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My CSA803 now passes the Self test and Extended diagnostics test.
In an earlier message I wrote that some if the 5 IR sensor groups were never selected during a full scan cycle. It appeared that this was due to a faulty 3-to-8 line decoder 74LS138. I replaced this IC and bingo.
In hindsight I am not 100% sure that it was the IC itself, or a bad solder joint to one of the 3 input lines. After replacement the operation of this IC was not stable. I resoldered the 3 input pins and that cured the problem. Previously I had checked everything for continuity with an ohmmeter and found no bad joints, but now with a probe I saw that the slightest pressure on the pins or on the board changed the results.

The repair time was nothing compared to the total time I spent to find the problem. I nearly completely reconstructed the schematics of the A9 Front Panel Control board and A10 Front Panel board, and the way all circuits are operating. Interesting and "on the fly" I got more insight in the CSA803. Once you know where and what to probe it's fairly simple to detect non-functioning IR pairs.
A silly mistake: after resoldering, all horizontal pairs worked well but still a few of the vertical pairs didn't. Then I realized that the distance from IR Led to sensor is much larger for vertical pairs than for horizontal pairs, and I used the bare board -- without the frame of "lenses" which are supposed to concentrate an IR Led light better on the corresponding sensor. With that frame in place all pairs worked fine.