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.