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This was accompanied by a lot of “clacking” sounds, probably the heads being lowered on the disk?
Second test
The second test was even less successful: the drive kept repeating there was an AC LOW FATAL ERROR. Apparently something broke… This seems to be done by this part of the circuit on the M7744 board:
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The 8881 is an equivalent of the 7439, and is open collector. I measured the signal on pin 4 of E7, and that remained at 0V, i.e. this circuit registered the AC failure, indeed. I also checked the 10V input, and that was fine.
This circuit should work as follows:
If the 10V is missing then Q2 will not conduct; its emitter is at 0v through the zener, and the base is near 0v because the 10V is missing. Because of that Q1 will also not conduct and thus not pull the exit of E7 high, i.e. the signal will remain low.
If the 10V is present (at least high enough) the emitter of Q2 will be around 3.3V through the zener. The base of Q2 should be slightly higher than 5V through the resistor ladder R22,R23 causing Q2 to conduct. This pulls the base of Q1 to below 5V, causing it to conduct, thereby pulling the DRV AC L line high.
Next step is to measure the voltages around Q2:
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Yellow = E7 Pin 4 (DRV AC L), light blue=collector Q2, purple=emitter, dark blue=base Q2
We see that the base is indeed > 5V, but collector and emitter of Q2 are both way above 3.3V. This indicates that the 3.3V zener has given up. After replacing the zener we have a proper signal when the test runs:
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Light blue is the signal on pin 6 E7, yellow is the signal on pin 4.
After this, and after messing with some floppies I got this:
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It looks like an entire pass tested OK 
During the test I could see the head moving; first track by track, after that reading random tracks… This part seems to be OK
Running the test with both drives filled with a properly filled floppy shows:
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