A Weird OSI 470 Repair

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glitch
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Joined: Mon Nov 28, 2011 12:43 am

A Weird OSI 470 Repair

Post by glitch » Tue Jul 30, 2019 7:02 pm

Full writeup here: http://www.glitchwrks.com/2019/07/30/47 ... -second-c3

So, on Friday, I started repairing this OSI 470 floppy controller:

Image

I've had it for years, it came with the Challenger III I've got, in the C3-OEM case -- the one I use for most of my OSI hacking. It didn't work, but I happened to have a 470B from another system that did work, so the 470B has been in the C3-OEM case ever since. Last week, I came across this board while doing some cleaning-up in the shop and decided to fix it...I mean, it's such a simple board, right? How hard could it be?!

Well, I ended up building another Challenger III in the process:

Image

The above system was operated basically as shown above (but with more cards) by its previous, original owner. He just had the backplane screwed down to a plank of wood, on a shelf, with modular power supplies and the CD-74 hard disk it lived with. There were a pair of 8" floppy drives in an OSI cabinet on the bench below the shelf, where they were easier to access. I have all the cards for said system, but since there was no case, I'd never really done much with it, and used boards from it to get the C3-OEM system going.

I decided to build up a bench system Challenger III since it looks like the C3-OEM backplane has a cracked trace or solder joint. That was why I was having weird RAM test failures with the GW-OSI-RAM1, using Mark's new universal RAM test. But, that didn't fix all of my 470 weirdness! It's documented in the writeup linked above, but basically a weirdly-faulted 6821 PIA was causing the 8" floppy drive to go past track 0, probably hitting a mechanical limit but still showing as on track 0. What a nutty problem!
Check out The Glitch Works
OSI Challenger 3, 510 CPU, 8" floppies, 74 MB hard disk system in need of restore
Parts bin Challenger 3 board set, never had a chassis in its time

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