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Next step is the plasti-dip treatment: make the motor turn slowly, and apply coats of plasti-dip every 5 minutes with a flat hobby knife until we reach the required size (around 10.5 mm diameter):
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Adding a coating every 5 minutes gets us this:
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Next step is to put a 1K resistor in parallel with the 200ohm resistor to increase the write current:
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After reinstalling the tape drive the capstan moves the tape, but sadly enough doing an fdf 0 will report an error 43. I also tried rew, but that was a bad plan: the drive did not see BOT and wound the tape off the wheel. Rik had warned me for that and of course he was right 
The unit makes a terrible sound while moving the tape, so clearly something is still amiss…
Time to get the oscilloscope. I put the yellow probe on U27 pin 3 which should be a tacho signal. The blue probe goes on U21 pin1+2 which is the SFD signal (Servo Fail Detect). Switching on then shows the following (the tape is accessed at startup and does its scream):
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That does look like there is no tacho at all. Next step is to check tacho in at U26 which is an 1820-1048, translating to the N8T20N, a monostable multivibrator. Pin 6 (not marked on the schematic but that is the - input) should contain the ATC Analog Tach signal; let’s put that on yellow now:
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That at least shows as something Enlarging that signal shows:
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58kHz!? It does seem a real signal from the wheel because when the motor seems to stop we see it slowing down:
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And it does the same at the start:
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Some further investigation showed that the -5V power was lost… And the 8T20 needs it so we need to fix this 1st.
I did check the PSU’s voltages when I got the machine, but clearly something has happened. Checking the PSU diagram it showed that the -5V is made in a very simple way:
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It’s simply a zener diode with a resistor on the -12V rail (which is present). Measuring the power over the zener with all power connections removed shows 0.2V over it, indicating a defective zener or capacitor. It turned out to be the zener. Replacing it with a 5.1V 3W version fixed the -5V rail.
And lo and behold, the tape unit now reacts more normal and starts to spool properly! Next step: trying to stop it from rewinding the tape off its wheel, sigh.