After spending a whole bunch of time on getting this deck to work properly and testing it (see other topic), and fixing various other things, I finally put the box back together. After checking everything, I finally went to give it a last test run and...deck's malfunctioning all of a sudden!
Symptoms are: In play mode, the heads move up but immediately go back down. It's not auto-stop. That kicks in 2 seconds later by moving the solenoid. In fwd mode, without a tape, the take up reel will spin but it's struggling. The brakes are engaged (took me a while to figure that one out). In rwd mode, without a tape, same thing, except other direction. It didn't originally do this, I had to loosen up the reel motor a bit with a separate voltage supply. I think this is irrelevant though, just a slightly tired reel motor.
So the one thing they have in common is that in neither mode, the brakes ever disengage. I can disengage them manually by pushing down the spring-loaded brake assembly. I also suspect that the heads not staying engaged might be due to the fact that the assembly that moves up is supposed to be caught by a moving plastic pin on the left side. I can manually push the assembly up and push this plastic pin in place, and then it will stay up.
Sensor-wise everything seems to work as expected. Hall sensor, the optical sensor with the disc, solenoid is operated fine I think (does the expected reset at startup without a tape).
Anyone had anything like this before? Looks like some mechanical linkage problem somewhere in the harder to access guts that I've so far managed to avoid. This deck's really giving me grief.. I lack the ability to just let go though!