I replaced the caps in my Pioneer from 1976, but there was a lot of distortion at less than half volume and it sounded like crap. I had flogged the amp hard for years before that and some of the speakers were difficult loads for it. I started out by cleaning all the pots. This got rid of the static, but not the distortion. There was also in incredible lack of bass. I also found a delaminated spot in the board, bypassed that with wire point to point, and the distortion was still there. The distortion had it sounding like an old cheap transistor radio on AM and it was weak overall. Once I changed the caps, including the large power supply ones, it sounded very nice. The rail voltages were also off and would not hold their value past one hot/cold cycle.
The beginning. The one thing that made this easier for a novice electronics guy is the old unit was designed to be serviced as it is comprised as specific/obvious subassemblies, which also makes troubleshooting easier. Also, all the main wiring used wire wrap instead of solder. I purchased wire wrap tools and the correct wire for that process. I also have a service manual for this unit, and I took plenty of before photos.
The power light shows blue but it is actually orange. The stock one had burned out many years ago. Here I was setting the bias etc. Everything was looking good.
Success. It sounded wonderful again. Tone controls now had a much more obvious effect comparatively as well and the distortion is gone. I pushed it and let it rip for hours, trying out different speakers and sources and then put it into play for about 6 months.
I had been to electronics school and have a minor degree in it. While I have forgotten a lot of it, I could recall the basics enough to know where and what info to look for, and I have a complete soldering setup/tools, heat sink tools and what not and test equipment, including oscilloscope. I also built a dim-bulb tester and have built amp kits, crossover boards and repaired/restored electric motors for a number of years as a day job so this was not completely foreign to me.
For anyone else that has to ask in the first place, I recommend working on broken things that are no good to you anyway with nothing to lose by learning on it. First and foremost, learn the safety protocols and at least have a knack for learning and fixing things. My boss is an EE. If I needed to, I could have had him help me in the event something didn't work out. Thankfully, I didn't have to ask him.
I'm not to be restoring any classics from that era anymore. I have multiple eras of amps/electronics from the '70s on up. The one rebuild is their last chance. I use them sparingly. Truth be told, the modern stuff sounds as good, or better, with the biggest improvement being the absence of analog pots. I have class A, A/B and class D, the latter of which is the quietest at idle and I like how cool they run, which ends up being a better environment for the actual electronic components overall.
The 20 yr old Denon AVR I am currently using sounds worlds better than the Pioneer did at 20 years, being it's pots had already been acting up well before that point. Older gear is nice to look at, but from a performance/reliability factor it's not practical for 'my' performance needs. I now appreciate amps that are stable down to 4, or even 2Ohm loads and with very little noise and being able to hook up any speakers I have up to the same amp.
People give up on modern gear way sooner, IME, with all the upgraditus going on so there is plenty of it left on the used market with a lot of use left in it. The hoarders and price gougers haven't really moved into that arena yet so there are IMO, the best deals to be had there, with much less risk.
I don't like buying previously restored gear unless I know who restored it and how, and with all the gouging going on from there, they can stick that where the sun doesn't shine. Even "factory authorized" is to be suspect without a warranty and the ability to back it up.