Commodore 128 colour problem?  - Page 1
Forum section: electronics / repair

author: Sanxion
author_rank: Contributor
timestamp: July 16, 2023, 12:00:49 pm
content: 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?

author: agent_power
author_rank: Regular Contributor
timestamp: July 17, 2023, 08:59:14 am
content: HiDo 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 spechttp://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c128/310378-2-right.gif

author: Sanxion
author_rank: Contributor
timestamp: July 17, 2023, 11:26:36 am
content: 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.

author: agent_power
author_rank: Regular Contributor
timestamp: July 18, 2023, 09:24:10 am
content: 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.

author: Sanxion
author_rank: Contributor
timestamp: July 18, 2023, 10:01:30 am
content: Yes, I was using a metal screwdriver.Do you know what frequency it should be set to?

author: mikerj
author_rank: Super Contributor
timestamp: July 18, 2023, 11:16:49 am
content: Frequencies are noted on the schematic linked above, 14.31818 MHz for NTSC, 17.73447 MHz for PAL.

author: agent_power
author_rank: Regular Contributor
timestamp: July 18, 2023, 11:43:10 am
content: 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

author: Sanxion
author_rank: Contributor
timestamp: July 18, 2023, 06:57:14 pm
content: 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.

author: TimFox
author_rank: Super Contributor
timestamp: July 18, 2023, 07:04:51 pm
content: Because the "actual" capacitor includes stray and device capacitances that are not knowna priori, and even with the correct capacitance there is some tolerance on the crystal frequency.

author: BrokenYugo
author_rank: Super Contributor
timestamp: July 19, 2023, 02:58:11 pm
content: 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.