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RK05 platter seating confusion

TJ_Mossman

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Dec 1, 2013
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Bristol, England
A few months ago I picked up an RK05j for my PDP-8/e, and I've finally got around to replacing the foam and testing the electronics, which so far seem fine.

However when trying to load a cartridge, it doesn't seem to stick to the spindle magnet correctly, as the platter hits the "duckbill" when the spindle is rotated.

Click image for larger version  Name:	Platter Wobble.gif Views:	0 Size:	9.3 KB ID:	1230660


At first I suspected that the spindle could be bent or that the disk is warped, but no matter what rotational position the spindle or platter is in when a disk is inserted, the platter hits the duckbill whenever the spindle is rotated about 90° in one direction and 100° in the other. I've checked the spindle with a dial indicator, and there's no discernible movement. Whenever the disk is inserted, a small gap is visible between the spindle and base of the disk (mating ring? I'm not sure if there's a term for it), which gradually closes as the spindle is rotated, at which point it bites against the duckbill.

Does anyone have any idea what the likely cause is and how to go about checking it? At this point I'm suspecting there's either a problem with the cartridge receiver (nothing stands out as being wrong though), or the alignment of the spindle shaft perpendicular to the heads.
 

djg

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MD
I've had a few packs that have that wobble problem. I haven't exactly figured what the problem was so don't take this as gospel. Originally I though it was damaged packs but got a good pack that showed it also. My guess is that the conical point in the center of the spindle is bottoming in the pack and preventing the pack from fully seating on the spindle. If the disk is sitting on the point the magnetic base will tend to pull it down with one side against the magnet and the other up. Yours is worse than any I have seen so could be different. I didn't think it was the cartridge holding the pack crooked since that would make noises if the pack was rubbing against it. Assume its some sort of tolerance on the spindle/pack.
 

TJ_Mossman

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That's certainly an interesting thought! My cartridge is a Scotch 910-16 rather than one of the official DEC ones (though they could have both been made by the same company for all I know).

I think I'll leave it alone until I can find a DEC branded cartridge and see if it fits. If it does I'll measure the spindle holes on them both.
 

TJ_Mossman

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Hello Tom,

I have a DEC RK05 disc you could borrow for test purposes. (NB. I also have a DEC Field Service Head Alignment disc if you need it).

Those would be useful if you're happy to trust me with them! I'd be curious to know if my pack works ok in your drive too.
 

SteveG

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Sep 30, 2020
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You are welcome. I have a set of 3 Diagnostics disks and an alignment pack which I want to keep safe but I think I have a scratch pack you could borrow.

NB. I have not had the time to get my RK05’s working so I can’t verify your disk yet. Luckily I have a friend who knows the drives very well (he was a field service engineer) and has promised to help.
 

Roland Huisman

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I envy that suitcase! I have a stack of these disk packs as well, the non DEC packs seem to have problems to open the lid in the drive sometimes. And when you push too hard you can damage/bend the opening lever in the disk drive. (I hope anyone understands my explanation) The early RK05 drives have a lower force to push the pack on the seating. I have had disks which made a scratching sound. When I pushed a little on the pack the sound disappeared. So I replaced the springs which are pulling the pack down with stronger springs. (That complete pack holding system is modified in the RK05J drives)
 

SteveG

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If it turns out that the early drives need a genuine DEC Pack then I do have some. I am reluctant to risk this pack as it contains some useful diagnostics software 20586D81-2901-4A7C-B4AB-DE24BADBA8BC.jpeg
 

TJ_Mossman

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With thanks to SteveG I now have the Scotch pack pictured above to play with!

It mounts and spins just fine, but on inspecting the platter I found a few dozen odd specs and bumps (corrosion?) that can't be removed with IPA and Kimwipes, so I won't be able to test the heads just yet.

Meanwhile I completely disassembled my original pack, so I could try mounting the hub on the spindle without the rest of the cartridge getting in the way. It still mounts on the spindle at a slight angle, but will stay flat if pushed down hard. Yesterday I took both packs to a local CNC shop so they can compare the two spindle holes with a laser scanner. (They'll also scan the whole hub casting, for a potential future project.) If the hole is smaller on my pack, they'll mill it out. Hopefully then it'll still work when I put i back together.

This is what the hub looks like, for anyone curious.
IMG_0139.JPGIMG_0144.JPG
 

Al Kossow

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That would certainly be an interesting discovery that early RK05 drives didn't follow the IBM 2315 spec and made the cone a different size, or their tolerances were wrong.
I've never had a problem interchanging packs from different vendors on Diablo 3x series drives.

The only thing I've ever seen which weren't interchangable in that pack style is later HP higher density drives which have thicker platters and
obviously the different sector slots.

If you decide to make new hubs with 16 slots, I'd be interested to hear how much trouble you have re-centering the platter on the hub.
Then, there is the problem with the foam they used inside the packs.
 

TJ_Mossman

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Bristol, England
This is a later drive, possibly one of the last ones made; I believe the spindle has a 1979 datestamp on it somewhere and some other bits are early 80s. If anything's out of tolerance I'd suspect it's the cartridge, but I could be wrong.

If there's any interest in manufacturing new 16 slot hubs and if it would be cost effective to do a group buy, that's definitely something I'd consider. There's also this hack, that may prove more effective: https://www.vcfed.org/forum/forum/genres/dec/39420-soft-sectored-rk05

I'm not too familiar with the densities and coercivity of the various 14" cartridges and disk packs. Would it be feasible get a partially crashed disk pack, and transplant the working platters into new cartridges?

The foam is intact and looks easy enough to replicate.
 
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