Although this thread did not attract a huge crowd, it has to be concluded one way or another. Maybe someone looks for a DC505A story 2 years from now. And stories that start but have no end are just frustrating. So here it is, the final chapter.
I was wrong (nothing new here). The short between wires 1 and 2 of the PCB connector cables I replaced was
not in my replacement cable, as I had assumed (it seemed so obvious). Making a new cable and replacing my first one didn't change anything: DCU
0 didn't count. The Set inputs of the FFs were at the exactly same voltage (3.8 Volts - between High and Low, obviously seen as High) as before. Out again went the cable, and on when my watchmaker's loupe. Now I saw it, a very fine glittery thing between the two pins. I removed it, and before soldering in the cable again, I made sure my multimeter measured a high impedance. I blame this on sloppy soldering/desoldering on my side, although it would explain the channel B failure from the start.
The only certain fault then was an open CR264 which made it impossible to set the trigger level for channel A such that it actually triggered.
I hardly couldn't believe it when I saw that thing actually showing a frequency measurement!
But there was another problem: The clutch on the display time potentiometer looked like it wouldn't last much longer. And it didn't. When I cautiously unscrewed the thing, it fell apart. With no replacement in sight, I decided to make an emergency repair (I'm not the kind that allows only original parts even in non-visible places). A piece of wire through the small slit in the potentiometer axle seemed enough for the needed torque. Not exactly beautiful, but works all right.
Another two hours were spent for assembling the beast. All in all, this was quite a harrowing experience for me.
What did I learn, though?
1. Make more photos! Document absolutely everything (would have helped tremendously with assembly).
2. Don't print just the schematics (especially for Tektronix!). Having to look up everything on a computer is very awkward.
3. Make some more photos.
4. Be meticulous. No, be
extremely meticulous.
5. Remember what you learned ages ago. Faults can be found, even in analog circuits.
6. Don't assume, even if everything seems oh so clear.
Always measure.
7. Don't be too afraid of rather hot ECL circuits and inaccessible spaces.
8. And finally: Make even more photos!
I can assure you that I'm relieved at last. And somewhat elated.