VPI SDS speed issue

Input line voltage/frequency fluctuations to the SDS should not change the output frequency - that's the whole point of the unit, it synthesizes a stable output frequency independent of the line.

When I was experimenting with using dental floss as a belt material, belt slippage caused all sorts of problems. As slippage got worse I had to keep increasing the frequency to get proper speed.
Try some replacement belts and it may cure your issues.

Regarding SDS output voltage. While the factory setting after startup is 72V, I actually run my SDS at 115V all the time (set per the procedure in the manual). I find this sounds punchier in my setup - but I use the original heavy TNT lead/acrylic platter with the single motor/flywheel, which is not a stock setup.

Tobes,
If I'm reading your post correctly, I'm actually confused on this issue. My understanding is that the SDS allows you to adjust the current frequency so as to obtain a correct platter speed, but what that "correct" frequency is is itself dependent upon the stability and frequency of the power supply in your wall. Since the frequency in your wall current fluctuates, you might have to adjust the output frequency on the SDS to get the platter spin dead on from time to time. I agree that if your wall current is relatively stable, once you find at what frequency your turntable reaches the most accurate speed, you might not have to adjust the SDS often, but since the current does fluctuate in most homes, occasional adjustments might be necessary. In my experience, I get the platter on my Classic dead on for 33.3 at 59.95 Hz, but have gone over 60 Hz a few times.

If the SDS knew precisely what frequency to feed to the table based on the current going into it, it seems there would be no need to have the function to adjust it on the unit.

The SDS is not a power line filter it’s an AC power regenerator (like a mini PS Power Plant). The SDS first converts the incoming AC to pure DC voltage and then digitally regenerates its own clean AC signal.

The SDS is designed to work with the AC synchronous motors. The speed of a synchronous motor is determined by the frequency it is fed. Logically, a device whose speed is based on the AC line frequency will function better when a stable consistent frequency is delivered.

Right, but this still doesn't answer my question regarding Tobes' post. Does the SDS need to be adjusted based on the fluctuations in the wall current frequency? I believe yes as otherwise there would be no need for the frequency adjustment on the unit. Tobes' first paragraph seems to suggest otherwise.

As far as I know, the SDS's ability to adjust line frequency in Hertz is not due to instabilities in line frequency coming from the wall, as this is usually very stable, but rather to fine tune deficiencies in platter, or pulley diameter, or other anomalies in the table's drive system itself.

Which brings me to this suggestion for the original poster.

First, find out what your ramp down voltage is set to. If 72V, this may perhaps be too low a setting for your particular table, and drive system.

That using Idler Pulleys, ultra heavy Platters, etc, 72V may not be sufficient voltage. You may need to select a slightly higher voltage?

Just a thought, and worth trying first, before condemning the unit as faulty. Mark