That's the track 0 switch. One pin pair is for an LED, the other is a phototransistor. Top 2 pins and bottom 2 pins are paired, usually the LED is on top. DONT remove the switch, it requires precise alignment

maxtherabbit wrote on 2020-04-15, 12:42:

That's the track 0 switch. One pin pair is for an LED, the other is a phototransistor. Top 2 pins and bottom 2 pins are paired, usually the LED is on top. DONT remove the switch, it requires precise alignment

Hi

Thank you. So this identifies the sensor pinout, but I also need to know to which leads of the broken-off flat cable right next to it should be connected, do you know? I just noticed that the flat cable has only three leads, maybe one is common ground?

Well eventually I think I will go hit-and-miss approach to see if any combination works, I cannot really make it any worse....

I have no idea. Try to find the service manual for that model drive online. If anything the common is probably the +5.

maxtherabbit wrote on 2020-04-15, 20:44:

I have no idea. Try to find the service manual for that model drive online. If anything the common is probably the +5.

Hi,

Well it occured to me that I can try and find any picture of this particular drive model and check if I can see the cabling and which lead goes where from it. So I actually found this article and figured it out. I add simple diagram to one of the photos

So I removed the flat cable connector from the main PCB, soldered in some wiring directly to the trough-pins, and this is my result

It appears to be a success, the drive now resets to Track 0 and seeks on PC startup. Seems to work normally.
I just now have to get some 5.25" floppy disk to try if it actually reads and writes, and I do not have any

nice clean soldering job. you didn't unscrew the sensor did you?

maxtherabbit wrote on 2020-04-17, 15:00:

nice clean soldering job. you didn't unscrew the sensor did you?

I did, unfortunately. It was impossible to get to it. You can see from the picture that the sensor is held in place by a screw, and a blob of paint/adhesive was added to maybe secure it in place (?).
In the picture, you can see the crack in the pain, some of the blob came off the metal with the sensor. I reckon I got it back in same position with maybe ±0,1 mm tolerance, how accurate does it need to be anyways....

Well we will definately see what happens once I get my hands on some disks...

1/10 mm is probably acceptable, any further would be problematic