The replacement parts finally arrived for my repairs to the 7623A. I replaced C811, and Q956 (which I think I killed by accident), and managed (I think) to reassemble everything (I will say that my respect for the industrial designers of the 7000 series has taken a serious hit from this experience: they did NOT make these things easy to work on, I had to disconnect half a dozen cables to get the power supply out, and then I had to unmount the transformer in order to be able to unsolder C811. Reassembly was also difficult because some of the connectors are very difficult to access without disassembly of the other circuit boards. I managed it, I think, but it was not easy).
Anyhow, the reason I have some doubt about whether I reconnected everything correctly is that when I power the instrument up now I don't get anything on the display. If I turn the readout intensity all the way up I get a diffuse vertical bar, about 1 division wide, that gets slightly narrower when I press BEAM FIND. If I continuously adjust the readout intensity through its range I can get a short line on the display (about 1 division wide and tall) whose slope responds to the vertical and horizontal position controls.
I checked the power rails on the LVPR board, and they all look good, except for the -15 V rail which now reads -21 V with 0.22 V ripple. That is a change from before, where the -15 V rail was -13.5 V with 2.1 V ripple, but I'm not sure that it's exactly an improvement.
It may be that either or both of Q945 or Q964 are damaged. I am in the process of adding a TO-78 socket to my component tester in order to check them.
It may also be that I did not connect something up correctly. I took pictures before I disconnected everything (thankfully) and I have tried to reconnect everything as the pictures show, but I have many doubts.
One specific doubt is that the core of the transformer may be shorted to the aluminum frame: some black paint flaked off the core where it meets the aluminum frame. I know that the core is insulated from the mounting bolts with plastic sleeves. Is it supposed to be insulted from the aluminum frame by some black paint? If so I'm sure that I can arrange some suitable insulation to replace the paint (teflon tape, or thin plastic sheet of some kind).
-- Jeff Dutky
