Scoutmaster runs very slow

Things to check, would be the Platter first. Have you ever re-lubed the Platter Bearing? Removing Belt, and rotating, can you feel any drag?

Belt Slippage. When was the last time you replaced the Belt? Try cleaning the Belt, and Platter.

Try plugging directly to wall outlet, and measure speed again, still running slow?

66hz is quite a bit to try compensating for any anomalies, with either Platter-Bearing-Belt-etc. I'm running my SDS with VPI MK-IV Platter at 60.17hz, and achieve dead on speed, which I can look at the KAB Strobe for countless minutes on end, and cannot detect even the slightest of drift one way, or another.

One other possibility, is your Ramp Down Voltage is too low. I have my SDS set to Ramp Down to 72 vac with no problems, but it is possible your motor, or something else is not permitting it to work at such a low voltage.

You can go into the Calibration Mode, the instructions are on VPI's website on how to accomplish this, and you can select a higher Ramp Down Voltage, and see how this works? If no success, I think a call, or email to Mike at VPI is in order. Hope this helps, Mark

It sounds like your motor has a problem. Can you swing by your VPI dealer and have it looked at?

Which motor do you have? If it's the standard 300 rpm motor that fits into the cutout section of the Scoutmaster plinth, make sure it is about 1/4 inch (check manual to be sure) from the plinth. If it already is that far, try moving it out just a tad more. If that still doesn't help, have VPI send you a new belt. How old is your current belt? Also, make sure the belt loops around the center of the pulley. If I remember correctly, the pulley has a couple of varying diameters to allow speed adjustment.
I found that, during the summer months, my SDS was set to 59.85 to achieve solid 33.3 rpm. During the winter, I had to dial it almost to 61 because my rig is in a room that gets cold (lots of windows).
If none of this works, I would suspect the motor itself.

Hope this helps.

Tom