Weird red-colour problem on my C64 Breadbin

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Lenkapo
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Weird red-colour problem on my C64 Breadbin

Post by Lenkapo »

So recently I´ve repaired a dumpster-found C64 breadbin rev.250407. It runs fine and everything is working until I´ve discovered that whenever it wants to display anything red or any colour that contains red for example purple, it just turns gray or some kind of gray-brownish mix. The C64 is a rev.250407
from '83 with a replacement 6569R4 PAL VIC II (wich probably got replaced sometimes in the past) from '87. Im using the S-Video capabilities of the C64 and a relatively good upscaler to display the video on a modern LCD monitor.
Picture: The upper colour should be red and the color below should be purple.
Another strange thing I´ve noticed is that the "upper" colour palette from 10-15 is also weird which means that most of the colors are either green,blue or this grayish stuff but never red. What is also really weird is that It runs fine without any crashes or other colours being bad ( I had a similar problem with a C64 in the past but that was just the colour ram and the system didn´t display only one colour wrong.
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eslapion
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Re: Weird red-colour problem on my C64 Breadbin

Post by eslapion »

Most likely candidates: the VIC-II (6569) or the color RAM
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Lenkapo
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Re: Weird red-colour problem on my C64 Breadbin

Post by Lenkapo »

I´ve already testet both of them in a different machine and they are both fine...
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Re: Weird red-colour problem on my C64 Breadbin

Post by eslapion »

The 4066 (U16) is responsible for switching the color RAM between an address range for the CPU and extra 4 bits to the VIC-II. It could be the cause.

However, the background color and border color are stored in registers in the VIC-II. Do these display all 16 colors fine ?
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Re: Weird red-colour problem on my C64 Breadbin

Post by Lenkapo »

The background has the same problem as the foreground. Could it be something else maybe in between the Vic and the RF Modulator or even inside the RF Modulator?
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Re: Weird red-colour problem on my C64 Breadbin

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Lenkapo wrote: Thu Jun 17, 2021 4:14 pm The background has the same problem as the foreground.
The picture you posted above says differently. Can you post an image to illustrate what you mean?
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Re: Weird red-colour problem on my C64 Breadbin

Post by Lenkapo »

I´ve just realized that what I said earlier isn´t true. The background and border colour have the same problem as the displayed text.
I´ve written a small basic Programm which goes through every background-border colour combination possible and every time it should display something red it becomes mostly the before stated grayish colour.
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Re: Weird red-colour problem on my C64 Breadbin

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Lenkapo wrote: Thu Jun 17, 2021 6:14 pm ... The background and border colour have the same problem as the displayed text.
The background color might be affected by problematic color RAM or problematic access thereto. However, the border color is a totally different ball game.

If the border color exhibits the problem you describe the problem is either between the VIC-II and the video output (including the RF modulator) OR it may also indicate a problem with the color clock signal received from the clock generator. Incidentally, the 250407 board does NOT use the 8701.

I suggest you check the frequency received on pins 21 and 22 of the VIC-II. On PAL systems, these should be 17.7344 MHz and 7.881 MHz respectively. PAL color encoding requires a high level of accuracy.
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Re: Weird red-colour problem on my C64 Breadbin

Post by Lenkapo »

eslapion wrote: Fri Jun 18, 2021 2:56 pm
Lenkapo wrote: Thu Jun 17, 2021 6:14 pm ... The background and border colour have the same problem as the displayed text.
The background color might be affected by problematic color RAM or problematic access thereto. However, the border color is a totally different ball game.

If the border color exhibits the problem you describe the problem is either between the VIC-II and the video output (including the RF modulator) OR it may also indicate a problem with the color clock signal received from the clock generator. Incidentally, the 250407 board does NOT use the 8701.

I suggest you check the frequency received on pins 21 and 22 of the VIC-II. On PAL systems, these should be 17.7344 MHz and 7.881 MHz respectively. PAL color encoding requires a high level of accuracy.
Thanks for the hint with the clocks, the colour clock was a bit too low but I´ve already adjusted the colour clock by adjusting the potentiometer R27 near the VIC so that it is spot on. I´ve also opened up the RF Modulator, removed some corrosion on the underside and re-capped the whole thing. But sadly still no change.
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