I got this chassis for a cabinet that I’m JAMMAfying. It’s the 25-29″ version of the chassis. The chassis arrived in a generic box that almost didn’t belong, and no manual or documentation of any sort.
The quality of the pcb looked ok at first glance, but then I discovered a number of issues. One high voltage capacitor was a pull complete with an old QC sticker from what appears to be the previous decade. Wires to the yoke were too thin, power plug wires were barely soldered in and seemed like no thicker than 22 gauge.
Then I couldn’t figure out whether the tube I had was correct since I had no info. Found some posts online about yoke acceptable yoke resistance:
- Horizontal – 1.3Ω to 2.5Ω
- Vertical – 6.0Ω to 12.0Ω
My tube had 1.4Ω horizontal and 4.1Ω vertical. I was worried about the vertical being way too low and blowing it. Turns out I was over reacting. Later on, I accidentally connected the horizontal yoke coil to the vertical connector and didn’t notice for a good minute. Chassis ended up surviving that.
These have a number of issues including issues with geometry, no degaussing coil, bloom artifact. They are also difficult to adjust with many knobs. Made a video about this one as well which may be useful to some.
I will list the things I did to correct the problem for each issue that I encountered. I will continue to update this page with more info as I deal with the issues one by one.
Geomery Issues
- Vertical can’t stretch all the way with max setting – replace LA7840 with LA7841 vertical driver IC
- Vertical too big at min setting – Replace LA7841+ vertical IC with LA7840
Color Mod
These chassis have a problem with colors washing out when the CRT is saturated. The problem occurs due to improper (in my opinion) AC coupling at the video signal input pins.
What ends up happening is, the bright colors saturate the AC coupling capacitors (top left corner of the schematics) and once fully charged the capacitors do not allow a strong RGB signal to pass through resulting in dull colors. I made a video on this with the recommendation to increase the coupling capacitors or eliminate them completely.
This mod reduces the blooming issues somewhat caused by brightness overdrive.
The capacitors may be there to prevent a DC offset from the monitor chassis to get into your equipment, so do this mod at your own risk.
Schematics
Will update this page with more info as I learn more.