Considering what you indicated in your last post, I suspect it is a dirty, intermittent contact in the source selector rotary switch. The only thing that has been moved is the selector when you changed from your previous input to phono. Those rotary wafer switches are complicated little buggers with lots of contacts. When you lose your right channel playing LPs, try rotating the input selector switch back and forth and see if the right channel returns. That would likely indicate dirty contacts in the selector that would need cleaning, or perhaps a failing solder connection where the rotary switch is soldered to the PC board. Both easy fixes by your serviceman.
Stereo mode in phone stage going out
I have a Pioneer SX-950 that had a near-complete restoration a few months ago. The issue I had taken it in for was the stereo mode when playing vinyl would cut out on the right channel. Thankfully, the mono mode stayed intact. Up until recently, all was well with stereo mode, but last week the same issue popped up again. My question is: would purchasing a separate phono stage correct this issue?