I modified a Harman Kardon T55C with good results. I did all the Van Alstine Audio Basics mods and packed the base with modeling clay. The arm is low mass so a high compliance cartridge is best, the Grado with a Longhorn mod works very well with HK tables.
The turntable mods will cost you less than $50 and make a big difference in regards to speed stability and imaging.
You can find out more here with photos:
http://www.wghwoodworking.com/harman_kardon/h-k.htmVan Alstine Audio Basics Turntable Mods1983/09 Harman-Kardon T-60 Turntable
Test reports on a very nice H-K turntable.
1984/05 Harman-Kardon T-25 Turntable
Description of the work we do to a H-K Turntable to make it free from hum and AC fields.
1985/06 Harman-Kardon Turntables 1 page
A few comments on improving the performance of the turntables with liquid silicon and the difference between the models.
1985/12 Harman-Kardon T-60 Turntable Rebuild 4 pages
Complete instructions to build your own external power supply for the Harman-Kardon T-60 turntable, thereby isolating your turntable from hum fields and mechanical vibrations.
1986/10 Harman-Kardon T-60 Update .5 pages
An update to the instructions for the H-K T-60 turntable upgrade project detailed in 12/85. The new instructions work with recent variations in the H-K power supply card.
Frank Van Alstine also sells the Grados with the Longhorn mod or you can make one yourself if you are handy (1982 Audio Basics pg 8-9).