After trying many things, pulling boards and testing, I came back to the PSU Control board.
I installed the PSU Control PCA from the working 2648 in the PSU of the defunct terminal.
As a result I was greeted by a "TERMINAL READY" message. So I knew that the fault had to be on the small PSU Control board.
Then I went through the diodes, capacitors, transistors and even resistors, desoldering when needed and taking measurements.
All parts looked good, I did not find anything unusual.
Finally I put both boards side by side on my workbench, powered them by 5V and used the two channels of the scope to simultaneously compare the signals.
Finding some differences in the "variable voltage one shot" circuit I finally traced it back to the frequency divider circuit.
The bad board showed only 1 V TTL signal amplitude between the two 7490 counters and the output of the whole circuit ended up in the 490 kHz range where it should have been 49 kHz.
I don't understand why I had not noticed this in my first tests where I noted the frequencies in this circuit as being o.k.
Well, after replacing the 7490 counter (of course not in my inventory, the last electronics parts shop in my home town closed 2 years ago so that I had to order it and it arrived just 3 days before Christmas evening) the PSU started to work again!
So: in the end the cause for the problem were the defective voltage regulator and the broken 7490 chip.
The terminal has a Universal RAM module (16 KB via the bottom bus) and I allocated a larger COMM buffer to allow for 9600 baud operation without handshaking.
With a 4 boards wide top connector I could probably use the full 32 KB via bank switching.
The terminal even has an HP-IB interface and for testing the associated escape sequences I used a small CP/M system.
I was able to set and read the time from a Digital Clock ([CP/M]-[RS-232C]-[Terminal]-[HP-IB]-[Clock]).
I also found a Composite Video board which outputs a signal to an external monitor or hardcopy device. Still have to check what an external monitor will show.
For the second terminal it would be nice to have an additional memory board.
It came with the smaller 8K bottom RAM module only.
- Does someone have a surplus top connector bridge with 4 connectors for 264x Terminals?
Then I could enable bank switching and use the full 32 KB capacity of the Universal RAM board.
- Does someone have a surplus 8K BOTtom RAM or a Universal RAM module (even without RAM chips)?