84V Mod on Color Classic - Thick light gray bars

[surfer150]

Hello together.

Mainly motivated by JDW’s excellent videos on YouTube, I decided to do the 84V VGA mod on my Color Classic too. To accomplish that, I cut the trace between pin 8 and pin 9, protected the copper with silicone (as i don’t have solder mask) and then connected pin 8 and pin 10 using a thick 1mm2 wire. Then I moved the 0 ohms resistor from J78 to J79. I ensured that nothing is shorted by taking measurements with my multimeter and compared the resistance of pin 8 and pin 10 with that of pin 8 to pin 9. I measured 0.05 Ohms vs. 0.25 Ohms.

After powering it on with the stock logic board installed, I noticed a wide light gray (lit) border followed by a dark border. Also the picture is a little bit distorted as noticeable in the second and third picture.

It’s also noticeable that the light gray border moves up and down as well when turning the vertical trimmer on the back on the device. Also it’s wider on the upper side than on the lower side. If I turn the horizontal trimmer, the screen content of the OS moves from left to right but no changes in the bars. Is the cutoff set too high or is this another issue? I know that doing the VGA mod involves readjusting the tube but I want to ensure if this is really only an adjustment related issue.

Also I’m noticing a humming/whistling noise coming from the tube which was not there before i performed the mod.

The next step will be doing the beefy components upgrade next week and putting in a LC575 logic board once it arrives from Japan.

 

[jonschultz]

Hi @surfer150, this is an easy fix. What you're seeing is that the G2 voltage (sort of like the black level control right at the tube) is too high and you're also starting to see "retrace lines". This is easily adjusted with the G2 potentiometer ("PL2 VG2"), on the left side of the analog board, just in front of the black heatsink for the horiztonal output transistor. You'll need a 2.5mm hex driver to make the adjustment. Just turn it down until the light grayish border disappears. This is a very commonly required adjustment any time something is changed with regard to the flyback transformer or B+ voltage. This grayish region will appear in any area that is actively being rastered because you're getting slight cathode emission when it's really supposed to be off. So it will move vertically with the "VS" control. The horizontal shift control doesn't actually change the position of the raster in the horizontal position, only where the video is traced in relation to it, and that's why the gray does not move with "HS".