How does the upper deck sound, do you notice any weird background noise in either play direction?
When I first brought the upper deck out of it's coma it did all kinds of weird things. Played muffled, often only on one channel, hummed and made crackling noises through the speakers while playing. The humming and background noise would get worse or better simply by touching any of the front knobs.
Long story short. Muffled issue was solved by adjusting the azimuth (which is simple once you know to use the holes through the circuit boards to the adjustment screw) but the humming and static I isolated to the selector switch circuitry that selects which pickups in the head to use. The switch was dirty internally and it was causing a ground issue and extra noise.
Not wanting to take the switch apart and remove soldered wires I opted to blast the heck out of it with tuner cleaner. Flipped the switch back and forth 20 times and whalla. Humming gone and channels work the way the are supposed to work on playback (both sides).
I have noticed on both my C100's that the upper deck will always have more static hiss in the background due to this pickup design. By it's very nature increases noise levels but it gets insane when the contacts get dirty.
Azimuth adjustment cleaned up any reverse track noises and crispness, deoxit on the pickup head switch repaired the humming. The top deck plays perfectly. Crisp highs and deep lows. I was amazed. Hope this helps