I've got a Hot Rod Deville 4x10 that's being a real pain. The clean always sounds great but every once and a while the drive channels will emit a loud shreik as if being sevierly overdriven. I initially found a 100v cap with it's top blown out and replaced it. That seemed to be the fix, for about three hours, then the shriek returned. The cap I replaced seems fine and all voltages according to the fender schematic are correct. The problem has gone away again for now but I would like to truly solve the problem. Any help would be greatly appreciated! Also, no footswitch in use
Music - Fender - Hot Rod Deluxe Guitar Combo Amplifier
Answers & Comments
It would help to know which cap you replaced. I see a lot of relays in the unit that are swtched "dry" that should a contact be oxidized might cause the problem. Such intermittents are hard to find the root cause. If the replaced cap happened to be one on the +/-48 volt supply, best to check that you have DC across it with not very much ripple using an oscilloscope. Even brand new caps can be bad... and those of course are polarity sensitive... hope you got them in right if those are the ones.
If C39 blew, that would indicate that CR11 might have shorted so verify you have the expected 48 volts. across C39. Another possibility was a power surge that pushed the 48 volts up to or exceeded the rating of C39. This could also have damaged R78 which is undoubtedly wirewound type... maybe in ceramic sand-coated case. Wirewounds CAN become intermittent so check this one out... Next check CR13 Zener... this really could cause a problem... You should find 16 volts across it and relatively noise free. C40 is also suspect for having LOW capacitance because it is being run outside of ratings for GOOD design practice (voltages across electrolytics should be in range of 50% to 80% of the marked rating)
When run at voltage below this they sometimes deform the dielectric and lose their capacitance value.