Turntable speed slow with quartz lock

If it's like i.e. the Micro DD decks, the built-in strobe is sync'd to the mains frequency, whereas the internal oscillator is not. So if it's by a very small amount, and you're using a mains-frequency strobe . . . It could be the mains that's ever so slightly off, not the turntable.

But if it's way off, then probably the TT is indeed at fault. Crystals do indeed drift with age, and sometimes surrounding circuit components can affect their accuracy as well.

First, try cleaning the speed selector switch with Radio Shack electrical contact cleaner. If you have variable speed controls, clean them too. I would not do anything else until you have done this- this is a very common problem.

Fixed it! Replaced all electrolytic capacitors in speed control circuit myself. Now runs dead on balls steady. BTW, the table ran slow with quartz lock on or off, so it wasn't just the quartz circuit. I suspected C117 in the speed circuit, but replaced all the electrolytics in that circuit (C1XX) while I was at it.