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Old 13th Oct 2021, 7:51 pm   #288
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 8,234
Default Re: Sharp MZ-80K help needed on repair

Sorry to hear that, would have been a nice simple remedy had it worked. Another two other possible scenarios -

-Thermal (time related) fault in the CRT monitor circuit
-Thermal (time related) fault on the horizontal sync generation circuit.

If you don't have any freezer spray you could try letting the machine cool right down (maybe overnight) so that the 'bent top' fault is not present, then go over the mainboard with a hairdryer set to a fairly hot temperature to see if that makes the fault appear sooner than it normally would. I would start with IC8 and move on to the other ICs in the 'Video Generator' section of the circuit diagram (PDF page 34 / manual page 32).

What about that problem with the 8255 pin 1, did you sort that? If there is a problem with that pin somewhere under the 8255 there could be problems with other pins which are related to the other problems you keep having. If you sort that problem out you could end up fixing or at least finding the reason for some of the other problems as well.
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