This is an Assy no. 250469 Rev. 4 shortboard.
The machine would boot normally, but could not gain access to the diskette drive/Serial Port. Loading a game from diskette would yield the ‘?DEVICE NOT PRESENT ERROR’.
Running the 64 Doctor diagnostic cartridge yielded faults on both CIA chips (MOS 6526’s) at U1 and U2.
As U2 is closely connected to the Serial Port I started replacing this chip.
This removed the User Port error, but the Serial Port error remained.
As the 64 Doctor cart pointed towards the CIA chip at U1 I also removed this and inserted a fresh new one.
However, the Serial Port fault remained, but I got a different message when loading a game from diskette. It would freeze after it started the search but never passed beyond this screen. I knew this fault could be related to the 7406 logic chip as in this repair job of a longboard (link).
I therefore replaced the 7406 logic chip at U22 with a new working one.
Unfortunately, it did not remove the fault. During a previous repair, the fault of the Serial Port was partly connected to the protection circuit of the Serial Port (link). I therefore started measuring the diodes to the right of the 7406 Logic Chip at U22 with the voltmeter of my multimeter. As the diodes (1N4148 or IN914) are placed on the PCB, the values cannot be used for much more than faultfinding. However, I was looking for some kind of outlier in my measements and guess what? I found five diodes out of eight that showed almost zero volts compared to the rest of the diodes (they had values of 0.5 V). 
Out with the five shorted diodes and in with some fresh new ones!
..and the error dissapeared…
…and the Serial Port was working again…
This is the pile of components that fixed the board.
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