Get the SERVICE MANUAL for it. There are Transistors on it that cause the problems you have and other problems. These are MUST REPLACE before they crap out and take the unit with them. REPLACE the 2sc1451's in the Main Amps (all 4 of them). And any 2sa725's and 2sa726's on any boards that have them. If you come across other pioneer's one of the 1st things to do is look for these and replace them.
2sa725/2sa726 = KSA992
2sc1451 = KSC3503.
NOTE and I cannot stress this enough. The pin layout on the NEW transistors is different than the originals. Download the DATA SHEETS for the new ones, and compare the pinout with the board and the old transistors. Adjust pins to fit in the CORRECT holes without touching.
Read this thread
http://www.audiokarma.org/forums/showthread.php?t=43186 pay particular attention to POST 11.
Once you've got the transistors done, rebuild the power supply (caps, transistors,) Test resistors with one side lifted. If out of spec replace.
with it being 40years old, I'd rebuild the amps, protection circuit, tone control board, and equalizer board (phono preamp). Mainly recap, but the troubleprone transistors get priority. No parts lists that I know of. So you'll have to make your own. Stick with a known good distributor like Mouser, Digikey. Use Good quality electrolytics, no need for Muse or Silmic II's. They won't make it a 1250/1280 in SQ. There may be small (<4.7uf/50v) SKY BLUE Sanyo caps in it. these are KLUDGE and have a tendency to short into resistors (capsistors). Get rid of them and install 50V panasonic ECQ-w stacked films. Never need to change again.
Mark the FIXER's "Rule of Thumb" regarding caps in PIONEERS'
Cap rules of thumb (nichicon)
I use upw, uhe for general caps including signal coupling in that order depending upon stock "cea"
I use ues, uep & uvp in that order for bipolar depending on stock
I use ukl for low leakage caps (in sensitive areas they reduce the noise floor)"ceanl", "cssa", "csza"
0.1uf to 2 uf I use stacked film where they fit: DFS and ECQ
solid aluminum and tantalum caps get stacked film when possible, or ukl's otherwise. "cssa" "csza"
c?B means it's an axial cap = one lead on each end
c?a = radial = both leads on one end
Download the
PIONEER "TUNING FORK" Service Notice's from this sticky thread Bunch of good stuff that explains the caps designations etc.
Service manual, schematics, and some general technical bulletins
here