Mac IIfx no floppy power on Pin 11 x2

mst3k

New Tinkerer
Aug 17, 2023
6
4
3
Greetings,
I recently acquired a Macintosh IIfx.
Everything works fine, except... the floppy drive(s).
I started simple and switched to a known good floppy. No luck.
I checked the voltages at the power connector and everything looks correct.

Case to pin 1 (yellow): 12v
Case to pins 2, 3, 4, 5, and 6 (red): 5v (+/- 0.01)
Case to pins 7, 8, 9, 10, 11, and 12 (black-ground): basically 0
There is no pin 13
Case to pin 14 (blue): -12v
Case to pin 15 (white): 4.46v (I've been told that 4.44-46 is normal on this pin but I don't know.

I checked the pins on both FDD connectors on the motherboard and everything looks correct, EXCEPT:

Pin 11 shows less than a volt. I believe it is supposed to be 5v.

FDD connector Pins 19, 17, 15, 13 = 12v

I've checked for obvious gunk, broken traces, etc.
I checked the two capacitors on the underside: C196 and C205 and they look OK.

I know just enough to know that I know very little.
Any ideas where I should be looking for my missing voltage on Pin 11?

Thanks
 

mst3k

New Tinkerer
Aug 17, 2023
6
4
3
Well, I found the issue.
It was a broken trace from L13 to Pin 11 on the FDD connector.
Not sure how to solder underneath a FDD connector, but at least I've found the culprit.
Cheers!
 

Volvo242GT

Tinkerer
Feb 7, 2022
296
161
43
Currently Duvall, WA
Firstly, forgot to say, that good to hear that the IIci has been working well for ya.

And, that you finally figured out why your IIfx was not reading floppies. I'd probably run a bodge wire from L13 to the underside of the board. Not ideal, but it'd get the connection fixed.