8 inch floppy head pad adjustment

Chris Zach cz at alembic.crystel.com
Fri Oct 9 19:17:30 CDT 2020

Thanks Jeff! Yep, one of the odd things about 8 inch floppies is they 
are always spinning (at least on RX01/02 drives) and thus the disc is 
always turning in the sleeve, and if the head is engaged it will wear down.

What they do is have a head on one side, and on the other is the 
solenoid with the shoe. Mine was way too low, so the head was basically 
always pressed into the disc. It's better now, and doesn't seem to be 
doing damage.

Sort of. One thing I am noticing is a lot of these disks came from the 
old Solarex plant. They have a lot of "data" that is probably test 
results from the wafer cutting systems and I'm debating keeping it. More 
to the point most of the disks are RX02 format so I can't read them with 
this RX02 running in RX01 mode (until I manage to fix a UNibus pdp111 
here, that's either the 11/24 or the 11/05. But the 11/05 might only 
have 24kw of memory so we'll see)

Anyway the bigger problem is this stuff was on the plant floor. Where 
the panels were cut. Which means there is silicon dust *everywhere*. 
Remember when my RL02 heads crashed? That was because the RL02 drive's 
filter was *clogged* with dust to the point where the heads couldn't fly 
and were wedge shaped from wear. Pretty sad. But for the floppies I can 
see that many of them have concentric rings in the data areas from where 
the silicon dust got on them and sliced into the data rings. Fortunately 
the RX02 didn't come from there, but since the silicon is embedded into 
the jackets it just wrecks the floppies the more they spin. So fire up 
and get the data fast is probably what I will have to do. We'll see.

However I do now have three RX01 disks with no errors, and one with 
about 20 errors due to a ding in the disk. I've tested them with DX0 to 
the point where I am happy it's reliable, and have made one a BRUSYS 
boot up floppy with RSX11M's VMR utility so I can boot from floppy and 
back up the system on the TK50 drive. Better than blowing a whole tape 
with nothing but BRUSYS. The other floppy I'll use to see if I can get 
the PDT11/150 working, I don't know the shape of that things heads 
either but I do know disk 2 is down. Oh well, never dull...

It's getting cooler outside, so I can get back to work on these systems. 
The VT52 is still working very well with no hum or whistle, the repair 
of the -12v supply seems to be holding out well.

On 10/9/2020 4:02 PM, Jeffrey S. Worley wrote:
> I've not worked on 8" floppy drives, but have on tons of 5.25" 
> single-sided drives.  Older single-sided ones (usually 35-track from the 
> 70's) had load solenoids for the pressure pads, as with a double-sided unit.
> The pad is to provide good contact between the media surface and the 
> head beneath.  I think older media must have ablated more than 
> latter-day media does, as it is rare to find a single-sided 5.25" drive 
> with a load solenoid.  The pad is always in contact whenever the drive 
> door is closed.  Double-sided drives of course retained the head-load 
> solenoid for some years, but eventually those were done away with.  So, 
> I think that unless you are using a drive 24/7, a pad in contact with 
> the disk should not be a serious concern.  That the pad IS properly in 
> contact is important of course, and pads should be inspected to see that 
> there is enough 'meat' left on them to provide the pressure needed.  Aft 
> of the pad, at the base of the head-sled pressure arm is a notch into 
> which the end of the spring rides.  Close examination of the sled will 
> probably show some higher and lower notches into which you can move the 
> end of the spring, to provide more or less pressure as needed, to tune a 
> particular drive.
> In the old days, someone running a drive on a BBS or other heavy 
> application might wear a pad out.  We'd just steal one from a cassette 
> tape and stick it on the arm.  The cassette tape pad was square and the 
> originals were round, but it never seemed to make any difference.  These 
> days there are no cassettes floating around to cannibalize, so I buy 
> felt pads for furniture from Amazon, trim them with a razor and stick 
> them on a drive I'm refurbishing.  Atari, Commodore, Tandy...  Many of 
> the 80's 8-bits used this very scheme on their single-sided drives and 
> this solution is good for all of them.
> I had someone insist to me recently that the felt pads I was buying were 
> acrylic and the originals were Rabbit Hair and that it was crucial that 
> the replacements be made of rabbit hair.  In practice, and that is 40 
> years of practice, any old pad will do just fine.  If it looks like the 
> right thing it will serve the purpose.   Just pick off  the old nub of a 
> pad and stick on your newly cut one and go.
> Common faults I've been noticing are that disk drives made in the 70's, 
> 80's, and 90's are failing in common ways.  I attribute these failures 
> mostly to lack of lubrication.  After 30 years they get a bit gummy and 
> the actuators have to work harder to move the head sled, which puts a 
> greater load on the darlington drivers which power the actuators, which 
> causes the drivers to fail.  Replacing the drivers will often restore 
> the drive to working order, but they will fail again in short order if 
> the original probelm is not resolved.  I simply clean the rails and 
> stepper bands, touch a little wd40 to the rails to free the head sled, 
> cycle the head back and forth a buncha times manually to exercise it and 
> distribute the lubricant, then follow that will a little white lithium 
> grease for a longer-lived lube.  Not only will the drive run better and 
> a lot quieter when lubricated, the loads on the actuators and their 
> associated electronics are greatly reduced, making for a like-new drive.
> The second thing that is happening quite often is electrolytic 
> capacitors are failing, leaking or not.  I had a pair of drives the 
> other day which made quite a racket when spinning free even without 
> media installed.  Replacing the electrolytics on the spindle motor's 
> board got rid of the noise and made it possible to properly tune the 
> RPM's, which had been just all over the map.
> best,
> Jeff

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