Well, that was a lot of information.
And good information, too!
I've been quiet so far because I've waited for replacement parts which now have arrived.
So I first swapped several of the 900 group of IC's, namely U901, 902, 906, 915, 916, 921 (the first three because they differed from the schematic and the last three because they are part of the measurement circuit).
Didn't make the slightest difference: OSC and PW still showed the same problems as before, RES and FILT were fine.
And so I decided to swap U805 and U814 as well. And guess what: OSC and PW are now tuning fine every single time I perform the tuning routine! I'll probably try to zero in later if either one was faulty.
But - weirdly enough - not all is well. After swapping the chips the filter became a bit flaky! Most of the time it's tuning fine, but occasionally one or two voices (3 and 4) fail repeated tuning attempts until they suddenly start working correctly again. I haven't discovered a pattern in this so far. What I find odd is that this never happened before.
Anyway, I now have a mostly working Xpander. A big "thank you!" to the group and especially r1n9o and Karl.
Another question: Is there a consensus about updating the firmware?
