NTSC modulator on old computer question - Page 1
Forum section: electronics / projects

author: alank2
author_rank: Super Contributor
timestamp: November 17, 2020, 12:10:39 am
content: I've been working on a commodore 64 for fun and have it working mostly, but the RF modulator has been a bit troublesome. There is a transistor in the lower left of the unit that if you touch it can make the picture appear or disappear so I wonder if it has a bad connection. If it is a PN2222 I may just replace it when I replace the caps with a cap kit I ordered.It has 4 adjustable pots. Two that are accessible though the metal lid cover and two that you have to take the cover off to access. Any idea what these pots would adjust for? I tried turning each 1/4 turn only in either direction and didn't see much difference (I only did a 1/4 turn so I could put them back there they started).

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: November 17, 2020, 03:10:04 am
content: There are many kinds of RF modulator on the 64 and the "modulator" also contains baseband driver circuitry for the video jack.https://www.nightfallcrew.com/wp-content/gallery/easy-trick-to-improve-a-little-bit-the-video-quality-for-the-commodore-64/modulator-251025.pngThe reason the RF signal disappears when you touch something is that your finger is pretty much a good conductor at those frequencies.It's not a 2N2222, and don't replace anything unless you have a reason. If it displays a picture just fine when you don't touch it, why replace anything?

author: alank2
author_rank: Super Contributor
timestamp: November 17, 2020, 03:25:30 am
content: Thanks Alex; I should have been more clear - there has to be a touchy connection with the transistor. Sometimes the picture works, sometimes not. Tapping the transistor brought it back.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: November 17, 2020, 03:26:57 am
content: It's a cheap and cheerful one layer board, so there are no plated vias, it's entirely possible that simply reflowing the solder joints will help.

author: james_s
author_rank: Super Contributor
timestamp: November 17, 2020, 04:22:50 am
content: I would bet that it's just a solder joint. While not impossible, it's rare for semiconductors to become sensitive to mechanical vibration.

author: alank2
author_rank: Super Contributor
timestamp: November 17, 2020, 01:17:39 pm
content: Thanks guys; I'll try to rewet all the joints and see how it does.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 17, 2020, 01:56:07 pm
content: Given how obsolete and low quality the "RF" output is, wouldn't it make more sense to just disconnect the modulator and replace it with a wire link to give you composite out on the RCA connector?

author: alank2
author_rank: Super Contributor
timestamp: November 17, 2020, 03:05:51 pm
content: I don't disagree Mike, they make modulator replacements that replace it with a s-video jack which is great, but my C64 is a very early one. I saw a way to try to get s-video out of the DIN connector that I am going to try. I'd like to get/keep the RF working just to keep it original if I can.

author: Benta
author_rank: Super Contributor
timestamp: November 17, 2020, 08:29:53 pm
content: If it's an early C64, it's probably only got the 5-pin DIN connector for video.This outputs CVBS, which is also fine. You don't really need S-video for color rendering.

author: james_s
author_rank: Super Contributor
timestamp: November 17, 2020, 08:40:09 pm
content: I don't really see the point in trying to improve things too much anyway. If you want perfect razor sharp RGB video just run an emulator on a modern PC. Part of the point of using original vintage hardware is to get the original vintage experience IMHO.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: November 17, 2020, 10:40:21 pm
content: Quote from: NiHaoMike on November 17, 2020, 01:56:07 pmDepends on the 64; the "RF modulator" also contains the transistors that buffer the baseband video signals. Removing or disabling the modulator means no more video output at all.Observe:http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c64/250469-rev.A-right.gifI'm not too sure anymore about which versions of the 64 have a separate discrete circuit on the mobo, there you can remove the modulator IIRC. I know the SX-64 has no RF output so there it's moot. IDK about the 128 either. They're like on my other desk.Quote from: james_s on November 17, 2020, 08:40:09 pmThere are different kinds of vintage enthusiasts. I am of the "original hardware" variety. I like buying the crap I couldn't afford or never even heard of as a kid. It makes no sense in any case. But that's what a hobby is!

author: alank2
author_rank: Super Contributor
timestamp: November 18, 2020, 02:09:34 pm
content: One question I have - how can I measure the composite signal to see if it is about the right voltage level?I connected it to my scope yesterday and the vp-p was 1.6V or something, but it would be lower if the color component wasn't as strong I think.I ask because a mods to make the signal stronger was changing R10 from 300 ohms to 120 ohms to match the schematic, but I am thinking this perhaps made it too strong. My LCD monitor shows a horizontal line through text on occasion and the capital I has a swirly sort of artifact around it. When I connect it to a real CRT, it doesn't show these things, but they do show up on the LCD.I'm thinking of trying something in between 120 and 300 ohms to see it how does, but it would probably be best if I knew exactly what part of the signal to measure.Also, there was no way I was getting the modulator box off, even with the FR300, I couldn't pull enough solder out to free it. I was lucky the loose connection in the transistor was accessible/fixable without taking it off.ALSO - I tried to do the svideo thing I saw where you use the composite as chroma (my c64 is too old to have a real chroma, only 5 pin video). I used a resistor inline on the luma (150 ohms) and chroma (330 ohms) and while the video was stable (again LCD svideo in), it did not look good and had a very rainbow-ish appearance. I would build a svideo cable if it would look good, but maybe composite is the best that can be done.

author: james_s
author_rank: Super Contributor
timestamp: November 18, 2020, 06:24:21 pm
content: The NTSC specs are widely available. Keep in mind devices expect to be driving a 75 ohm load so if you connect a scope without terminating the line the voltage will usually be considerably higher than it is when driving a monitor.