I had the same symptom, and it turned out to be in my Power Amp (Marantz 140, ca. 1977). Yup, at low volumes, after the system ran for maybe 30 minutes, the right channel would start to drop out; working the preamp's volume control would put it right for maybe 20 minutes. Naturally I thought it was the preamp--tried pot cleaner, no difference. Finally I tried another (new) preamp--and the same thing happened. I even tried a third preamp (for other reasons) and still experienced the symptom. Lately I haven't noticed the symptom, possibly because I keep the power amp turned on all the time.
Marantz 2500 Volume Control - Help!
MY MARANTZ 2500'S VOLUME CONTROL OCCASIONALLY GETS QUIET ON THE LEFT CHANNEL ONLY . ROTATING IT A FEW TIMES RESTORES THE SOUND . WILL A SPRAY , CLEAN AND LUBE , CLEAR UP THE PROBLEM OR IS THIS A SIGN THAT THE VOLUME CONTROL WILL NEED TO BE REPLACED FROM AN OXIDIZED WINDING ?I APPRECIATE THIS SITE AND THE VAST KNOWLEDGE OF IT'S MEMBERS . THANK YOU .