There are three signals which matter for this coming out of the readout board:
The X/Y shutdown goes first to the horizontal channel switch on the horizontal amplifier board, and then to the vertical interface circuit on the interface board where the plug-ins connect. The readout vertical signal goes directly to the vertical CRT amplifier.
Thanks, I hadn't seen the X-Y inhibit coming from the readout board on the schematic. Just in case it's useful for anyone with similar issues it comes out of the U2126 timer along with the Z axis signals.
It turns out the plug (P38) for the X-Y inhibit input to the main interface board wasn't plugged in properly, must have come loose when I removed the rectifier module earlier. With this fixed the readout display doesn't move with the trace anymore, but the vertical of the readout is still collapsed.
I've been probing around the readout input into the vertical amp board and I'm not sure what is going on. I can see the signal at the inputs to R414 & R408 (see below) but their outputs are at constant
-15V -8V. Both sides of R429 are at -15V DC. Is -15V the correct bias or should it be closer to -50V?
I've measured most of the resistors in circuit and they seem okay, PNP transistors Q407 & Q415 (2N2907A) also test okay with a multimeter, I substituted them for general purpose parts too but no change. Could it be the hybred IC U450? I can measure the diode drops in the transistors between pins 1 & 2 and 4 & 5.
I feel I must be missing something obvious, otherwise I might need to start removing components to test them...
R414:
R408:
edit: I found a schematic in the 7623 user manual which shows the DC levels, mine seem roughly correct. I'm measuring -14.4V instead of -14.1V on the input of J431, but I assume that's a schematic error? Otherwise Vbe of the transistor internal to U450 has to be 1V...