1.4MB drive won't detect HD disks

[schrockwell]

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.

 

[schrockwell]

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.

 

[schrockwell]

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.

 

[SpocksBeer]

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.