Floppy drive track-0 issues

Tony Duell ard at p850ug1.demon.co.uk
Sun Jan 10 07:05:57 CST 2010

> Hi guys,
> I've got a Citizen Z1DE-55A (3.5" 1.44MB PC) floppy drive hooked up to 
> the disc analyser (it seems my Sony MPF520 has a duff head; it's reading 
> garbage for one side of the disc). Here's the problem...

It may not be the head, but it shouldn't he hard to check...

Firstly, clean the heads -- not with a cleaning disk (no matter what the 
manuals may say), but with a cotton bud and propan-2-ol. I once spent an 
afternoon chasing a non-existant fault on the analogue board of a CBM 
2850LP that turned out to be drity heads.

Secondly, most dosk heads have 4 connections -- if there are actually 5 
wires on the connector, then the last is most likely a screen (do any of 
the pins on the drive pCB connector test as grounds? If so, it's almost 
certainly a screen). The 4 connections are the 3 wires of a centre-tapped 
reand/write winding and one end of the tunnel erase head, the other side 
of that goes ot the centre tap. 

Just occasionally you come acrooss other arrangements. One is to have the 
erase head totally flating wrt the read-write head, so 5 wires. Another 
is to include some or all of the switching diodes in the head assembly. 
Look for SOT23 pacakges on the flexiprint to the heads. Sony once made a 
double-sided head assembly with 6 wires and with the all the switching 
diodes on the flexiprint, but they went away from tlat on more modern drives.

You cna test the head windings for continmuity quite easily. If you're 
worried you may have magnetised the head by so doing, you can demagnetise 
it using a tape head demagnetiser. The head windings are low-resistance 
and will essentially show as a short-circuit between all 4 head connections.

If the head checks out, trace form the head connector on the PCB to 
switching  diodes, often doube diode units in SOT23 packages. Check them, 
replace if necessary. The centre tap pin most likely foes to a 
transistor, part of the head swtiching circuit, I've had that fail before 
now. Again, easy to check and replace.

That Sony model number sounds somewhat familiar to me. Is it a normal-size
(half-height, techniclly 3.5" unit with a logic board iwth 2 large PQFPs
on it (one for the analouge circuitry and one for the digital circuitry)?
And a positiion at the rear right corner of the main chassis that could be
used to mount an eject motor unit, like a Mac drive? If so, grab the
HP9114B (must be B) diagrams from http://www.hpmuseum.net, IIRC that uses
a similar drive. 

> - When the drive is selected, irrespective of head position, TRK00 is 
> always inactive.
> - If the drive is given a seek command, it will seek to track 0, assert 

Those 2 seem inconsistent. The first says that TRK00 is never asserted, 
the latter gices a time when it is. Which is it?

> TRK00, but the INDEX and READ DATA outputs will go idle (float high).
> - The drive remains stuck in this state until power-cycled.
> Has anyone seen this kind of thing before? The MPF520 had a head-step 
> lockout (if track0_sensor == active, then ignore STEP commands) but 
> didn't crash out if you asked it to step.

I've seen floppy drives that do odd things if, for example, one of the 
input signals is asserted on power-up. But never this. I assume the 
control logic is one ASIC :-(, so you can't investigate it.

What signals are you asserting when you try the seek? You don't have 
write-gate asserted or anything silly, do you?

Silly suggestion. Connect it to your PC and do some seeks. Use a logic 
analyser to see what the signals to the drive are doing. Then see how the 
signals from your excersiser differ.

> For what it's worth, the drive works fine in my PC... I just can't see 
> why it's locking up like this.
> Also, am I right in thinking that pins 12 (DS1) and 16 (MOTEN) are the 
> two I need to pull low to select the drive, if it's connected before the 
> twist in the cable? And that DS0 and DS2 would be the select and motor 
> enable pins (respectively) if the drive was located after the twist?

Yes, that sounds right.


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