[DMcMahon]

That is a good diagnostic. The driving torque in Play mode is fairly minimal just enough to collect/wind the driven tape onto the Take Up spool, so normally should not upset the playback speed.

Suggest you remove the top plate and check the brakes on both reel tables are releasing correctly and then check that both tables can be manually fairly easily rotated in both directions when Play, FFD & RWD selected. For RWD (Rewind) the Supply table rotation will be somewhat stiffer as it then will be driving against the see-saw drive and correspondingly for FFD (Fast Forward) the Take Up table rotation will be somewhat stiffer as it then will be driving against the see-saw drive.

If nothing obvious found then suggest you check things out to the service manual, like the clutch set-ups. Sounds OK from your results above but double check that the tape is making good contact with the capstan shaft in Play mode, the pinch wheel is not loose and the heads are really clean.

David