Disk III only works when the analog board is held in the air

[philcollins]

Hey y'all,

I'm in the process of repairing a Disk III for this Apple III project I've been cooking with and I'm experiencing a really strange phenomenon. I was wondering if anyone out here on 68kMLA has seen a Disk III do something like this:

When the analog board is screwed into its base, the drive makes a loud grinding noise on any access and the A3 reports an I/O Error. The A3 Confidence Program won't even recognize the disk as being online in this state; however, when I unscrew the analog board and hold it in the air, the disk works just fine, passing all confidence programs.

I've replaced all the ICs on the analog board with tested good ones and have fully recapped all the electrolytics, but to no avail. What's particularly weird here is that, as soon as I as much as gently lay down the analog board on top of its mount during any disk activity, the grinding noise resumes and the disk fails.

It seems like a grounding issue of some sort, but the mount is made out of plastic, so I'm uncertain if that's the case.

Any ideas on what could be cooking here?

Thanks so much for the consideration and all the best!

 

[philcollins]

I’ve created a quick video to show what’s happening with this unit; it is a weird one for sure:

 

[bibilit]

The only thing i am thinking is…maybe stretching the data cable is connecting something loose. I will check for a bad solder joint (connector side) or broken cable.

 

[stepleton]

Fascinating, please carry out more experiments. Hold it from a different part of the board. Prop it up on something or suspend it from something (non-conductive and not statically charged) instead of holding it. Flex the board as you hold it. Press gently on connectors or ICs as you hold it or while it's resting in place. See what happens when you only barely lift it off the board. Raise it by a centimetre and try again. Repeat. Keep trying things that help you narrow down just what about holding up the board is enabling it to work...

The idea to check solder joints is a good one.

 

[Forrest]

If you have some 1/2 inch, 3/4 inch or 1 inch standoffs, you could mount the board at different heights and record the results. Maybe mounting metal shielding between the drive and board, between two spacers.

 

[nathall]

The only thing i am thinking is…maybe stretching the data cable is connecting something loose. I will check for a bad solder joint (connector side) or broken cable.

Yeah, this screams bad ribbon cable or ribbon cable connector to me. When extended it’s able to make proper contact but not when relaxed.