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Commodore 128 color problem

S

Sanxion

Member
Joined
Aug 25, 2018
Messages
21
Hello all,

I have a problem with my Commodore 128 that I have not been able to make any progress with and I am hoping someone out there can help...

The colors are faded to the point where it looks like a black and white screen. Although, after a few minutes, some "colour" will appear at different points on the screen, usually around the border.

I can discount the following:

1. The monitor
2. The cable
3. The capacitors
4. The VDU IC
5. RGB vs RF issue

So...my question is, what do I look at next?
 
1. The monitor - I have tried a different C128, C64 and C128D. No problems with those.
2. The cable - As above.
3. The capacitors - I have changed them.
4. The VDU IC - I have tried a known good one from a different machine. Problem persists.
5. RGB vs RF issue - Problem occurs with both connections.
 
Some questions for you.
Has the problem suddenly appeared on a previously working system?
Is your system PAL or NTSC?

Note: the 1701 monitor does not support RGB or RF.
 
Since I acquired this C128, the colour has never been constant and correct. It would fade in and out when first turned on and eventually settle. However, it has progressively become worse and now the colours are constantly pale. This led me to think that a component had worn out or failed.
The system is PAL.
Regarding the 1701, I have tried a different monitor and a standard TV with RF. All the display devices exhibited the same problem. I am therefore certain the problem is with my C128.
 
Sanxion said:
Since I acquired this C128, the colour has never been constant and correct. It would fade in and out when first turned on and eventually settle. However, it has progressively become worse and now the colours are constantly pale. This led me to think that a component had worn out or failed.
The system is PAL.
Regarding the 1701, I have tried a different monitor and a standard TV with RF. All the display devices exhibited the same problem. I am therefore certain the problem is with my C128.
Click to expand...
Its possible the colour subcarrier frequency is off, or drifting. If you look closely at coloured areas when the colour is low, do you see 'Hanover Bars'? These are fine horizontal lines of dim and brighter colour. Wikipedia has an article with pictures of what they look like.

There is a trimmer capacitor (CT1 in the C128DCR), located between the colour crystal and the 8564 chip. While it really should be adjusted with proper test equipment, try adjusting it carefully with a non metallic screwdriver. Aim for the least amount of Hanover Bars (preferably none), and maximum colour stability where the colour is consistent every time the C128 is switched on, and does not change when the C128 is cold or hot.
 
I actually replaced that capacitor a few weeks ago...when I adjust it, I can eliminate the Hanover Bars to the point where the borders appear smooth. It does not however seem to affect the colour at all.

Bizzarely, when I switched the C128 on today - after days of inactivity - the colours were perfect. Rich and vibrant. I couldn't believe it...so I carefully reassembled the machine, replacing the shielding and keyboard, turned it on, back to the same problem 🙁
 
Machines that don't work and suddenly do are suspicious. And I certainly would not assemble them in the hope they keep working. You must have heard of a guy named Murphy? He must have played a trick with you.

OK, serious now. I don't believe in parts that don't work suddenly will work. Broken is broken. But I do believe in bad solder connections or broken lines. Disassemble the board, start it up and start bending it a bit in several directions and see the results. No guarantee this helps but better than doing nothing.
 
If it's a luma/chroma connection to the monitor, you may losing contact with the chroma pin. If only luma is making it through to the monitor, the image will be black & white.
 
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