Vintage St4 In Excellent Condition Needing Answers?, Claibration info Options

[tubeman]

I just recently acquired an ST4 in near perfect shape along with its accompanied EV 915 Crystal Mic. The unit has never been touched with a soldering gun and all the tubes are original to the unit and test NOS on my Hickok 752. So, I put her on the variac last nite and let her run up all nite long. She seems to be quite accurate as is. I was wondering if anyone could tell me what that securing trimpot mounted to the chassis just under the motor would be? When I calibrate the tuner, the indicator on the first line will stay in the middle, but does not appear clearly. Its more llike a burry blob that you bring to a rest. Is this normal?

What does that secured pot on the inside of the unit do?

I have experience with tube amps and high voltages, so I'm not worried about any of that.

Thanks in advance.

I tuned my guitar to standard 440 and play along to some Pink Floyd and the tuning is rock solid!! Seems like I have a good unit here and wanna see if I can get her to display a bit clearer. I will not touch that pot until someone tells me what it does.

Steve