[G6ONEDave]

I was almost convinced that it was the psu causing a problem but since you not only replaced the STK chip and then swapped the 2 psu's around to no avail, that confirms that the psu was not responsible for the trouble.

Although frustrating that the trouble still exists, there is some progress being made. We now know that the psu is OK, we also know that the reel motor is OK and also the reel motor driver chip is OK.

The intereference spikes are being caused by the reel motor drawing extra current to drive the tape from the supply spool to the take-up spool. This is confirmed by the fact that they don't exist to the same extent with the blank tape. I am a little surprised that the filter caps fitted directly to the reel motor had no effect and this brings me to my next suggestion.

I wonder what would happen if the chassis/earth/0v rail was slightly resistive. It could be worth checking all the 0v lines from the psu to the main pcb, the end sensor, deck terminal pcb and then to the mechanism metalwork. The ohms readings should be 0.1 perhaps up to 0.3 ohms depending on the resistance of the meter leads. Certainly not anything above 1 ohm. You could also try running a wire in parallel with the existing 0v lines. Also remember to check the pcb tracks involved with the 0v lines.

The only other item not swapped in the involved circuits is the syscon chip but let's not go there just yet.

Dave