Commodore 128 colour problem?

[Sanxion]

Hi all,

On to my next issue - ah, the joys of owning classic machines  - my C128 seems to have a colour problem in that when the machine is first turned on, it will sometimes appear in black and white. After a while the green of the C128 main screen will appear but not in a uniform manner. There will still be patches of white along the border. If left on long enough, all the green will appear.

So far, I know the lead, monitor and power supply are fine as I have tested the with another 128. I have also checked the large 4 capacitors on the board near the power switch. They all tested fine but I decided to replace the 220uF and 100uF as I has replacements for those. None of the ICs seem to become very hot or even very warm.

What should I check next?

[agent_power]

Hi

Do you have this issue with the RGB or Y/C / composite video out?

Check the video clocks and adjust it with C20 if they're out of spec

http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c128/310378-2-right.gif

[Sanxion]

I have the problem with both RGB to Luma/Chroma and UHF.
I did adjust C20 turning it anti clockwise and for a brief moment the dark green appeared but once I removed the screwdriver, it was gone. Could the fault be with the C20?
I have also tried a known working VIC II with no difference.

[agent_power]

Are you using a metal tool to adjust it? You can try to use contact cleaner and exercise it.

I would attach a frequency counter or accurate enough scope to monitor the clock frequency while turning the trimmer pot.. It needs to be set precisely.

[Sanxion]

Yes, I was using a metal screwdriver.
Do you know what frequency it should be set to?

[mikerj]

Frequencies are noted on the schematic linked above, 14.31818 MHz for NTSC, 17.73447 MHz for PAL.

[agent_power]

The stray capacitance introduced by the metal screwdriver will influence the frequency while it's on the trimmer pot. If you don't have a plastic tool to adjust it you need to measure the frequency after  you remove it

[Sanxion]

As a side question...Why use a variable capacitor between 4-40pF at all? Why not use the "correct" value one so there is no need for the adjustment.

[TimFox]

Because the "actual" capacitor includes stray and device capacitances that are not known a priori, and even with the correct capacitance there is some tolerance on the crystal frequency.

[BrokenYugo]

If you lack proper alignment tools the basic screwdriver type is easy to carve from a disposable chopstick or other scrap piece of plastic/wood.