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Lew
A crude but effective test for RFI which is perhaps worth trying.
If you have a portable AM radio. Tune it off station and move it around your room listening for increased static. Maybe something there that is not at first apparent.

How many solder joints are there to check Banquo?

About a million of them, Halcro :).

Here are some quotes from my tech to perhaps narrow things down.

The PCB is two sided but without plated through holes. It looks like they put eyelets in and then solder on both sides. I found one that was open but by the time I pulled the PCB it was connecting. I resoldered it and now the 45 light comes on and that speed works. The 33 light never comes on so I think there is something wrong with that flip flop....

Doing just the few that looked suspect did not work (it did work but only for a day and then the gremlins reappeared), so after trying several other types of fixes, he ended up doing them all:

There were no bad parts, just bad connections. I ended up resoldering all the feed thru eyelets, then flipping the board and removing the solder with a sucker, then resoldering them with much less solder. They all look pretty clean now. I know other companies that used the eyelet method had reliability problems. For some reason the solder cracks around the eyelets, probably related to different coefficients of thermal expansion.

So, look for the joints that utilize 'eyelets' (I'm not sure whether that's a subset of all the joints or all of them). I think it took him a couple of hours to do them all, so not too bad.

Dear Richard, If anything, the tt itself generates RFI internally. I think Halcro noticed some related issues when he ran his "naked", without the metal casing. In fact, I think we all concluded that one function of that casing is to shield other components from RFI. But how could extraneous RFI account for my problem? (By the way, I do use the AM radio as an RFI detector. Moreover, my TT101 malfunctions exactly the same in our kitchen on the first floor as it does in the basement, which I should think is a much more inherently protected environment, because it's all below ground.) I don't say you cannot be correct; I just wonder why and how. Your further thoughts are welcome.

Lew
I know that it is a leap but, we have experienced problems with servo feedback systems in our, hydraulic industry.
This where shielding has been inadequate. ( yes I thought of Halcro's birthday suit) while simultaneously being exposed to RFI.
A famous one was where the Cats hydraulic lifted stage in London would move all by itself. Problem was traced to poor shielding letting in transmissions from the ubiquitous London cabs outside.

When the obvious causes are exhausted we need to explore the path less travelled.

Good luck