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New arrival – Sharp MZ-80K

160802-IMG_20160802_202212Like so many of my machines, the Sharp MZ-80K was out of my reach the first time round and by the time I could have afforded it, I was on to something else.

This example was passed on to me recently and had previously been used for developing a pharmacy system years ago.

The years have not been too kind to this one and there is some surface rust but it’s not too bad cosmetically.

160805-IMG_20160805_185525It’s not without problems however. When I first gave it some power I saw the screen on the right. As well as the general mush there are vertical lines running up through the characters. That made me suspect the character generator ROM and sure enough, it has a broken pin.

160805-IMG_20160805_203905 I snipped a pin from a surplus IC and solders that in place. The vertical line has gone and I’m just left with the mush.

MZ-80KSignalsDiagnostics continue.

Fortunately I have a copy of the service manual and on page 11 it shows a series of expected ‘scope traces. At pin 8 of IC7 there should be an 8Mhz wave. Sadly, I see about 760Hz. Way off.

MZ-80KClockCircuitIt doesn’t seem possible as the video is rock solid and all of the video signals are derived from that 8Mhz clock signal. I’ve checked several times and I always get the same result. I was suspecting the crystal, the 74LS93 or the 74LS04 so I replaced all three but to no avail.