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.
Turntable speed slow with quartz lock
I have an 30 year old Harman Kardon T-60 turntable that runs slow (according to strobe) while in quartz lock mode. I would chalk it up to a bad unit except my friend's unit does exactly the same thing. The coincidence is too great for me to ignore. Any ideas on how 2 units that use a quartz referenced oscillator could both run slow. I have recapped the power supply on mine with no effect.