520 STFM Floppy Issue

[lilwashu]

Hi

I have recently acquired a couple of 520STFMs and initially both had floppy drive issues - one of them would make a loud spinning noise when a disk was inserted and not read it, and one was moving the head a bit but not spinning the disk.

I swapped the drives between the computers - the drive that was not spinning then worked fine (Sony drive), and the one that made a loud spinning noise did not spin at all (Epson drive).

I seem to have a 520STFM that will not allow its internal floppy drive to spin up. I have tried:

Testing for voltage at FDD power connector - all good (solid 12 and 5V)
Swapping power supplies over just in case - no change
Testing continuity between FDD pins on motherboard and the end of the floppy cable in case of a break - no issue

Either disk drive lights up and the head moves a bit - if I move the head away from the parked position it will move back to its default when power is applied.

Does anyone have any ideas on what the issue could be? Most FDD related issues I have seen appear to be related to the drive itself or the PSU, which doesn't seem to be an issue here, so I guess something on the motherboard is at fault.

[czietz]

Drive apparently being selected (because the LED is lit and the head moves) but not spinning likely points to the MOTOR ON signal. Have a look at the schematic: https://docs.dev-docs.org/htm/search.ph ... 28Rev+A%29. This signal comes from the WD1772 (floppy controller) and is driven by a 7406 IC. In the best case, only the latter IC (or some track or solder joint along the way) is damaged and not the WD1772.

[lilwashu]

Thanks - I've had a probe with my multimeter and I think I am hitting the limits of my rudimentary electronics skills.

I get continuity between pin 20 (motor on) of the WDC1772 and pin 9 of the 7406, which doesn't seem right according to the schematic (it generates a reading to pin 13 also but doesnt beep). I'm a bit lost as to where to go next!

[czietz]

Atari could have swapped the inverters of the 7406 compared to the schematic. To which output pin of the 7406 is the MOTOR ON signal on the floppy drive connected to? Or you could have miscounted the pins on the 1772 by one: in the schematic, pin 21 is connected to pin 9 of the 7406.

EDIT: As soon as you've figured out the correct pins for your STFM, you can use the multimeter to debug it while it is running. I would work backwards: Does the voltage of MOTOR ON change at the floppy drive while trying to access a floppy? (I guess not.) Does it change at the output of the 7406 inverter? Does it change at its input? Does it change at the output (pin 20) of the 1772?

[lilwashu]

OK I have done some more digging and with the help of another thread on here have tested:

Pin 20 - Pin 9 of 7406: OK
Pin 8 of 7406 - L24 inductor: OK
Pin 8 of 7406 - Pin 8 of external FDD connector (motor on): OK
Pin 8 of 7406 - Pin 16 of internal FDD connector (motor on): NOT OK

There is what looks like a suspect trace running next to the FDD cable to near pin 16. Can I run a bodge wire from external FDD pin 8 to internal FDD pin 16?

[PhilC]

@lilwashu post a photo of the dodgy track perhaps?

[lilwashu]

As requested, see pic - there was corrosion hiding under the clip which I have cleaned. Thanks for all the help so far.

[Tomswork]

Did you remove the strain relief clip from the cable

Tom

[lilwashu]

Yes I did. I should probably add that external floppy drives work OK and the other thread I was referencing is here: https://www.exxosforum.co.uk/forum/viewt ... 7&start=10

[Tomswork]

If you have a spare cable I would try it old cables get brittle and flaky.

Tom