Tandon TM-100-1 electronics repair

Ethan Dicks ethan.dicks at gmail.com
Fri Apr 28 12:01:35 CDT 2017

On Thu, Apr 27, 2017 at 11:17 PM, Mike Loewen via cctalk
<cctalk at classiccmp.org> wrote:
> On Thu, 27 Apr 2017, Ethan Dicks via cctalk wrote:
>> Additionally, for a testing framework to poke signals through the
>> drive for debugging during the repair...
>    You should check out a couple of disk utility programs for the Model 4:
> Super Utility 4/4P
> Hyperzap 3.2C

Thanks for the tip!

>    If you hook up a working drive to boot from, one of those should help to
> exercise/exorcise the bad drive.  :-)

Right.  I have one perfectly working mech and one that's got read problems.

I did locate the other boxes of TRS-80 floppies I was searching for (I
put them away, on a bottom shelf.  Doh!)  In there was a set of
(original) Dysan test floppies and an original Floppy Doctor disc
(instructions at

I learned a couple of things last night - On the working drive, the
speed (which looked OK on the strobe disc) was clocking in from
199.8-201.2 ms per index.  The other drive is around 198.9-199.8, a
bit slow but not much.  The not-working drive passes the "A" test
(write protect, status bits and index strobe), but fails the "B" test
claiming there is no data received from the drive.  I did swap the
boards between the two mechs.  The problems stay with the suspect
mech, and the good mech works with either boardset.  Since the index
detector and the motors are good in the bad drive, I'm left with the
hypothesis that the head has failed.  Moving a scope upstairs will
likely be highly informative.

So that's where I'm at - the drive was working before the power was
jiggled and now it's not and it doesn't appear to be the electronics.

Thanks for all the responses.


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