Re: Sharp MZ-80K help needed on repair
When it's doing that in a repeatable way, that is the time to try disconnecting pin 14 because that will tell us whether that mysterious 0V on V-Gate is being output by the 8255 or whether there is something else in play here - intermittent low resistance or short between V-Gate and ground.
From your most recent observations it appears that the V-Gate line can be at 5V even without the pullup resistor - this is the condition which we and the RAM TEST code expect, that is why the RAM test code does not bother to set up and use 8255 pin as an output. We need to know where and why low voltages sometimes appear on that pin.
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