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midibob

Sys80 PSU

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Hi chaps,

Still beavering away at trying to get a Sys80 system going and despite finding many bad chips and broken/corroded tracks I've yet to get any I/O and CPU combination to actually get the reels spinning.:(:headache: Looks like I might be jumping in the Thames again.:lol:

I thought I'd have a breather as I've been bashing my head against a brick wall for the best part of a week already.

I've started looking at the later type PSU's and noted there are at least two revisions F & J. We have docs for the rev F but not the J. One of the minor differences is that there is a 5v adjust pot. If you follow the info in the manual it states to ground both the +5v and +12v regulators to the heatsink. On a couple of J boards here that has been done but then it shorts out the ground pin of the 5v regulator so the pot becomes redundant. Surely that's not right?

Also on a rev F board there is a wire link L8 which has been removed and a diode put in. This lifts the 5v ground rail by 0.6v which I assume is a mod to get a slightly higher voltage to the CPU as there is a bit of a voltage drop.

 

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Not arf! :)

Flippin watchdogs, 100Hz pulse feeds, 2.4KHz and 100Hz interupt routines I think I've been through the lot.:lol:

It's always the same when you have all faulty items and you're trying to get a working system, it could be anything. It won't beat me but might give me a damm good thrashing!!

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I've lifted the 5v regs and put thermal pads under them and now, as expected, the pots do what they are suppposed to do. I've adjusted them to give about 5v on the I/O board.

The other reg board (rev F) with the diode fitted (1N4001) was a bit to high (5v6) so I've swapped it for Schotty diode which only has a 0.2v drop.

After that brief interlude it's back to the grindstone to see if I can get the system working.:headache::pray:

 

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I think system 80 runs OK under 5V on the I'O board.

what roms are you using and is it a full machine?

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I've put all the info back in my workshop thread so that it all stays in one place.

I assume the later PSU reg board was designed just to give a touch more of the 5v to the I/O and CPU as I was finding it dropping to about 4.8v before. It hasn't made any difference of course but I think TTL bottoms out at 4.75 so it's a bit close for comfort.

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