Power filter capacitors fail by shorting or opening.

lytic caps can go leaky, this can cause hum. But usually they go high ESR or sometimes the capacity falls, either of which causes hum, and in either case piggybacking them would make the hum go away. So its not certain but most likely the problem lies elsewhere than those caps, since piggybacking them did nothing.

Hum is more likely to come from a bad signal ground connection than bad caps. You've some some real troubleshooting to do.

NT

They can short and they can also develop high series resistance.

They can even explode, spray goop everywhere, and disintegrate, at which point their resistance is a bit moot.

Electrolytic capacitors usually fail by (1) drying or chemical change of the electrolyte that gives the capacitor most of its characteristics, resulting in loss of capacitance and/or excessive series resistance. Sometimes, this is due to age or non-use (eqpt being stored for many years without being used). High leakage current through the capacitor is common. (2) exposure to excessive heat, whether external or internally generated. Internally generated heat is usually the result of the capacitor developing excessive series resistance. The current through the capacitor's series resistance generates heat, and will cause rapid deterioration of the electrolyte and failure of the capacitor. (3) Exposure to excessive voltage, beyond the rated voltage of the capacitor. The result of extended exposure to higher than rated voltage is usually punch-through of the electrolyte, resulting in catastrophic failure of the capacitor (a short).

If the capacitor is used as a power supply filter, and the capacitor develops a short, then the most likely result will be overheating of the power transformer. If not caught in time, the transformer will be destroyed, usually accompanied by a large volume of pungent smoke. If the equipment has a power line fuse, the transformer may be saved. A failed rectifier can absolutely cause a capacitor to fail. Since the capacitor should be exposed only to DC voltage, if there is an AC source connected (caused by a failed junction in the rectifier) it will fail in short order. Violent explosion is possible in this case. If the capacitor fails with high series resistance, the bad effects will be more subtle. As indicated in your post, excessive 120 Hz hum might result. Sometimes the capacitor develops high leakage current (1). Bridging a good capacitor across the bad one can sometimes reduce the hum. There are other things that can cause hum in audio equipment. Open shields in the signal path can certainly allow induced hum to be amplified. Poor connections of circuit components can allow hum to pass. Poor grounding of components can cause hum. Missing cover plates or other chassis components can allow hum to be picked up.

If this is a tube amplifier that you're asking about, then it's not uncommon for one of the tubes to develop a heater-to-cathode short. That will allow the AC filament voltage to be superimposed on the signal. A tube tester can easily identify defective tubes. Find a friend who has a tube tester and test all the tubes. Alternatively, substituting the tubes with known good tubes can quickly identify the bad tube.

Cheers!!!!

-- Dave M MasonDG44 at comcast dot net (Just substitute the appropriate characters in the address)

Some days you're the dog, some days the hydrant.

Thanks to all and thanks to you Dave for the detailed answer.

This is a background project, and new things came to the fore today, so it's going to take me several days at least to do all the testing I see I need to do, and to read this thread thorougly as well.

So I plan to get back to you all in a few days.

Thank you.

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