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8” CDC Floppy Drive Repair

jacobtohahn

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North Carolina, USA
Hey,
Just today, I got my hands on this cool 8” drive that I plan to use with my Zenith Data Systems Z-110 to transfer data between 5.25 and 8 inch disks.
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When I bought it, I already knew that it was very dirty, and that it was also missing two capacitors for the power circuitry, which I have already bought and am replacing tomorrow, but there are a few more issues that I had not anticipated that I could use some advice to try and fix.
First, the belt on the back of the drive had become brittle and snapped. I’m not sure where I can get a new belt or what kind I need. Also, the power switch on the front of the enclosure doesn’t click back and forth like it should, and instead loosely moves between the on and off positions, as if it’s not catching the physical switch mechanism. I’d love to know what kind of switch I could use to replace it, and if there is a way that I could short past the switch until I get a replacement. I have photos of the belt and switch attached below. Just ask if you need any more info or photos!
Thanks!
Belt (w/ 8” disk for scale):
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Front of switch:
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Back of switch:
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Chuck(G)

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The switch looks like an ordinary illuminated rocker switch Like this However, you'll need to measure the hole size to get an exact fit.

As far as the rest of the drive goes, it would be helpful if you could post the model on the nameplate. CDC made endless varieties of these things.
 

jacobtohahn

Experienced Member
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Messages
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Location
North Carolina, USA
The switch looks like an ordinary illuminated rocker switch Like this However, you'll need to measure the hole size to get an exact fit.

As far as the rest of the drive goes, it would be helpful if you could post the model on the nameplate. CDC made endless varieties of these things.
The switch looks good, thanks. I’ll be sure to measure it.
Here’s a photo of the information on the drive, made by Magnetic Peripherals, Inc, which was a subsidiary of CDC. I drew over the image to make the stamped-in numbers more readable:
37B31C4B-06EE-482F-B94E-99A758FA0563.jpg
 

Chuck(G)

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It's not going to be easy. If I recall correctly, these are rubber-reinforced fabric belts. At this point, they're probably strictly a custom item. There is a forum member, MicrocomputerSolutions, who specializes/specialized in repair of 8" drives. If he's still around, you might drop him a PM.
 

jacobtohahn

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North Carolina, USA
It's not going to be easy. If I recall correctly, these are rubber-reinforced fabric belts. At this point, they're probably strictly a custom item. There is a forum member, MicrocomputerSolutions, who specializes/specialized in repair of 8" drives. If he's still around, you might drop him a PM.
Thanks, I did that. Apparently he hasn’t been online in months. Any other potential solutions in case he doesn’t reply?
 

jacobtohahn

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Thanks for all the info! I was able to find a part number for the belt and contact a company that reports 11 in stock, so I’ll have to wait for a reply. I also contacted isellprojectorbelts.
 

jacobtohahn

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Here I am again. I’ve completely repaired the drive, new belt, capacitors and everything. I also crimped my own IDC cable to connect to my Zenith Data Systems Z-100. The drive powers up and the spindle rotates, and if connected to the computer and a disk is installed, the head “clamps down”. However, the computer doesn’t recognize the drive in Z-DOS at all. I’m not exactly sure what to do, and there is a 4 switch block (red block in the 3rd picture) on the drive PCB. The head motor seems to want to move, as it rotates just a fraction, but doesn’t move any further. I can move it with my fingers, but it definitely has some resistance. Any ideas on what I can do?
Photo spam:
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Chuck(G)

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See here, page 31 (PWA diagram). Note that you have 4 DIP switches instead 4 shunts. One of these needs to be closed to access the drive. On PCs, this is normally the second drive select, but I'm familiar neither with your cable nor with the Z100, so your mileage may vary.
 

jacobtohahn

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Thanks, that’s helpful. I must have missed that when I read the manual earlier. I have, however, closed one of them and didn’t get much progress. I’ll try again with each one tomorrow, but I’m not sure which address I’d need. The cable is just a 50 pin Shugart card edge connector to a 50 pin IDC connector that I put together myself. I’m not exactly sure if software could be an issue, either. The computer runs Z-DOS, which is Zenith’s version of MS-DOS, but I also happen to have an IBM compatible MS-DOS that runs on my Zenith. I’ll try that with the drive also.
 

Chuck(G)

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You'll have to figure out which drive select the Z100 expects--that's something I've never fooled with. If you get nowhere, there are some things we can try.
 

MikeS

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If it's the only drive on the cable, why not close all 4 drive select switches for now.

Touching a grounded wire to the STEP pin (36) of the interface should move the head; if not, try it with the DIRECTION pin (34) grounded.

Congratulations on getting this far; how did you ever manage to find that belt?
 

jacobtohahn

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If it's the only drive on the cable, why not close all 4 drive select switches for now.

Touching a grounded wire to the STEP pin (36) of the interface should move the head; if not, try it with the DIRECTION pin (34) grounded.

Congratulations on getting this far; how did you ever manage to find that belt?

Once I get home today, I’ll try your trick for moving the head. I had tried to close all four switches, with no luck. Maybe there’s something else going on?
As for the belt, I was able to get it with lots of luck. A company called Compro Computer Systems somehow had 11 of the original belts in stock, and I just received mine yesterday and it works great.
 

Chuck(G)

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A lot of what Mike said holds true in general. To test the accessibility of the drive, you don't need a computer at all--just by grounding various inputs you can observe the function.
 

jacobtohahn

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Messages
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Hey,
Until recently, everything seemed to be working just fine on the drive. I tested my cable to see if there were any shorts (none) and I tested the head with a grounded wire, and it didn’t move (but it tried). I lubricated every piece that the head slides across, and it moves a lot easier now. However, after plugging the drive back in to test the head again, none of the interface pins would provide any reaction. I checked the logic board and found that one of the chips, the largest on the board (pictured below) was so hot when power was applied that it almost burned my finger. No capacitors, resistors, or any other components seem damaged. I think this might be the end of the line for this drive unless I can find a new chip to test with. Anyone have any information on it at all?
2CE4FC50-8EC8-4D88-999E-E59EC83C0B0B.jpg
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jacobtohahn

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Messages
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Location
North Carolina, USA
That's too bad; that IC is likely a custom one for this drive.

I’m not quite sure how, but luck seems to be helping me out a bit! There just happens to be a listing for a very very similar MPI drive on eBay, for much cheaper than I would have thought, with the exact same PCB model. I may just replace the entire drive and sell the one I currently have to make up for the cost of the new drive (so I’ll probably only have a net loss of 20 bucks or so), as my current one is very dusty.
I’ll post again in a week or so when I receive the new drive.
 
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