I am a long time ST-70 user. It has been awhile since I have done any work on my unit other than changing tubes. Last week the left channel started making a crackling noise. Changing preamps didn't remove the noise. The 7199 on that side has always been sensitive. I pulled it and replaced it with a spare. The noise got worse. I removed the original PC-3 and replaced it with a 6GH8A board that I have had since the 90's. The unit was quiet for ten minutes, so I turned on the preamp. The crackling noise came back and stayed even after I shut off the preamp. I switched the tubes left to right and the noise stayed in the left channel.
Does anyone have any thoughts? Am I looking at a quad filter cap, power supply or what?