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Old 27th Sep 2021, 12:13 am   #140
SiriusHardware
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Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,485
Default Re: Sharp MZ-80K help needed on repair

Revised version of Tim's suggestion above: If the 8255 is in a socket which it sounds like it is, remove the 8255 and carefully bend pin (14) out at an angle so that when you plug the chip back into the socket, the pin hangs out over the edge of the socket so that it is not connected to anything.

Then try measuring the waveform on 8255 pin 14 again - it it still only going between 0V-1V or is it now going between 0V and ~5V?

Your other problem - half the screen showing scrambled characters instead of the correct ones - seems like a different new issue with the video display system, specifically,

-The system is displaying the contents of the screen RAM correctly but is no longer able to write data to the first half of the screen memory.

or

-The system is showing the second half of the screen memory correctly but is no longer able to show the first half of the screen memory correctly.

You will need to fix this problem before fixing any other because you need a fully working display in order to run diagnostics on the other part.

The introduction of this new half scrambled display fault sounds like a case of cause and effect - what exactly did you do just before this fault appeared, did you replace the CPU socket or the MROM socket? If so, you need to do a close inspection of what you did and if necessary meter out all the tracks going from whichever device's socket you replaced just before the fault appeared.
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