Tandon TM-100-1 electronics repair

[Ethan Dicks]

Hi, All,

I'm cleaning/refurbing a TRS-80 Model 4 I picked up at VCFe that was
in dirty-but-mostly-working shape.  I've completely dismantled,
cleaned and reassembled the keyboard, I fixed the bad cable to the
floppy controller, I cleaned and lubed the TM-100-1 floppy drive, and
had it all nice and working, then I bumped the power strip it was
plugged into and the momentary surge took out the electronics on the
floppy drive.  I swapped it out with another TM-100-1 unit (borrowed
from a Model 1) and it's booting again, but when using this toasted
drive as :1, I get either an ERROR 3 or ERROR 4 from TRSDOS 1.3 (my
primary testing disk for the moment).  The drive still seeks and spins
but it won't read disks that it used to read before the power hit.

I also have an original NEWDOS/80 disk and a copied MULTIDOS disk.  I
have not yet fixed up a PC with a 40-track 5.25" drive for making
fresh disks, but it's on the list of solution paths.

I have the TM-100 service manual PDF (which includes schematics), so
it shouldn't be difficult to work through the functional subsystems of
the drive electronics.  My question is are there any specific issues
with the parts on the TM-100 PCB to look for?  There are a handful of
reasonably common ICs, and dozens of discrete components.  Of course I
can trace through each section looking for where the results are
unexpected, but for such a common thing as a TM-100, perhaps there are
known pain points and perhaps someone here has repaired a few and
could highlight what parts might be "fragile".

Additionally, for a testing framework to poke signals through the
drive for debugging during the repair, what's a good platform?  A PC
running MS-DOS?  The TRS-80 Model 4 itself?  Besides doing
directories, are there any good bits of software anyone can recommend
for exercising floppy drives on a sub-system-by-subsystem basis?
(move the heads, do a read, do a write...)

I expect like the last repair (shorted tantalum filter cap), this
repair is going to be a small number of components.  Parts of the
drive are known to work - the motor turns on and off when it should,
and it does seek back to track zero when manually moved off of track
zero prior to doing a DIR :1 or when booting it as :0.  At first
glance, something appears to be toasted in the read electronics.

It's not impossible to find another TM-100-1 or replace it with a
TM-100-2 (more common, owing to its appearance in the IBM 5150 PC),
but I'd like to just repair this one and get back to TRS-80 hacking.

Thanks for any tips or pointers.

-ethan