Encountered an unexpected way for a tpdd to fail.
I have a Purple Computing drive I got a couple years ago that never
worked. But it almost worked. The cpu and sensors and motor control
actually all worked fine. You can send it commands and it responds
correctly, you can read the state of the sensors, you can make both
motors run on command. The only problem was actually reading or writing
data never worked.
I always assumed that it was going to be some exotic problem with the
head read amplifier circuit or something like that that will require the
scope to hunt down.
I was in the process of swapping the guts out from a parts-only busted
up tpdd1 from ebay and along the way the big pulley and main shaft
basically fell out.
So the problem all along was the disk grabber hub was spinning freely
loose on the main shaft. Turns out the disk grabber hub is plastic and
just pressed onto the shaft, and the plastic had cracked around the
shaft. Overall the hub is still whole, just the bore is no longer an
interference fit, it's now merely an exact fit.
It's hard to see this problem because all the parts you can see are
working fine. There's almost no way to see any of the hub or the disk
when it's working to see that it's not spinning when the big pulley is
spinning.
The way you can tell is if the big pulley can be pulled out at all.
Normally it can spin but not pull in/out at all.
I don't know what's a good way to glue the hub back to the shaft. It's a
smooth polished bearing surface shaft, so I tried super glue but I
expect it'll just break free as soon as I try to use it, but I'll let it
sit for a day before I try.
Here's a couple pics of the parts since you can't normally see these parts.
The main spindle has a single ball bearing and a plain bushing or
possibly sintered bearing. The brass part in the middle is both the
holder for the ball bearing on the disk side, and the plain bushing
bearing on the belt side.
https://photos.app.goo.gl/QbirJowcGEEdV3xb9
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bkw
