I feel that I must give a word of caution here.
On the forum here we see many people with no knowledge or experience of electronic things, who will make a statement like:
"I have a Super Wizzo ZYZ123 which makes a funny noise. What component can I change to fix it?"
In the full expectation that from the vague description of the faults, the electronic gods here will know instantly what is wrong and can identify the single component which needs to be changed.
Unfortunately real world electronics is often (mostly) not like that.
Some will jump in and say "it is the electrolytics, change them", I believe that I half way went there myself:
As others have suggested, suspect the power supplies, especially the electrolytic capacitors.
The thing is, when faultfinding, the first thing is to physically examine the thing, look for bad solder joints. We learn with experience that components which get hot, or are large are the ones where bad solder joints are to be found.
(Components which are large and hot, well, nine times out of ten, there is your bad joint!)
Having done that, then consider the electrolytics, the bigger they are the more likely they are to be a problem. The will dry out, spill out their insides and become nothing more than a high value resistor.
These faults are obvious.
But caution!
Earlier this year a friend of mine called around and gave me a 40 year old spectrum analyser, sadly it was not working.
Following my faultfinding logic as given above it was obvious that many of the big electrolytic capacitors in the power supply were in a bad way, their end seals were split and had leaked electrolyte many years ago.
However measuring the supply voltages and checking the ripple voltages with an oscilloscope, showed that the power supplies were well within specification. So there was no point in splashing out a fistful of fivers to replace these caps when it was possible that the whole thing was scrap due to a whole load of other faulty components, many of which could be made from unobtainium.
As it turned out, I was able to get the analyser going quite nicely after much head scratching, and I did eventually replace all the electrolytics in the power supply, but did that improve anything?
No!
I guess the moral of this rambling tale is:
When fixing something, do the following, in this order:
1 Physical inspection
2 Check the power supplies
3 Hard core electronics faultfinding
If you skimp on steps 1 and 2, then step 3 is a waste of time.
And often step 3 is the only way to fix it.
JimB