Thanks for the replies from everybody, here an update;
Michael & Albert, thanks for your measurements at the 5V1 & 5V supply. Mine 5V1 is 4,715V (U670) and the 5V is 4,96V (U668). This is a bit lower but almost the same as yours. Keep in mind that the 5V1 is not only for the A4 display board but also for the 7475 latches (TTL) and the 8T06 BCD decoders. A bit strange design that should need normal Vcc TTL power supply from at least +4,75V
Mark, VR210 is spot on 8,99V (the unit is working correct with normal output signals, only the display is functioning weird). And all the new caps are low ESR types from the known best brands. I knew that was important because of the "switcher" inside.
Michael, I checked P390 on mine and it?s almost the same as yours; pin 1 turning the VAR knob from left to right gives 91,4V to 108,8V and pin 3 is spot on 8,99V. U400 is tested on a component tester and is OK.
And many thanx for al the pictures Michael, this must be really a lot of work! Is there a way to download them all together in one time from this apple icloud site or is this only possible one by one?
As I said before, the unit is working normal only the display is not working correct.
Here is a table from the display with the VAR knob out and various settings at CCW (7 o?clock) to CW (17 o?clock), so turning from left to right.
VAR DISPLAY
7oc 1.6 steady
8oc 0 steady
9oc 5 steady
10oc 7 steady
11oc 8 steady
12oc a bit fuzzy, all segment lit (random unstable output from the BCD decoders?)
13oc steady display, randomly generated and last result from the varying output from the BCD decoders
14oc no difference, same display as 13 oc
15oc no difference, same display as 13 oc
16oc no difference, same display as 13 oc
17oc no difference, same display as 13 oc
As I said before, the LOW led is always on and the HIGH led is never on.
It seems that this is the corresponding result of the output signal from U460; fully CCW till centre is a sort of triangular wave, then a sawtooth in the centre position, and +16V from centre till CW.
Sadly till now no change in pin-pointing where the problem is :-(
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