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Old 7th Jan 2021, 6:18 pm   #65
SiriusHardware
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Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 7,513
Default Re: Non-working Commodore PET 3016

Yes, I missed that one. Hidden in plain sight. Your circuit diagram reading skills are a lot better than you originally thought. Better than mine, in fact.

That brings us up to 8 ICs (discounting the CPU which you have already shown is not the source of the short) which could be where your short is.

Continuity between CLK1-out on UG5 and all of the other IC pins which that signal should go to will be one to look at later, but unfortunately the priority remains to find and clear that short between CLK1 and 0V / GND.

Once you've cleared that short then - (if UG5 has not been damaged by having its output shorted to 0V or if indeed UG5 is not the actual source of the short) - then you should be able to go to each of the other IC pins which CLK1 goes to and you should see 1MHz on each of those pins.

I've noticed that your new meter only seems to have one resistance / ohms range on it so obviously it must auto-range, does the old meter have manually selectable ohms ranges - maybe 200 Ohm / 2 K / 200K / 2M, that sort of thing?

Going back to what refugee mentioned, it would be good if we had a 'second opinion' about the actual resistance value of that short - if possible could you measure between UG5 pins 7-8 and say what the resistance is according to your old meter (on its lowest Ohms range, if it has more than one Ohms range).

If there is any significant resistance at all (more than 1 or 2 Ohms, say) that would make it much more likely to be a short inside a chip, but if the resistance is as ultra-low as your newer meter says it is then it remains more likely to be a physical short.

Last edited by SiriusHardware; 7th Jan 2021 at 6:23 pm.
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