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Commodore 64 video problems

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First post, by vkcpolice

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Hi all i have had this beautiful commodore 64 sitting around for a while now as it has this video output problem where you can see in the images that the output has this sort of rainbow effect across the screen.
you can see it in the basic text on screen. it seems to be where the jail bars go down the screen. i have been told that you can buy a luma fix for the video chip i have one of those in my other commodore computer but that was to fix the jail bars not a rainbow sort of problem.
also in the pics you can see some of the capacitors have some white marks on them. not sure if this is normal or not or if they are causing the issue.
i recently bought a brand new video chip for the computer but it had no change on the rainbow effect.
the issue happens on all outputs s/video composite and rf.
aslo the computer itself is 100% working with no issues with software or anything the video output is the computers only problem
if anyone know if this is an easy fix or not please let me know.
or if a luma fix would fix this issue thanks all

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Reply 1 of 26, by zyga64

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I'm not sure about old mainboard, but for me (C64S - short board with separate color ram) replacing RF-modulator improved the picture significantly (on my C=1084S).
https://github.com/tebl/C64-Modulator-Replacement - this one is for old/long board
I also have lumafix, but It doesn't help at all (in my case).

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Reply 2 of 26, by Deunan

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Yup, the C64 output is poor on modern LCDs and even older HD CRTs. The RF modulator tends to inject some noise into the signal and the video out must pass through the modulator module because there's a 2-transistor amp in there. The point of the replacement "modulator" is to get rid of all the TV frequency stuff and have just the video amp, possibly also with some small improvements. I've created my own replacement for a C64 mobo that came without a modulator at all (some people remove it for swapping parts since they also tend to die completly) and it works pretty well now. So, unless you need your C64 fully original or you want that TV antenna output I'd also recommend replacing the modulator with just an amp.

I've seen people try and tweak the original modulator to improve the signal quality (less cross-talk from RF head to the video out) but results vary a lot, and that requires desoldering the modulator and messing with it anyway.

Reply 4 of 26, by badmojo

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Not much to be done about it in my experience but the cable you're using does make a difference I've found. I tried a few and one of them - which was cheaper than the rest - had a noticeably better output. Sadly I can't remember where I got it, sorry!

And yes the lumafix thing can improve the image quality but my experience was that if you made one set of colours look better (blue on blue boot screen for example) then another would look worse. So I gave up on it after a while - better for the image to be consistently stripey than variable I decided.

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Reply 5 of 26, by vkcpolice

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ok thanks i know its not the cable as i use it on my c64c that has the luma fix installed and that has no rainbow effect down the screen. i am no good at soldering so not sure what i can do here.
mscdex thanks i had seen that vid a while ago but was not really gonna mess around with stuff incase i broke the computer.
seems to been a luma leak i use s/video so it looks a lot worse than on composite but composite is just so blurry i never use it

Reply 6 of 26, by Jo22

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Deunan wrote on 2021-10-11, 19:35:

Yup, the C64 output is poor on modern LCDs and even older HD CRTs. The RF modulator tends to inject some noise into the signal and the video out must pass through the modulator module because there's a 2-transistor amp in there. The point of the replacement "modulator" is to get rid of all the TV frequency stuff and have just the video amp, possibly also with some small improvements. I've created my own replacement for a C64 mobo that came without a modulator at all (some people remove it for swapping parts since they also tend to die completly) and it works pretty well now. So, unless you need your C64 fully original or you want that TV antenna output I'd also recommend replacing the modulator with just an amp.

I've seen people try and tweak the original modulator to improve the signal quality (less cross-talk from RF head to the video out) but results vary a lot, and that requires desoldering the modulator and messing with it anyway.

By coincidence, I also tinker with the C64 at the moment.
I got mine by a friend who passed away many years ago.
It was the PLA chip which was defective on his unit, so I replaced with an alternative address decoder.

From what I have learnt, it's not really the RF emulator that's the culprit.
I do have many old devices that provide pretty pictures via RF.
It's the C64 itself who is an RF mud-slinger.
The worst users can do is to permanently remove the shielding for convenience.

Edit: The RF level increases eight (8) times without that metal/paper shielding.
Source: https://www.c64-wiki.de/wiki/EMV

Anyway, the video circuitry itself (on C64 pcb and inside RF modulator block) is also to blame. The Chroma signal pollutes the Luma signal, so according to some web sites, moire patterns can occur if using b/w monitors and a chroma/luma cable.
That's why I thought about cutting the Chroma connection inside my C64, making it monochrome only.

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Reply 7 of 26, by Deunan

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Jo22 wrote on 2021-10-12, 12:45:

By coincidence, I also tinker with the C64 at the moment.

Well, it's true that the main issue with using S-Video output on C64 is cross-talk between luma and chroma. But the reason why this cross-talk exists it due to modulator, it needs to combine the two to make a TV singal out of it. The combined result, before it's shifted to VHF/UHF, is also output as composite. The best thing to do is to remove a resistor or two from the mixing path, thus loosing both composite and HF output, and prevent the cross-talk from happening. And since this makes the modulator somewhat useless why not just get rid of it and re-make the video amp properly? This way also any HF feedback/noise is removed, and power usage is sligthly lower as well. In fact there's little need to shield such amp in a metal box, from experience it doesn't cause or receive any extra noise the TV would pick up.

