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Sometimes its simple

Gary C

Veteran Member
May 26, 2018
Lancashire, UK
On the bench is one of the Museums fat 40's and I have been tasked with getting at least one of the pile of disk drives working.

First task was to see if they complete the startup diagnostics and light the centre LED green. Well, out of 10 units, four are showing the expected startup routine ending with the green LED, four just light it red, and two exploded ;)

One, when I had my head right next to it, that made me jump I can tell you. Its the old tubed RL network mains filter turning into a little cannon and ejecting its guts. Thats the 5th one of these units I have had explode. Next time I'm just going to jumper it out to save my nerves (well, more like save the nerves of the people around me, but that last one really did make me jump)

The best condition unit is an 8250. Connected up, and no joy. Not a single blip from it. 'Device not found' is all it says. Time for the scope and some lid open typing while checking if ATN is pulled low.
Well, I could confirm that ATN was being asserted, so I stuck my CBM disk emulator in, as its serial terminal shows you which lines are being asserted which is quite useful when testing, but again no activity. Odd.

Ah ha !, ATN is going low at the edge connector, but its not getting to the IEEE 'plug' (is it a plug when its sort of male and female in the same unit ?) and there it was, the edge connector was so dirty, no signal is getting through. Quick clean and the emulator is flying. 8250 plugged in and responding, but the drives need an overhaul as some of the eject mech is sticking and I think the heads are a bit dirty.

Joe will be pleased. He's been after a working PET/Drive combo to display since I started.