Floppy Disk radial alignment

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Floppy Disk radial alignment

Postby RickNel on Tue Nov 02, 2010 9:24 am

I'm following up here on the post in Tezza's blog about Philip Avery's method for fixing FDD alignment without a proprietary alignment disk.

I tried to add my comment to the blog, but the form claimed I was trying illegally to post "more than 3000 characters" - a slander, but maybe this intelligent site detected that I am an undercover Aussie. BTW there is no Australian site like this, as far as I know...

The blog post was very timely for me (thanks Tezz and Philip). I have a pair of these Tandon TM100-2 5.25" floppies in my S-100 machine and one had drifted off alignment over a long period.

I started following it through, and have now got my second drive nicely aligned, but with some deviations. Let me suggest a couple of extra points.

First - use ImageDisk for the control and testing - it's free and very versatile. I don't have a TRS80 so thought I might have to write some code to goose that read signal for the scope. Then I had another look at the Alignment module that is part of Dave Dunfield's excellent ImageDisk free utility (DOS platform - reads, makes images and writes to any floppy format incl CP/M). Easy to Google it.

Hooking the Tandon up to a PC, I used the ImageDisk align utility to quickly find out that my problem was not just alignment, but one of the heads not firing at all. I took the controller board off the drive, and noticed that the diskette had a buckle in it where it passed between the heads. The plastic frame that sits on top of the aluminium chassis may have shrunk a bit over 30 years, or I may have pressed it back into position too firmly after cleaning out the drive, or maybe a combination of both. Anyway, with these drives it is worth checking that the disk slot is exactly at the right level for the fixed head. Problem fixed by levering one or two of the plastic holding posts about 1mm out of its snug hole in the aluminium chassis. The test for correct position is easy - the disk slides in without pushing, shoving or twisting!

I had hooked up my scope as instructed, but it turned out that ImageDisk provides an easier check on your console screen. You can navigate the heads to any cylinder/track and read one head at a time for data. It just keeps reading the same track till you tell it to do something else. Once I had adjusted backwards and forwards till I could read data consistently on both heads at track 20, I just stepped through all the tracks, both sides, and made finer adjustments to ensure they were all reading correctly.

Then ImageDisk can do test reads and writes for any track. I'd recommend confirming outermost, innermost and centre tracks at least, before you are satisfied and start tightening up the screws again. Keep the drive in the test rig and keep testing while you gently tighten the screws one at a time - the tightening can be enough to nudge it a fraction off alignment.

The scope test will give a more fundamental hardware spec alignment, but read and write were enough for me!

I also recommend going the extra step of taking the controller board off the drive for the adjustment process. It is only held by two screws then a slide and a wiggle. Unplug all the cables first.

Carefully unlace the two sets of head cables, that come from the front of the drive, from their cradles in the plastic frame. You can then re-connect the board standing on its end at the back of the drive, loose from the chassis. Then run your tests and do your adjustments with all the mechanics of the drive in full view, and with most electronics out of the way of screwdrivers, crowbars, chainsaws or other tools you may be using. Just be careful that there is some slack in the head cables while the board is in that position, so the cables do not put any drag on the head carriage and inhibit its movement.

I found it much easier to control the fine adjustment of that pretty crude cam screw with the board out of the way.

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Re: Floppy Disk radial alignment

Postby tezza on Tue Nov 02, 2010 10:23 pm

Hi Rick,

Thanks for posting this. This seems like an extremely useful modification of the technique! I'll link this post to the Main blog article as a postscript! I do have imagedisk. Next time I have a shonky drive I'll give it a go.

Welcome to the forums BTW!
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