21-06-2026, 07:09 AM
I just want to share an update on my restoration project. Thanks to all your great feedback, help and ideas the status now, is that both machines is more or less alive.
The 16K model had a defective Z88PIO which is replaced now. All of the keyboard keys - except "x" - are non-functioning
They measure between 50k to 500k ohm when pressed down and 6M Ohm unpressed. But shorting the pins on the solder side result in the corrosponding character being printet. Also the 6545-chip is partly defect - it displays an image but it tends to get distored and has these purple artifacts. However if I replace the chip with a 68B45 I had lying around, the image is stable. But the 6845 apparently does not have the registers needed for the keyboard scans, so it only helps in diagnosing the defective 6545.
The 32K model also have a defective Z80PIO and a fully defective 6545. The MREQ line wasn't present on the core board even though it was present on the CPU. I replaced the IC33 and IC19 just in case. With the 68B45 chip it now turns on but sometimes the basic promt disappears or random characters appears. Could that be due to another bad logic chips in the video circuitry - maybe one of the 74LS157 or the 74LS175? I have checked the videoram.
The keyboard seems to be as dead as on the other machine.
Besides from sourcing 2 new 6545 it would seem I have to start cleaning the keyboard switches. Is it correct that I need to desolder each switch, dismantle it and clean the parts before refitting it? It's the black variant and I tried to remove a random key and clean it. Now it shows ~1k ohm when pressed.
The 16K model had a defective Z88PIO which is replaced now. All of the keyboard keys - except "x" - are non-functioning
They measure between 50k to 500k ohm when pressed down and 6M Ohm unpressed. But shorting the pins on the solder side result in the corrosponding character being printet. Also the 6545-chip is partly defect - it displays an image but it tends to get distored and has these purple artifacts. However if I replace the chip with a 68B45 I had lying around, the image is stable. But the 6845 apparently does not have the registers needed for the keyboard scans, so it only helps in diagnosing the defective 6545.The 32K model also have a defective Z80PIO and a fully defective 6545. The MREQ line wasn't present on the core board even though it was present on the CPU. I replaced the IC33 and IC19 just in case. With the 68B45 chip it now turns on but sometimes the basic promt disappears or random characters appears. Could that be due to another bad logic chips in the video circuitry - maybe one of the 74LS157 or the 74LS175? I have checked the videoram.
The keyboard seems to be as dead as on the other machine.
Besides from sourcing 2 new 6545 it would seem I have to start cleaning the keyboard switches. Is it correct that I need to desolder each switch, dismantle it and clean the parts before refitting it? It's the black variant and I tried to remove a random key and clean it. Now it shows ~1k ohm when pressed.