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Commodore 128 - No chroma/luma video output? Help!

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First post, by Durandal

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Hey all,

I recently bought a Commodore 128 with a 1901 monitor and can't seem to get it to display anything using a chroma/luma/audio cable. Whenever I try using the cable it displays some intermittent flashing diagonal-ish stripes of colours (looks like green and purple mostly) that eventually sort of "die down" and almost go away completely. I have to turn the monitor and C128 off and on again to get them to show up in full force.

Further info + what I've tried already:

- the cable is an official 8-pin Commodore cable and is confirmed to be working perfectly with my C64C and the 1901 monitor

- RGBI in 80-column mode on the C128 works fine with the 1901

- Just to test it, I stuck some wires into the video socket connected to an RCA jack on the other end, and successfully got composite video on a regular TV in 40-column mode

- RF also worked fine on that same TV

- checked for cold solder joints on the video port and everything has perfect continuity

So what gives? 😢 😢 I tried doing the same thing as I did with the composite wire, but for the chroma and luma wires, and it didn't work. I got the exact same symptoms as when using the official Commodore cable.

Other than this problem, the C128 is working fine in every other way - I've tested it with 3 different disk drives and a datasette in both 128 and C64 mode via RF and it's all working great...

(PS. also posted on Lemon64)

Reply 1 of 3, by retardware

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Was the 80-column output actually accessible on the composite output?
Iirc back then only 40 (at most 64) chars/line were considered acceptable when using composite color, because of its low color bandwidth.

Maybe the C128 detects whether a 1901 is connected via RGBI, and uses a different frequency/sync pattern?

Reply 2 of 3, by Durandal

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retardware wrote:

Was the 80-column output actually accessible on the composite output?
Iirc back then only 40 (at most 64) chars/line were considered acceptable when using composite color, because of its low color bandwidth.

Maybe the C128 detects whether a 1901 is connected via RGBI, and uses a different frequency/sync pattern?

No, 40-column only it seems in composite and chroma/luma mode. I don't think the C128 knows when it's connected via RGBI, apart from the 40/80 column toggle switch.

Pretty sure I figured out the problem though thanks to a genius suggestion from a friend. He said that even though every other form of video output was working, the RF modulator is likely still the problem. So I opened up my C64C because I had a hunch that it would be using the same modulator - I was right. My friend told me to bridge 4 connections from the C64C modulator to the C128's, plug the chroma/luma cable into the C64C's port, remove the C64C's VIC-II chip, and power up the C128.

Low and behold:
5evRm7u.jpg

So apparently I need a replacement modulator. Anyone got a spare from a C64C or C128 they can sell me? 😊

Reply 3 of 3, by FuzzyLogic

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You can probably fix it as it's not too complicated. Or you can just remove it and put in a bypass circuit to get the chroma/luma/sound out.

Be careful when removing the modulator. It's easy to pull out a copper via or two when unsoldering it. I had to make one by wrapping a thin copper sheet around a thick sewing needle and gluing into the through-hole. Or you can use rivets if you find the right size.

Check out the Lemon64 site for your Commodore needs.

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