Hi Ben, Do you have an oscilloscope, it's the only way to adjust/check any player. You have to be able to "see" the signal coming off the disc to correctly set up. You say someone has twiddled the pot's, that could be a problem and I think you need access to the service manual. Depending on the machine there may be adjustments for focus and tracking gain, EF balance and PLL lock. As a generalisation you can try this. If it will play at all, if you turn the focus gain up you may hear "white noise" coming from the optical pickup itself, adjust up to the point that the noise is apparant and then back off slightly. Tracking gain set initially to around mid point. EF balance is very critical, you certainly need scope for this. Try moving slightly each way and see if playability improves. PLL or phase locked loop again very critical, you need the set up procedure to adjust correctly and a frequency counter. You can set it to the midway between the points at which the player "loses it" assuming it is a pot for this adjustment (sometimes a coil, if it is don't touch). If you have a 'scope you may be suprised to see that the RF signal or eye patern coming off the disc always looks the same, it does not matter what the content of the disc is, the only exception is the "gap" between tracks tracks. Good luck
Regards Karl