68kMLA Classic Interface

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1.4MB drive won't detect HD disks
Posted by: schrockwell on 2026-06-20 07:15:00
Hi folks. I am restoring a 1.4MB SuperDrive, an MP-F75W-01G. It won't detect high-density disks.

After cleaning, re-lubing, and replacing the eject gear, it reads and writes 400K and 800K disks great. However, it treats every disk as double-density.

I immediately suspected SW3, which is what detects HD disks. Sure enough, there's an issue there. Luckily, I have a working drive for comparison.

The high side of SW3 reads 5V, which is correct. On the low side of SW3, when the switch is open, it should be pulled to ground. I measured 0V on the good drive, but 2.4V on the bad drive. Ouch. That would explain it.

Where can I begin debugging this? There are no obvious visual flaws or solder joint cracks. I would suspect either a bad cap or a bad switch, but I don't know where to begin. Thanks in advance.


CleanShot 2026-06-20 at 10.04.46@2x.png


Floppy schematics.jpg
Posted by: schrockwell on 2026-06-20 08:00:51
Some more info. The resistance across SW3 when it’s open is 10k on the good drive but only 2K on the bad one. Now this is in-circuit, so not precise, but could be a clue.

Next step is to pull out the switch and test it.
Posted by: schrockwell on 2026-06-20 08:18:51
So the mechanical switch has been ruled out. Removed from the circuit, it measures straight open/short as expected. And the open pads on the PCB still measure 2K across them.

After putting the switch back, I confirmed that the switch remains open with an HD disk inserted, so it’s not a misalignment or mechanical issue.
Posted by: SpocksBeer on 2026-06-20 14:14:25
Welp I'd be suspecting the transistors either side of that arrangement, marked 5 and 7 in that schematic. One or both might be damaged and switching to ground when it should not.
Posted by: shirsch on 2026-06-21 06:25:53
Agree. One or both of those transistors is shorted or extremely leaky.
Posted by: schrockwell on 2026-07-01 15:00:43
Appreciate the ideas. I lifted both Q5 and Q7, no improvement. Also lifted the switch again to be sure.

That just leaves the pulldown resistor and pin 37 of IC1 (CXD1603Q) connected to that net. And that pin appears to also be connected to pin 48 through the IC, so it might be something on that side.
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