Tandon TM-100-1 electronics repair

Tony Duell ard.p850ug1 at gmail.com
Thu Apr 27 12:58:38 CDT 2017

On Thu, Apr 27, 2017 at 6:42 PM, Ethan Dicks via cctalk
<cctalk at classiccmp.org> wrote:
> 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".

Floppy drives of that vintage consist of a number of almost independant
systems (there's a common enable/drive select but that's about it).
It looks like a problem in the read chain, but is it? If the spindle was
turning at the wrong speed I think it would fail to read.

So I would check each subsection of the drive, just in case it's not
the obvious one.

In the case of the read amplifier, you could start with a differential-
input 'scope on the pins used for head alignment (reading a normal
disk). If there's a good signal there the head select diodes (AFAIK
these are present even on the single-sided drive) and first amplifier
IC are OK. If not, then debug that part.

I don't know of any obvious common failures, and anyway
murphy's law will ensure that your drive has something odd failed :-)
Stock fault lists are useful if you have many indentical units to repair,
you might get 90% of them off the bench quickly but fairly useless for
one-off jobs.

> 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...)

There are/were drive exercisers but no idea where you'd find one now.
If you were nearer to me, you could use mine....

More seriously, the signals involved are fairly slow speed. Before
I had a drive exerciser, I linked the important drive inputs (select,
motor on, step, direction, etc) to a PC parallel port and wrote a
short program (I think in Turbo Pascal, but it was a long time
ago) to turn the motor on, step the heads around, etc. I think
I monitored some of the drive outputs using the printer port input
lines too, but a logic probe is all you really need to check index,
track 0, etc.

You need a 'scope to debug the read amplifier of course (that
applies even if you have a drive exerciser).

Do check the spindle speed. Often there is a strobe disk on the
spindle pulley in these drives (if it's missing, I am sure you can
get an image of it somewhere, print it, cut it out and stick it on the
pulley). But also check ('scope) that you are getting 5 index pulses
per second, just in case the darn thing is running at half speed or


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