Intermittent channel in TT setup - ideas?

Sorry for the long windedness - usually I can figure issues out on my own, but I believe this one is truly "weird".

For the past year, I've experienced the strangest channel cutting-out issue in my vinyl setup. It's now been observed with different phono stages, tubes, and cartridges. Indeed, last time it happened I immediately swapped cartridges and it remained constant - so that's not it. I'm *fairly* sure the dead channel follows the phono cable L/R swap, though I need to verify this next time - that would seem to eliminate downstream gear (i.e. phono stage, preamp, amp).

Here's the symptoms: while listening to music (and it always happens WHILE music is playing), 1 channel will suddenly "cut out" - actually there's still sound coming out that speaker, though perhaps 40-70dB down from the signal level. This must be reflecting the crosstalk limitations of the phono stage, since it kind of sounds like a "ghost" image of the good channel. The problem lasts a few to several minutes while I fiddle with reseating connections, flapping my arms, etc. Signal ALWAYS comes back, though not always on my preferred timetable (i.e. immediately). Once "fixed", I've never had it reoccur in the same listening session. It occurs perhaps once every 2 to 5 listening sessions (~2 hours a piece). Always occurs nearer to the start of a session.

I've suspected the DIN connection from my phono cable, since the plug doesn't fit well into my tonearm's barrel (ugh, this tonearm even has odd square-ish pins). I think that is a more likely culprit than the headshell connection.

Here's the curveball - last night it occurred, and the phono DIN re-seating didn't do the trick. I've always MUTE'd my linestage during the process, but not this time. When I spin up my SOTA turntable (w/ old vacuum pump supply) with mute OFF, it will induce a moderately loud POP (plus a few softer clicks) in my system. So I've always kept mute ON until after the needle's dropped. Since the system wasn't muted here, I heard it - and after that loud POP, I noticed that the "dead" channel suddenly came up in its noise floor to a normal level (typical tube white noise from a high gain system) - as if it came to life. And of course, soon as I dropped the needle both channels were back to full glory. I'd never noticed the noise floor thing before. In retrospect, I think there may have been times with the channel seemed to cut out in response to a "pop" or "thump" sound from the record.

So before this event, I was pretty sure the DIN connection was the culprit. But I'm not sure whether this new information supports that argument. How would an intermitted phono cable connection affect the noise floor - shouldn't that all be fully determined by the chain AFTER the phono input? How would a "pop"-type signal affect a poor connection? Any ideas on what could explain this behavior, or ideas on the issue in general?

Fortunately it's not been a significant factor in my enjoyment of music. Still, I've love to get it figured out!
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Yes, that's all pretty vexing.

An intermittent phono cable connection could conceivably affect the noise floor in several ways, although one of those ways would affect it in the opposite direction from what you observed.

First, if the connection opens up, the low impedance of the cartridge coil would no longer be connected across the phono stage input. Under normal conditions, the low impedance of the coil would tend to "short out" noise that would otherwise be generated or introduced at that point, and be amplified by everything downstream. That is the opposite of what you observed, though.

Second, if the cable has an intermittent short, when the short is present it would tend to kill noise as well as signal at the phono stage input.

Third, if the cable has an intermittent open, noise that may be picked up by the cabling "ahead" of the point where the problem exists would not make its way into the phono stage when the open is present.

Given that you are not yet 100% certain that the problem follows a channel swap at the phono stage input, though, I would also be suspicious of the possibility that a mute circuit in a downstream component may have an intermittent problem. If so, given that you've already changed phono stages, the line stage would seem to be the likely culprit.

As to why a "pop" type signal might have affected a marginal connection, I suppose the answer would have to be mechanical vibration resulting from the sound coming from the one speaker that was functioning at the time.

Also, I see that your turntable is located such that heat from your tube equipment will raise its surrounding temperature significantly. Perhaps that is why the problem only appears near the start of a listening session. Eventually the temperature rise fixes the marginal connection, or whatever else is causing the problem.

Finally, it may be helpful to use a multimeter to perform checks for continuity between the rca plug at the phono stage end of the cable and the corresponding headshell leads, and also for the possibility of a short between the signal pin and the ground sleeve of the rca plug, while jiggling various points on the cabling. DO NOT, however, use a multimeter while the headshell leads are connected to the cartridge, as you don't want the test voltage supplied by the meter to be applied to the cartridge.

Hope that helps,
-- Al
Thanks Al - good ideas there; this is indeed helpful.
I hadn't considered the line stage (especially its muting circuit, which is relay'd via remote), but now that certainly seems at least as suspect as the phono cable, if not more. Vinyl has been my sole source in this system for over a year, so I wouldn't have had a chance to eliminate the phono links of the chain. My speakers have even been loaned out to other systems - with no issues there - so at least the transducers at both ends should be in the clear (definitely prefer it that way). Certainly, my first thoughts a year ago were "oh no - cartridge failing :(".

If indeed it is either linestage/phono cable, I've received superb support from both manufacturers on past occasions, so I'd be well covered. Actually, thinking back - I had that linestage factory upgraded around a year ago. I think I'll go lookup the dates...shipping 60 lbs. of tube gear via UPS is always risky.

Next time the issue occurs, I'll gather as much new information as possible - *especially* the L/R phono swap. I knew about keeping meters away from connected cartridges, but that's a very very good warning to repeat :)

Thanks again - very thoughtful post, as always.