Twiggy froggy foam replacement

[ried]

One of my Twiggy drives had a disk inserted and clamped while powered down for several decades - which has permanently compressed the foam by a significant amount (maybe 30-50%?). While the drive is clean and works, it is unreliable and my suspicion is that the froggy's compressed foam is a contributing factor - not providing enough clamping force as originally designed.

Does anyone know the froggy foam specs and what may serve as a suitable replacement? The two pieces are illustrated in the Twiggy manufacturing instructions PDF available from Bitsavers, but no details are provided.

[sigma7]

When I first started working with Twiggies ca. 2008, I found the foam pads on the froggy only interfered with reading Twiggies due to increasing jacket friction, making it more likely the drive belt would slip, so I removed or disabled them.

However, as mentioned elsewhere, I've found one older Twiggy disk that turned out to be much more readable with the foam in place (for the front/lower head). (I've also mitigated the belt slipping issue which may or may not be relevant here.)

I don't know if the important effect of the foam was to affect speed variation, the angle across the head, or some other dynamic or geometric consequence. Perhaps it was subtly correcting a warped jacket or liner of this particular disk.

Incidentally, the way I discovered this was by pressing on the jacket beside the front head with my finger, experimenting with how the angle of the media crossing the front head affected read success.

So although I still postulate that it is better for media longevity to not use the foam (and no other floppy drive design uses it as far as I know), I admit there are circumstances where having the foam is useful.

My perception is that the foam is among the "softest" I've ever seen, so I tried the softest foam I had on hand and found it worked for the troublesome disk.

Hence this may not reflect the original specs of the foam, but it is what worked for me for the one particular disk.

The original foam is open cell, clearly low density and very soft to the touch.

The material I have on hand that is most similar is 5mm thick with adhesive on one side. This thickness seems about right.

I cut a rectangle out of it, slightly undersize for the front recess in the froggy: about 7.5 x 23mm

I cleaned the froggy with IPA and the adhesive already on the foam was sufficient to attach it.

I didn't apply foam to the rear position as this particular problem disk read fine with just the front foam.

So... how to characterize a random piece of foam?

I cut a length to make about 2 cubic cm of foam and weighed it on a scale having a resolution of 0.001 g but unknown accuracy:

0.070 g (with adhesive) (0.103 g with adhesive and backing paper)

which makes the density about 35 mg/cc

I might be able to manage a compression test to estimate the force applied by the foam.

[ried]

That is an impressive assessment of foam function, with the data to back it up! Thank you, sigma7.

This particular drive has trouble reading and writing disks with consistency. I have several new old stock 3M FileWare disks that work just fine in other Twiggy drives, but not this one. Generally, the Lisa prompts me to format / initialize the Twiggy disk and it fails halfway through the initialization process - LOS complaining that either the disk or the drive may be damaged. Sometimes, though, initialization completes successfully and the disk mounts on the desktop as normal. After a reboot or two, the disk inevitably fails to read properly and I am prompted to initialize it again.

Rinse, repeat.

My amateur brain guessed that ICs on the analog board that amplified the signals from the read/write heads might not be working properly, too weak for the old media in most cases, and that replacing those ICs would fix the issue. I replaced every IC on the analog board and... nope. Same behavior persists.

So, it seems there's another reason why it's not reading and writing the disks consistently. The foam is the only thing that's obvious. Otherwise, the drive is clean, has a new drive belt installed, and appears like it should be working fine.

[AlexTheCat123]

I know very little about the physical construction of Twiggy drives (I've never even seen a Lisa 1 in person other than in museums), so there might be a really obvious reason why this wouldn't work, but could you try pressing down on the disk with your finger to simulate the foam pad and see if that fixes all your problems? That way, you don't spend a bunch of time trying to find replacement foam if the problem ends up being something else entirely.

[ried]

Good thought, Alex. Wouldn't hurt to try! The foam is still present both front and rear, it's just permanently compressed to perhaps 50% of its original height.

[stepleton]

I think I've mentioned here long ago (though I'd be hard-pressed to find it): I've definitely used the tongue-depressor method to achieve better results on my #2 Twiggy drive, which needs some adjusting. I think some of sigma7's posts about servicing the drives will help me do that, but I'm holding out for an easier way to operate my Twiggy drives on the bench, which may be forthcoming thanks to Alex's work 

Until then, I recommend using BLU to investigate further. I'd enable verbose reporting over the serial port and then try reading, writing, and formatting disks. The stream of stats may show patterns that characterise the way the drive is struggling with operating on different portions of the disk, and you may also be able to achieve virtually instant feedback if you gently manipulate the disk (tongue depressor etc.) while the read/write/format is underway.