Humming through the speakers or does the amp hum?
I'm guessing the amp by your description, which would have nothing to do with the power supply caps. The only thing inside the amp that would hum is the transformer but why when the power is turned off unless it has some standby circuits that always get power? Most amps switch off the power before the transformer which would make it impossible to hum.
Transformers can hum for several reasons:
1. they are overloaded, doubtful when it is off
2. the mounting screws or screws that hold it together are loose
3. it sits on a rubber pad that has collapsed or dried out
4. the laminations in the core have separated,
5. torroidals can hum if there is noise on the line like from light dimmers
You can check 2 and 3 yourself. Tighten all screws on and around it. Be sure it is unplugged and has been sitting for a while before you take the covers off. If it is 4 you are out of luck unless it is because the bolts holding it together are loose.
I'm guessing the amp by your description, which would have nothing to do with the power supply caps. The only thing inside the amp that would hum is the transformer but why when the power is turned off unless it has some standby circuits that always get power? Most amps switch off the power before the transformer which would make it impossible to hum.
Transformers can hum for several reasons:
1. they are overloaded, doubtful when it is off
2. the mounting screws or screws that hold it together are loose
3. it sits on a rubber pad that has collapsed or dried out
4. the laminations in the core have separated,
5. torroidals can hum if there is noise on the line like from light dimmers
You can check 2 and 3 yourself. Tighten all screws on and around it. Be sure it is unplugged and has been sitting for a while before you take the covers off. If it is 4 you are out of luck unless it is because the bolts holding it together are loose.