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Some further investigation showed that the -5V power was lost… So before I can continue - let’s fix that…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.