Here is a picture of the problem(s) before I get into details.
Background
- I have a CRT that has at least two issues - colour distortion and pincushion failure.
- This board has previously been worked on by someone who did a poor partial recap with ChongX type capacitors, leaving the board with significant damage, especially to the pads.
- This post was originally written for Reddit, where I'd been posting my progress, but it wasn't getting much traction. I've edited it to post here in the hopes of getting more advice.
- I have finished replacing all the problem caps, including upgrading the common problem caps as specified in the service manual.
- I have reflowed several key joints.
- I identified a broken trace that meant the image processing chip (IC404 - CXA1478) did not have the Green and Blue Ground Pins connected to Ground. I have fixed this (see pics).
- I identified a broken trace to capacitor C541. This appears to be a filtering cap connected to several pins on IC507. See pics for the repair.
- I identified a broken trace around capacitor C1500 that I have repaired with a jumper wire. This was a tricky fix and it's a bit messy but I think it's ok now. I have tested with the multimeter and the only reason for my uncertainty is the difficulty in following the low-quality schematics and scans in the service manual. This cap appears to a filter on the 72.HL-DL line which I believe is part of the deflection circuit. (pic of repair)
With the things I found above, I was certain that I'd fixed at least one of the problems. Unfortunately that isn't the case - but the problems at least seem more "consistent" now, rather than appearing after a warm-up period. The problems are:
- When the screen should be blank, I am seeing a green scan-line effect.
- Pincusion failure.
- Front panel brightness, chrome, etc. dials appear to be working as you would expect them to.
- With underscan enabled, I can see the RGB lines where they should be, so that circuit is working and the system is capable of displaying all 3 colours.
- When in "blue only" mode, the green tint seems to turn blue. However I can also see a blue tint when overscan is enabled. I think it's possible the blue tint is always there but I just cant see it past the green.
- There is significant discolouration on the screen with the OSD.
- Looking again at the board, it is clear that IC404 has been removed and replaced at some point, as there is some minor damage around the pins and the soldering there was of poor quality.
- I think the pincushion failure is probably a failure in the circuit around IC509, most likely Q503 or C579, based on other people's posts.
- I now think the green tint might be a short somewhere allowing signal to bleed into the output, most likely around IC404.
- I am less sure about the blue. The green seems to be contained within the "screen space" while the blue seems to travel beyond, which makes me wonder if noise is bleeding into the blue signal line somewhere.
- I have ordered replacements for IC509 and Q503, and I'm optimistic that replacing one or both of those will fix the pincushion failure.
- However I am slightly concerned given I'm worrying about signal bleed and noise that I need to track down the source of the green and blue tints. My next course of action here is to use my oscilloscope to take readings around IC404 and compare them to the service manual, along with manually verifying connectivity around IC404.
- I think I also want to double-check all the filtering caps and make sure that they have continuity and are the right way around, especially working back from the neckboard on the blue signal line.