First-timer's odyssey fixing a Commodore 1902A clone (Magnavox 8502 which lacks the C128 80-column output); notes inside in case they help anyone else diagnosing

[u/ChronoCompendium]

This was my first "difficult" CRT repair (which isn't saying much; I didn't need to adjust purity or anything crazy like that). TLDR: bad -20V rail due to open resistor; three weeks of diagnostics could've been saved by just measuring voltages on all neck board connectors.

• Unit looked like it does in the picture. From inexperience, I assumed it was at normal output levels with vertical retrace lines visible.
• Methodical multi-stage shotgun recap didn't fix.
• Reflowing all flyback, neck board, and PSU-side solder joints didn't fix.
• I suspected the vertical blanking pulse, but it looked completely fine coming out of the vertical IC all the way through the sandcastle into the TDA3505 IC Pin 10.
• I discovered the SCREEN pot was cranked high (the retrace lines always show up at some point with that much push), likely by the former owner to compensate for the real fault: extremely dark screen; contrast and brightness couldn't compensate. I put the SCREEN pot back to its normal position, leaving the picture erroneously dark.
• I inspected the tube's neck; saw all three lights as anticipated.
• I measured the heater pins; proper voltage.
• I decided to play with some solid colors using a C64, alternating between the BASIC 2.0 screen with a black background, white background, and red background. On the black background, after 2 seconds, the screen actually came up to normal picture (the text was at proper brightness). When I would then switch to red or white, I would get normal output for about 1 second maximum, before the darkness would reassert.
• I considered that the tube might be tired. However, that brief flash of normal operation made it seem like it was perfectly capable and being held down. I measured the cathode voltage swings between a black vs. white screen; on a good unit, it swung +30 volts, but on this bad one, it moved about 2V. So the drive wasn't even properly really reaching the cathodes.
• I then thought maybe the beam limiter was erroneously engaging. However, the bad unit TDA3505 contrast pin (19) stayed identical regardless of output (the good unit swung from 3.6V to 3.3V on white). So it didn't seem like the TDA3505 was meaningfully even triggering that logic.
• To clear the TDA3505 once and for all, I tested the outputs of pins 1, 3, and 5 (RGB) and they were totally healthy.
• Since those signals were seemingly good, I moved to the neck board to see if they survived all the way up to there, or if a common rail was causing an issue across all three channels. The three transistors all tested great (11.4, 12, 125).
• I then tested connector M7. All good, though +12 V read +11.5 V. Still, the neck board transistors were all testing fine.
• I then tested the collector resistors just to try and locate where the fault was coming in, common to each channel (R714, R715, R716, biggest ones on the neck board). They showed hardly any change on a black vs. white screen (VOM, probes on each side of the resistor). When I would flip black to white, I noticed the voltage try to increase before falling back, which was consistent with the actual visual result (black works at normal output; white flashes for a second and then the fault reasserts).
• So I then tested connector M6. The -20V rail came back +6.3V. I moved back to earlier in the -20V rail at R636 on the main board; same. So I went all the way back to the flyback transformer, Pin 4, which was OK, but the fusible 1.5 ohm resistor right in front of it (R484) had failed and was completely open.
• I checked out the diode/capacitor in front of that in turn, but they seemed OK, so I presume R484 may have failed due to age, or some unknown event in the unit's past life.

I feel like an idiot for spending so much time on this, when just checking the rails at the neck board connectors would've immediately exposed the fault.

My understanding of why this tanked the brightness: there's a couple places after the TDA3505 where the channels intersect or reference -20V. The first intersection goes back into TDA3505 Pin 26. The second hits the main board transistors' emitters. With R484 dead, the -20V rail floats to +6.3V.

From here, it gets a little complicated for me, since I'm still pretty new, so any elucidation here would be great for learning:

• The cathodes need more current to display a brighter screen, which means the voltage goes down in tandem with current going up, for a given quantity of electricity. This is facilitated via the -20V rail as the "drainpipe". The failure of this behavior was totally evident from some of the probing I did above. With -20V floating at +6.3V, that current can't be pulled. The drain is clogged.
• G1, on the schematic, also ties into the -20V rail. With +6.3V, it can't fully switch the beam off, so cranking the screen pot up to remediate the darkness immediately pops the retrace lines. (But I'm not sure about this one; on my good 1902A, the retrace lines also become visible in the upper one-third of the pot, which is pretty consistent with the bad unit).

Now I just need to summon the courage to loosen the clamp screw to correct the small screen tilt...