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5.25 FDD - Well now this is a fine kettle of fish

A

ac7kh

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Aug 4, 2018
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Location
Junction City, OR
Refer to the photo

Got a Mitsubishi 4851 1/2 height drive, ebay with the following problem.

The drive door (lever) cam at the point of the pencil will not rotate on the shaft. The cam locks the door and positions the head to read the floppy.

Very frustrating as this was to be the second drive on my M3 48k system.

Any suggestions for a fix ????

Cheers

RossMitshibushi 4851.jpgMitshibushi 4851.jpg
 
Yes I do hear the click.

I didn't make it real clear in my previous post but the cam does not rotate with the lever shaft when closed. The shaft/lever does not rotate at all without a disk being inserted (as it should).

Ross
 
I took a look at my Mitsubishi drives--unfortunately, they're the M4853--and they use a different retaining mechanism (push diskette in; push eject button).

Looking at a Qume 142, however, a similar scheme is used as the M4851. On the Qume, there's a small setscrew on the cam to retain it in place. I don't know if the same applies to your drive, but there must be something similar. Perhaps a small wedge to fix the cam on the shaft?
 
I tried some super glue, standing the drive up on end. held until the drive spun up and I guess the vibration (floppies are not balanced) made it break loose. Green loctite sound promising, got to go into town tomorrow and will get some. No set screw unfortunately. either the shaft is knurled or has some type of retaining system (can't think of what it is called) that engaged when the shaft is slid through the cam. There are actually three cams on the shaft, so that makes it challenging to pull out and replace. Beside where to find a replacement would be a wee bit of a challenge in and of itself.

Ross
 
There are two types of green loctite that should work here but you'd need to experiment to see which one does better. You only get one chance though, and there's no real penalty for failure.

The kind you'll find in stores is a threadlocker like what we all think of loctite as. If that fails, the other kind is a bearing retainer, which is designed to do exactly what you're trying to do. You won't find that in retail stores, unless you have a Grainger or Fastenal store which will sell to the public. But online retailers will have it. If not Amazon, then MSC or some other industrial supplier.

I'm actually surprised that super glue didn't work. The only reason I didn't suggest it is that it's too permanent in my opinion. If you ever have to disassemble this thing, super glue will prevent that if it holds. If you're okay with never being able to disassemble it, then thoroughly clean it, apply a generous amount of super glue and then add baking soda to the super glue and quickly stir in with a toothpick. That should hold. But, again, if you ever need to disassemble, you'll probably destroy the cam.
 
I now have been able to lock the cam in the close position. The shaft just turns within the cam and does not open the drive. The drive is working just can't remove the disk unless I press the head down tight against the disk and pop the cam loose. At the moment this is not a bad thing as it gives a solid position to apply the loctite.

My wife has gone to town with instructions on which type of green loctite to get (crossing my fingers and toes).

Being in an AG community I may have the ability to order through NAPA, IH, or John Deer. I seem to remember the type you are specifying from working on harvesters as a lad.

Ross
 
I think it's probably a weak solution at best. The cam will be subject to quite a bit of torque and will break loose in short order, as it's plastic-on-metal, regardless of the threadlocker used.

I'd strongly advise inspecting the original assembly to see how the original was secured. In other words, is the shaft strictly round or is it asymmetrical?
Is the shaft knurled at that point? Is it perhaps slotted?

I suspect that the ultimate solution will be to machine a replacement cam.
 
As a note on Chuck's last statement. The cam doesn't have to be made of plastic. Easily machined brass will most likely work well.
I'm wondering if it originally had a pin? Can you look carefully at the working drive.
Dwight
 
That would be my first idea even if a pin is lacking--drill a small hole, say with a #55 bit and insert a length of wire/rod. That would be a permanent repair.
 
I agree. But what I thought might be a problem might be the solution: A broken drill is as good as a piece of music wire, provided it actually makes its way into the shaft.
 
First let me thank all of you for helping me brain storm this issue. As of this coming Friday the drive becomes officially junk and goes on the shelf for future projects, I have another disk arriving (a full size mitsibushi drive) and I have my fingers crossed that it will work as drive 0. Then I will have a full size and a half size in the M3.

I like the idea of putting a pin in the shaft, will have to set up my dremel drill press and dial it down real slow, and give it a shot.

Again thanks to all

ttfn
Ross
 
Well this morning I was examining the cam closely under a bright lamp and found the crack, so I filled it with super glue and now 5 hours later have an operating 5.25 drive. Don't know for how long, but I will probably place it into my M3 as a third floppy (1 full sized and two half sized). Thanks again to all !!!

Cheers

Ross
 
I have no idea if its the same drive or not, but a long time ago I replaced the 2 full height Radio shack drives with 4 x half-height Mitsubishi drives, and I absolutely loved them. Besides being double sided, they would read and write alot faster because the stepper motor that moved the head could be set to 5ms in a dos setting somewhere. This also made it alot quieter. The mechanism to eject the floppy also made this very satisfying "thump" sound somewhere. They worked, looked and sounded like superior-quality drives in every way.

Good luck with yours!
 
I recall having a stepper test program. It would run the disk in and out and set the step rate to the fastest level that wouldn't cause errors. I recall it made the stepping sound quiet when it was at optimum. I don't recall but I think it was a DOS program. I resisted Windows for a long time.
Dwight
 
Well I just got a message on eBay from the vendor who sold me the original drive (the one that had the broken cam) saying he will send a replacement and apologizing for not responding to my message saying there was a problem with the original. Any suggestions for an external case w/ps that will hold 2 "1/2 heights" ????

Cheers
Ross
 
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