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It is actual 2 boards; the middle keyboard is a separate one connected by a dip flat cable.

Its schematic diagram:

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And part 2:

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Initial state

I removed the CPU and display boards from the case, and cleaned up the goo left by some foam between the keyboard’s keys.

I connected the CPU board to a 5V power supply and checked current consumption (around 1.6A). Nothing got hot, so I checked the CPU clock (worked) and the R/W and address lines. The CPU seems to run.

After that I connected CPU board and display board together and powered them up with 5V and got display:

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and after that lamp test like display:

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The initial display seems to be a lamp test, and from that we can see that some displays do not work: the three alphanumeric ones on the left and the two on the right. The “channel” one initially did not work either but that was just a loose connection.