Hello Owen, the symptoms you've mentioned are common for amps from the 70s and 80s, that are still in use today. Most electronics, hi-fi equipment included, generously use electrolytic capacitors as part of their circuits. These generally need to be replaced over a period of time, as the liquid electrolytic inside them, dries up and hardens (at times, becomes like powder) over time, thereby reducing capacitance. High value electrolytic capacitor banks are generally used in the power supply section for filtering power supply ripple and if these fail, the ripple gets through in the form of 50 Hz hum. The first symptom of failing electrolytic capacitors is the amplifier will play well when started cold and after warming up, will start acting funny. Looks like the filter electrolytic caps in your amp, are on the way down.
What you will have to do is very simple.
Step-1, open up the amp and look for physical damage of electrolytic capacitors - either bloated (a slit plastic outer cover is a good indication) or leaking or check whether the PCB area under the capacitor features some corrosion. These are capacitors that need to be replaced anyways.
Step-2, try to get hold of a capacitance meter and check the values of the other electrolytic capacitors. This will be easier if you have a service manual of the amplifier so you can refer to the capacitor values. If not, you can read them off the print on the capacitor itself. The value that you read on the meter should be close to the one printed on the capacitor, the maximum tolerance you'd probably afford is a plus or minus 1% or 2%. However I would say its better to replace all those who do not match the listed values.
The high value capacitors in the power supply section need to be handled with care as you may end up getting a jolt from electric discharge. Let your amp cool down for about 30 minutes, before putting your hands into it
Also while replacing electrolytic capacitors, ensure that you get the polarity right, reversing the polarity can lead to some nasty consequences. Not a bad idea to take a lot of good pictures of the component layout before attempting to do anything.
And finally, don't discard your hi-fi equipment due to capacitor troubles. The guy who takes them on will get them replaced and put your amp back into service anyways. Why not let that happen while you continue to own and enjoy your hi-fi components. Replacing electrolytic capacitors is a very easy task and not that expensive too. It will take an evening's work, that's all.
Note: if you are not comfortable with DIY, please ask someone who has testing and soldering skills to help out. Also, it is better to replace electrolytic caps that show slight variance as they will go down, sooner or later. The sound quality of our equipment can also be bettered with a new set of caps. However with new caps, you will have to give your amp about 40-60 hours of burn-in time for things to settle down.
