Stereo mode in phone stage going out

Nice full featured receiver. Did you let the shop know the problem is back? They might have had a plan B you are unaware of.

Some diagnosis ideas

You have aux in, as well as two tape in, all could be used as line in, for the output of any Phono stage that is complete including both signal boost and RIAA EQ.

You could prove it is not the cartridge/wires/arm/tt with a simple inexpensive Phono stage. I found one that sounded better than several highly praised phono stages I tried and returned. It’s designed to fit anywhere, and keeps you listening while you search for one that you prefer to it.

Pyle MM Phono EQ, $15

In Stereo Mode, does one channel (always the same channel) drop out randomly, intermittently? Or, Permanent problem again for what input(s)?

You could have a problem in the cartridge, headshell wires, the tonearm’s wires, within the TT, and the mono mode is simply repeating the info from a single channel in both.

Is it ok for a while, then happen as things warm up?

You could have an intermittent bad trace on a printed ckt bd that didn’t act up in the shop, perhaps heat related, one thing to try is a quiet fan to keep the temperature low, see if it only happens when the fan is not running, or pull the unit out into open space where heat cannot build up to the degree it might be doing?

You have two phono inputs, does the problem occur using either?

However: Stereo/Mono switch should affect all inputs, so if it is the switch, an external phono stage would not solve it.

You have external jumpers allowing you to check the pre section separately from the amp section.

Jacks/Cables: to rule it out, while playing in Stereo Mode, gently move both ends of any jacks at the TT and in/out of the receiver, Phono in, the jumpers, a connection.

Your TT cable: captive, prove they are not the problem? Loose, change, problem remains or gone.

Make notes, before/during/after: it's easy to forget what you were sure you would remember.

Given the age of the unit, it’s probably a dirty or faulty switch. Before you buy anything, I would query the guys who restored it for you first. If it’s the switch, that is a very easy and inexpensive fix. I have one question along the lines of Elliot’s post, when you are in mono mode, do you hear signal from both speakers?

Forgot to mention, do you have this problem when switched to any of the high level inputs?  For example, CDP or tuner?  The stereo/mono switch is typically in the linestage circuit, not in the phono stage circuit, in an integrated amp or receiver. By that reasoning, the loss of a stereo signal ought to be happening  when you are listening to any source component, if the switch is at fault. 

Thanks for all the suggestions.

Over the past week, when playing vinyl, usually about the 4th/5th song is when the (always) right channel goes about 98% out in stereo mode. Flipping it to Mono restores the sound in both channels. 

All other programs (CD, Tuner, Streaming device connected via Aux) work fine in stereo mode. Never a problem there.

I have not connected the TT to Phono 2 but will try that.

The TT is a Rega 1, four years old with a 2-year-old Ortofon Blue cart. 

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.

I had a sx1250. It’s the stereo/mono switch. 
you need some deoxit to clean it. Then you should be good. 

I tend to favor porchlight’s suggestion that the fault is in the selector switch, because OP says it doesn’t happen with other inputs. on the other hand, if the stereo/mono switch is only in the phono stage circuit, in that case it could be that stereo mono switch. But I think most of those old receivers and integrated amplifiers would have the stereo mono switch in the line stage circuit.

@nicholsr 

usually about the 4th/5th song is when the (always) right channel goes about 98% out in stereo mode

When you say 98% out, is that 98% of the time or 98% of the volume?

I suspect a loose contact around the base of the tonearm cable.  Could it be that as the tonearm approaches the inner tracks, the tonearm cable is stretched enough to slightly dislodge a connector?  Or maybe a wire in the tonearm is broken, but normally remains in contact - until reaching those inner tracks?

Otherwise, it seems that the fault occurs as the amplifier warms up for a few tracks.  Try starting playback at track four.  What does this do to the onset point?

Simplest test is to switch amplifiers - maybe borrow one from a dealer for a trial listen!

Been a while since I posted here, but having restored a Pioneer SA-8500 II, thought I'd chime in. Some great suggestions so far. 

The thought of a dirty switch is a good suggestion, however, this unit does not have a rotary selector switch - it is all push buttons (pushing in one source pops out the currently selected source). See this pic of the 950 (its a beauty): https://www.classicaudio.com/forsale/pio/SX950.html

Trying out the phono2 input to see if it has the same problem would likely determine if the problem exists on the switch board. All of these switches (rotary and push button) need to be cleaned in a restoration on these older units (and not just by spraying deoxit all over the place - they need to be pulled out, taken apart if possible, crud cleaned off, and the appropriate contact cleaner/enhancer applied).

Also try connecting the turntable RCA leads in reverse (left to right and right to left) and make sure the sound in the right speaker still goes out. If the left channel goes out in that case, then the problem is before the 950 as @elliottbnewcombjr suggested - either the RCA connectors, the cables, or the cartridge leads or the cartridge are the issue. 

If it is not one of those, then it is could be something on the phono stage itself - the phono stage in this unit is very simple and uses six transistors, all of which are known to fail eventually. They have readily available and easy replacements too. This fellow on audiokarma had the same problem and fixed it by replacing those six transistors:

https://audiokarma.org/forums/index.php?threads/pioneer-sx-950-no-right-channel-on-phono.1032652/

If you end up replacing the transistors, might as well replace all the electrolytic capacitors too on the board as well while you are in there.