Hum when initially turning on Stereo

[danswan]

I Have my PC linked (via stereo minijack -> RCA) to my old (approx 25 years) technics stereo. It used to have 4 component parts but I've disconnected the CD and tape deck, leaving just the amp part & tuner.

It's always worked pretty well and the sound is actually really good. Just recently, though, when I initially turn on I'm greeted with a very noticeable hum. After around 5 mins it's faded down to nothing.

It's not a connection, I've tried with nothing connected at all (except speakers) and it still happens.

Is this likely to be something that will get worse, and should I be looking for a replacement? Other than this hum, and the concern that it will get worse, I've got no other complaints

 

[lucid]

The fact that the problem goes away once the amp has been on a while suggests something improves when the amp gets warm. Capacitors have this trait.

When cold, any intermittent faulty capacitor is more likely to display symptoms. Once warm though, the cap' will operate as normal until the fault becomes more pronounced.

Hum with nothing connected but the speakers would rule out ground loops; so we're back to capacitors.

Theres also the age factor. Capacitors dry out as they age. As they do they become more 'leaky. That means in some circuits where they're being used to block AC mains from getting through that they become less effective in that function. AC present in the DC power amp side of the circuits will present as hum. The medium-term prognosis is that will get worse. Longer-term, the amp will fail.

If you know what you're doing then changing the caps isn't difficult, but it can be very time consuming in not only sourcing decent components at the right types and values but also ensuring that they're the correct physical size.

A simpler solution is to change the amp.

 

[danswan]

The fact that the problem goes away once the amp has been on a while suggests something improves when the amp gets warm. Capacitors have this trait.

When cold, any intermittent faulty capacitor is more likely to display symptoms. Once warm though, the cap' will operate as normal until the fault becomes more pronounced.

Hum with nothing connected but the speakers would rule out ground loops; so we're back to capacitors.

Theres also the age factor. Capacitors dry out as they age. As they do they become more 'leaky. That means in some circuits where they're being used to block AC mains from getting through that they become less effective in that function. AC present in the DC power amp side of the circuits will present as hum. The medium-term prognosis is that will get worse. Longer-term, the amp will fail.

If you know what you're doing then changing the caps isn't difficult, but it can be very time consuming in not only sourcing decent components at the right types and values but also ensuring that they're the correct physical size.

A simpler solution is to change the amp.

Thanks for the prognosis, nice to get some understanding. Time to start saving for a replacement I guess.