Technics SP-10 mkII speed adjustment question

Hi,

I'm on my way to complete my Technics SP-10 mkII project. Actually, a friend of mine, a professionnal audio technician, is working to upgrade the PSU, which is done but a small adjustment on the speed must be done and he need some cue on this issue.

We already asked Bill Thalmann, Artisan Fidelity and Oswald Mill audio. Plus, I'll post on DIY Audio today. We'd like to get the answer as quickly as possible to finalized this for the week-end. Hope someone on Audiogon can help.

Here's the message from my technician:

"Hello,

I'm an electronic technician and I do repair for audio equipments, vintage, hifi pro and more. I have a client here that brought me his turntable Technics Sp-10 MKII to fixed. I have a little question about it and he gave me your email because he pretended that you have some experience with this kind of materiel. So, hope that you can response my technical question.

I replaced all capacitors in the power supply and a big solder job. I checked for defect solders or capacitors on the circuit boards inside the turntable and I tied to do the adjustments . Everything seem good right now, the turntable work fine. I tried do do the period adjustment with the VR101 and VR102 potentiometers like in the service manual ( see attachment, Period adjustment method). When I looked the stroboscope at the front of the turntable, It's pretty stable but I can see a tiny rumble at 33 1/2 and 78 speed. 45 is the more stable speed for the stroboscope. So, I fixed the phase reference with T1 at 18us of period and I try to do the period adjustment at the point test T and S on the board with the O point for reference. When I put my scope probe on the T point, I can observe the stroboscope running. It is not stable at all. If I pull off my probe, the stroboscope is stable again. So When I have the 2 probes at point S an T at the same time to do the adjustment, it's impossible to fixed the wave T because it going right to the left on my scope. When I turned the VR101, the T wave going faster or slower but never stable. I tried to ground lift my scope, plug it into the same power bar and try to pull off the reference at the O point. I can't have a setup that I can see a stable T wave in my scope with the one that I can do the right adjustment. Why? Is there a problem with the turntable or maybe it's a incorrect probe or ground setup? Please let me know what you think.

Best regards"

Thanks for help,

Sébastien