Check the resistor pack thats connected across the reel opto inputs on the board.
You will get reel A fault with no reels connected ! with nothing plugged in all reels will appear to be jammed.
If all the reels are spinning normally and your getting reel A fault, look in the back of the machine and see if you can see the reel opto lamps ( early cabs actually had incandescant bulbs for the light source in the opto sensors) lighting up. Sometimes they can die. If a reel is shuddering or not moving its more likely to be a drive transistor or one of the ICs buffering the reels (74LS04s, ICs 12 to 15)
if the reels are running, and you can confirm that its not the reel backboard at fault as it has a bunch of resistors on board that can fail, then you are looking at replacing T55 , and then checking IC17, the 74ls138 controlling the opto sensor select, is working.
if thats working then you look for pulses on pins 10 -14 of IC5, the 6821 thats interfacing the optos to the cpu. One of the pins will always be stuck as no reel connected to it ( 4 reel machine) but 4 of them should slow pulse as the reels go through the optos. Every time the hole in the reel passes the opto sensor it should pulse ( I think it goes LO to HI briefly but that could be the other way around, to put simply you should see 4 pins pulse when 4 reels moving , if 2 are stuck then thats your problem!)
Also change the smoothing caps on the regulator board because bad caps can cause reel jitter and cause the reel to be in the wrong position when the CPU looks for the optos.