Tandon TM-100-1 electronics repair

[Ethan Dicks]

On Thu, Apr 27, 2017 at 1:58 PM, Tony Duell <ard.p850ug1 at gmail.com> wrote:
> On Thu, Apr 27, 2017 at 6:42 PM, Ethan Dicks via cctalk
>> (diagnosing and repairing a) TM-100-1 floppy drive...

> 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.

Yes.  I did consider the spindle speed, but I'm reasonably certain it's good.

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

Well, yes.  I'm really just asking about known weaknesses as a starting point.

> 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.

Right.

> I don't know of any obvious common failures, and anyway
> murphy's law will ensure that your drive has something odd failed :-)

Perhaps.  The last failure with this unit had the most obvious cause.
I literally tested one component and it was exactly that part that had
failed.

>> Additionally, for a testing framework...

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

Right.  I don't have a drive exerciser (and I live in the States) so I
didn't figure that was an easily achievable path.  I'm figuring on
having to use a live machine to make the drive dance and I don't fancy
writing my own code on top of it all.  Rather than go to that extent,
I'm more likely to toss the drive on the "to be fixed someday" pile
and get to it (much) later.

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

Yes.  Of course I have a scope.  At least one.

> Do check the spindle speed. Often there is a strobe disk on the
> spindle pulley in these drives

There is.  It works.

> 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
> something.

Worth verifying, but there's a visual difference between 300 RPM and
150 RPM (plus they sound different).

-ethan