caps act similar to batteries, storing voltage until needed by the circuit. If the voltage is not there, the volume drops. When first turned on, it might work fine but after warming up, it no longer works, maybe due to heat expansion making it lose contact. It could also be that the heat causes chemical changes in the dry electrolyte.
Same thing was happening to my Wurlitzer electric piano, same cure. Replaced all the caps. It only lasted a few seconds while the amp would work for 10 minutes or so.
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The volume dropped way off like it has a bad connection. I cleaned the jacks and that did nothing. When I opened it up to check it out I discovered that a previous owner had attemped to reflow a few solder joints. I got it to work for about 30 seconds at normal volume and then it would drop off without even being touched. It did this repeatedly. The previous owner obviously had the same issue and then dumped it on GC for a credit. It must have a bad component. I'll take it back Monday and have them put my credit towards a new one.