I know that some C64 models also have their own issues with the video chip and its internal cross-talk, as well as the subpar filters between this chip and modulator. That too can be re-touched to improve signal quality but I found (granted, with very limited sample count) that just replacing the modulator box with a decent amp for S-Video only makes a world of difference.

Reply 8 of 26, by Jo22

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+1

I just meant to say it's the RF Modulator™, but not the RF modulator. :)

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Reply 10 of 26, by Deunan

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In essence, this: https://github.com/tebl/C64-Modulator-Replace … ator_simple.png

This is just a part of the original C64 circuit inside the modulator box. I strongly suggest getting rid of composite mixing, so at minimum C6 should not be populated. This will let you have pure luma on composite output and easy way to add chroma back if you need it. Better yet is to disable composite entirely and get rid of C6, R8, R11 and possibly L2 (though L2+C4 might have some other benefits like better rejection of harmonics for luma - so in my own circuit I left it be).

Reply 12 of 26, by Deunan

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Yes. In fact if you take all of those out then S-Video will be the only thing you'll have. If you want to try and mod the modulator then you can either remove (actually, just lifting one pin is enough) C6 or R8, that's the minimal change required to disable mixing of the luma and chroma signals. This alone should greatly improve S-Video output, if it does not then perhaps your C64 also needs other mods in the video chip output filter circuitry. But if it does work then you can decide if it's good enough or if you need remove the rest, or even replace the modulator as a whole.

A word of warning, this is a diagram for one version of the C64, there were some other modulators in there too (depending on the mobo/video chip). In general the layout and inner workings are the same, but part numbering might not be, so make sure you are removing the right part. Preferably desolder it without damaging in case it needs to go back. Also this particular replacement amp, as many others on the net, is kinda broken. The author left the D1 diode in but removed the power bypass capacitors - there's one electrolytic and one or two ceramics plus a choke, but that choke is not requied without the HF part of the modulator. The omission is most likely due to these parts being drawn as part of the HF section of the modulator, so people just cut that away and also end up removing power filters. If you end up building your own amp I'd recommend looking at original schematics.

EDIT: Ah, one more thing. It seems many modern TVs don't like the pretty high drive of the chroma signal that this circuit does. I changed that for my own amp, basically replace R4 (68 ohm) with 180 ohm, and R5 (220 ohm) with 150 ohm. These are the values I went with and I'm pretty happy with the results.

Reply 13 of 26, by liqmat

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These photos are back in 2015 when I was restoring a slew of C64 and VIC-20s. As you can see, the image on that LCD TV is crystal clear with no modifications made on the Commodore systems themselves. Used a composite A/V cable I bought on Ebay.

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Reply 14 of 26, by vkcpolice

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do you have to put a wire between where the resisters used to be or do you just completely remove them and break the trace?
also where are those resisters located are they the tiny ones inside the modulator box or the ones in the video box near the video chip?

Reply 16 of 26, by Jo22

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I haven't tried yet, but to my understanding, disconnecting C6, by removing or lifting one leg, could get rid of the noise comming from the Chroma signal without completely breaking the RF modulator or the normal AV cables.

Both would still work and output clean video in grayscale, because the pure Luma signal with its sync+blanking signal is still there (VBS).

Also, as a bonus, the RF generating part of the modulator circuit would still have something meaningful to do,
which means it would not start oscillating in an uncontrolled fashion.
This could happen if it was left powered on and if its input pin was just left floating.
Anyway, no big deal. Just came to mind.

So an old school TV could still be connected via RF or a crisp video monitor via AV cable.
Whilst a modern colour TV could use an S-Video cable..
Or a Luma/Chroma cable, which essentially is the same except for the RCA connectors and slightly different voltage levels (resistors/caps can adapt them).

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In what to one race is no time at all, another race can rise and fall..." - The Minstrel

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Reply 17 of 26, by vkcpolice

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i had a look at some of the mods those resisters are way way too small for me to work on.
i tried doing the mod that was in the vid that was posted but there is just no way i can do it.
my c64c does not have this luma problem nor did my old breadbin so it seems to be related to this board that is a pal board rev A

Reply 18 of 26, by Jo22

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liqmat wrote on 2021-10-13, 20:51:

These photos are back in 2015 when I was restoring a slew of C64 and VIC-20s. As you can see, the image on that LCD TV is crystal clear with no modifications made on the Commodore systems themselves. Used a composite A/V cable I bought on Ebay.

c64_basic.jpgc64-vic20_restoration.jpg

Um, does that screen use a scaler maybe?
On my b/w tv from the 70s there is noise, with vertical stripes (using the stock RF modulator box). 🙁

"Time, it seems, doesn't flow. For some it's fast, for some it's slow.
In what to one race is no time at all, another race can rise and fall..." - The Minstrel

//My video channel//

Reply 19 of 26, by liqmat

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Jo22 wrote on 2021-10-26, 17:48:
liqmat wrote on 2021-10-13, 20:51:

These photos are back in 2015 when I was restoring a slew of C64 and VIC-20s. As you can see, the image on that LCD TV is crystal clear with no modifications made on the Commodore systems themselves. Used a composite A/V cable I bought on Ebay.

c64_basic.jpgc64-vic20_restoration.jpg

Um, does that screen use a scaler maybe?
On my b/w tv from the 70s there is noise, with vertical stripes (using the stock RF modulator box). 🙁

I don't have any of that anymore, but this is the model.

Paired with one of these.

It was a crystal clean image.

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