I got this from the GeoFX website.
Intermittent problems are some of the most frustrating ones to solve. The amp works fine until - something - happens, and then it acts up. Most of the effort in debugging this one is to make it act up so you can cause it to happen when you want, which will then in turn let you find the problem, and verify that you have really fixed it. Intermittents always mean that something is just at the very edge of failing and it takes the causing event to push it over. When the causing event or condition is not present, it works fine, or maybe will "reset" when the amp is turned off or cooled.
Notice carefully what makes the intermittent happen, if you can. Very common events that institute intermittent problems are:
Mechanical vibration - it only happen when it's banged or shaken by being on top of speakers
Heat (thermal stress) - something only edges over into failure when it gets hot
Voltage stress, perhaps combined with heat
This one is probably only going to yield to the laundry list technique, so here goes:
Failure only happens after a longish time of playing: this is most often thermal, as something fails when it gets hot enough. Good places to look:
Tube develops a problem when it gets really hot
Bad solder joint opens up when it gets hot enough
Resistor or capacitor goes bad when it's hot - these usually show signs of overheating to a visual inspection. Power tube screen resistors are a common place this happens.
Mechanically damaged part opens up under thermal stress. Broken resistor bodies can be held together by the lead's springiness and only open when they get hot. Capacitor leads may have the same problem, as can soldered wires and joints.
Failure only happens when the amp is sitting on top of speakers
Bad solder joint or broken part
Failure only happens when the amp is cold
Bad solder joint or broken part, or badly drifted resistor value.
Failure only happens when the amp is taken off standby: Since the B+ voltage rises in standby, this often means that the higher voltage is preaking something over. This may take the form of the amp only coming on slowly after a delay when the switch is thrown, or of a squeal or pop after the switch is thrown, or ugly sounding distortion for a while until it "gets better"
Preamp decoupling capacitors
Signal coupling capacitors
Dirty, contaminated, or arcing tube sockets.
Amp stutters or cuts out when driven really hard: overdrive is causing the output tubes to go into grid blocking after being over driven; this is caused by the signal causing a temporary bias shift.
Depending on how desperate you are, you might want to apply a shotgun technique: Methodically remelt every single solder joint in the amp, adding in a bit of rosin core solder as you do. A milder form of this would only remelt the ones in the circuits you suspect. This sounds horrible, but really doesn't take all that long. The worst part of doing this is that you never really find which one caused it, just that the problem quits.
I have a friend who repairs amps for a living and who said he once fixed an intermittent problem on a Fender where the amp was cutting out intermittently. He found that the wires to the output tube sockets had been put in the correct places but never soldered!
