Adrian's RF modulator video cleanup

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rmzalbar
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Adrian's RF modulator video cleanup

Post by rmzalbar »

This evening I applied Adrian's RF modulator fixes to one of my 250407. Two capacitor filters disabled and an inductor bypassed. A major improvement, definitely worth doing. Video quality looks much closer to my C64C.



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banman
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Re: Adrian's RF modulator video cleanup

Post by banman »

Hi rmzalbar,

Well done!

I don't suppose you have before and after shots of the C64 video output for comparison from your modded machine?
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Re: Adrian's RF modulator video cleanup

Post by rmzalbar »

I don't, but my results match what you can see in Adrian's video. I can post some after:

This is Y/C mode, not composite, but there is a huge difference. Nice square pixels, certainly to to the limit of the monitor's dot pitch.



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Last edited by rmzalbar on Thu May 07, 2020 10:01 am, edited 2 times in total.
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Re: Adrian's RF modulator video cleanup

Post by rmzalbar »

I mean.. just LOOK at that. The monitor's sharpness setting is set to zero!


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Re: Adrian's RF modulator video cleanup

Post by banman »

Hi rmzalbar,


Yes! I'll pay that,.... :D

These pictures are a very compelling argument....

How hard did you find doing the mod and are there any potential pitfalls in doing this for ones self?

My prowess with the side cutters and soldering iron are doutable at best....
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Re: Adrian's RF modulator video cleanup

Post by rmzalbar »

With the NTSC version there are two capcitors to clip and an inductor to bypass. Clipping the capacitors is easy, even if you don't have precision flush cutters to reach the leads, you can just break the ceramic capacitors. I did solder a wire across the inductor, leaving it in place, but it's a little easier to just clip it off and solder to the stubs.

If yours is a PAL model, you only have one capacitor to clip and no inductor, so it's even easier. Just watch the video to see the process. He didn't get as much of an improvement with PAL as he did for NTSC, but he also shows on his oscilloscope that his PAL VIC, ceramic, puts out a weaker signal, so that may just be his particular chip.
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Re: Adrian's RF modulator video cleanup

Post by banman »

Hi rmzalbar,

Gee,the Adrian's video is good. Gives me some confidence to try to do a mod myself.

I like the way he explains how it's all done and the schematics.
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Re: Adrian's RF modulator video cleanup

Post by banman »

Hi rmzalbar,

I was looking for something about how to troubleshoot the C64's rf modulator. I came across this....




I have one Longboard breadbin C64 that has symptoms very similar to what Adrian has.

I get sound from the 8 pin video connector and nothing from the RF modulator....


I might try this method out and hopefully will get it working again.
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Re: Adrian's RF modulator video cleanup

Post by rmzalbar »

Sorry I didn't see this before. Yes the potentiometer tunes the carrier frequency for the RF audio encoding. One thing to note if you are dealing with a PAL C64, is that there is a jumper to switch between German PAL and UK PAL, and what that jumper changes is the audio carrier! Look for a trace marked G.PAL / I.PAL if memory serves. It may be near the RF modulator.

Edit: Yeah..
2020-09-01_10-36-20.png
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Re: Adrian's RF modulator video cleanup

Post by banman »

Hi rmzalbar,

No stress. I hope you are well in these uncertain times.


Thanks for the information. I will have a good look at this.
I would like to get the audio going thru the RF section again.

It must be a relatively easy fix. The SID chip works fine using the the 8 pin AV connector.
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