I¡¯ve been troubleshooting the oscilloscope further, focusing mainly on the Vertical Amplifier board. I¡¯ve attached pictures of the boards and the schematic diagram from the service manual, with two connectors marked because disconnecting them impacted the behavior (you need to log in to view these pictures):
Main Board:
/g/TekScopes/photo/299721/3870949/Verical%20Output%20-%20Circuit%20Board.jpg?p=Created%2C%2C%2C20%2C2%2C0%2C0Thick Film Hybrid Board:
/g/TekScopes/photo/299721/3870951/Verical%20Output%20-%20Thick%20Film%20Circuit%20Board.jpg?p=Created%2C%2C%2C20%2C2%2C0%2C0Diagram:
/g/TekScopes/photo/299721/3870950/Verical%20Output%20-%20Schematic%20Diagram.jpg?p=Created%2C%2C%2C20%2C2%2C0%2C0Here are my findings:
1. Disconnecting P442 completely resulted in a horizontal line at the screen's center. Adjusting the Vertical Position knob had no effect, and pressing the Beam Finder switch only shortened the line without changing its position.
2. Connecting only Pin 5 of P442 (+75V) gave the same result as disconnecting it entirely.
3. Connecting Pins 3, 4, and 5 of P442 also had the same result.
4. The line disappeared if Pin 1 or Pin 5 of P442 was connected.
5. Disconnecting P385 while P442 was fully connected caused the line to reappear.
6. Connecting only Pin 1 of P385 moved the line slightly upward.
7. Connecting only Pin 2 of P385 kept the line at the center.
8. The line disappeared if both pins of P385 were connected.
9. When the line was centered, voltages on the upper and lower rails matched closely.
Looking at the schematic, the 'Thick Film Hybrid' board (btw, it's a PITA to solder on it) connects to the main vertical amplifier board via diodes VR363 and VR463 and through P385. Additional findings:
1. Feeding +12V to L369 and +75V to L385 simultaneously seemed to have caused the vertical deflection to malfunction.
2. Voltages at the emitters of Q314 (~6.27V) and Q414 (~3.67V) suggested the trace was off-screen.
3. When the horizontal line was centered, both emitter voltages were around -3.85V, a notable difference.
At this point, I feel close to identifying the fault but am unsure which direction to take next. Any insights or suggestions would be much appreciated!
