I have one very corroded M90 deck that I'm attempting to bring back to fully working condition, but it has logic board issues that first need attention before I dig into the mechanical side of it.
After carefully documenting and bagging parts over nearly 20 steps I've got the board separated from the mecha. One IC was removed to clean up some severe corrosion and dirt, but thankfully the PCB is largely intact, which is good because it's got plated holes, printed traces and about 60 printed resistors. The conformal coating mostly held up with only R710 visually compromised.
Only rewind, fast forward, stop and play do anything at all, and rewind only works when the heads are up (i.e. reed switch is closed). Stop will engage the solenoid (and stay engaged) even if you haven't pressed play yet. Actually the solenoid turns on and stays on with all four of my working buttons, but I've yet to figure out if this is normal, because FR704 gets too hot to touch if it's engaged for more than even a couple of seconds. The solenoid does not engage when you turn on the unit, but does click on shutoff.
So far I've done a bit of testing on the control chip to try and understand if there is an issue in this portion of the circuit. I'm currently focusing on the reel motor, since I haven't seen it spin yet, even though FF and REW buttons are "working."
The deck has extreme corrosion in some areas, the pinch roller is totally shot. However many steel parts are nearly intact. You can see how interfaces are corroded, like moisture got stuck in the crevices. Nothing so far gone as to be unusable. A previous owner or tech was in here and replaced the belts, stripped a screw and lost a couple, but nothing damaged.