Day given the fact you have the schematics you could continue the series. You said that you we'll continue the series as soon as you have time, When we could have the next video we have to wait a lot of time?
Despite that, The first think I would do is to adjust the -18V rail to -18 and no like now ( from part 2 the -18V rail is to -19.2).
I still think that the dac and adc are good but the op amps (change all the same op amp that you have arleady replaced) are bad.
Best luck and let us now when you'll continue the series. (You could do a live show repairing the instrument).
As you could understand I really like the repair video, I wish to have other videos on instruments reparing.
If you ever buy a TDS 540 tektronix scope and If you need the schematics, tell to me (I have the original no revision TDS540 schematics).
Best regards, Alberto.
Happy Christams and happy near year.