Two SE 1/20 do not accept floppies

[GeekDot]

Hey Gang, so here's a IMHO tough nut to crack...

Just got two SE 1/20. Cleaned them etc. and both boot fine off their screaming original 20MB HDDs.

What puzzles me is that both(!) refrain to accept every floppy-disk being fed to them... either instantly ejecting or offering to format them. Yes, the disks work fine on other Macs.

Trying to format a fresh 1.4M or 800k disk results in happy grinding but ends with an Error.

So out went the original MP-F75W-01G, complete clean'n'grease, positively checked with another Mac and back into the SE. No change.

Ok, got a MP-F75W-12G from 'the stack(tm)'... no different.

So these are the ROMs:

...and the PCB printing:

Any ideas? As I can't add any software to them, I'm not able to check the ROM id if it's actually an FDHD one (ROM ID: $037A)...

 

[Bolle]

Check if the resistor network next to the floppy port is ok on your boards. Also make sure you have the correct cable for 1.4MB drives. Not sure if this is a thing on the SE but there were Macs that needed the one with the yellow stripe for 800k drives and the one with the red stripe for 1.4MB drives. The drives would not work if used with the wrong cable.

Those ROMs are FDHD and the floppy controller in that picture is a SWIM as well.

 

[GeekDot]

Check if the resistor network next to the floppy port is ok on your boards.

Do you mean the filters M/C 8908L 115-0002 (yellow DIPs)....  these seem to be compatible with https://www.bourns.com/pdfs/601.pdf, still hard to find, and no idea how the check the internal R-C-R network of each pin-pair...

The floppy cables are "red-stripes".

 

[Bolle]

Apple did not always install filters with R-C-R networks. Some boards have simple resistor packs instead of the filter networks.

Start by checking the series resistance of each pin pair. If you desolder the filter you can also measure the capacitance for each pin. The actual value will be off though because the resistors. You could do some maths to calculate the actual value if your multimeter comes with a proper datasheet. I usually only check if no capacitor is shorted out and if the capacitance is in the same range for all of them.

 

[Bolle]

edit: -got confused-