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RF Interference on Composite modded 2600? Homebrews only


AtariLeaf

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This is an odd problem. I purchased a composite modded Vader from a forum member and all seemed fine. Today however I went to play Draconian and I get what looks like bad RF interference. I checked some other games, original games and they all seemed fine. However other homebrews like Fall Down and Lady Bug are also showing these wavy lines. Any idea what this might be and why it seems to only affect homebrews? Like I said this didn't happen when I first got this machine, even with the above mentioned homebrews. I've tried various video cables but no change. Power supplies swapped and also no change. Is it even RF interference and if so how can that be when the RF is not even in play here?

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Edited by AtariLeaf
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I have a similar issue with an original four switch. All original carts were fine although Pitfall 2 was had some light lines. For the homebrews, SF2 looks like your picture and Stella's Stocking kept resetting in the main menu. I have a couple othe homebrews that are fine. Wierldly Stay Frosty 2 would play perfectly from the Harmony cart. I bought a six switch (I wanted anyway) which plays them perfectlly so its some strange homebrew cart/console specific thing. It would be nice to isolate the problem, but I don't intend to modify the four switch as it is my original console from 1981 and plays the original library. Do you have a mother console to try?

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Edited by 5x7
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I've always had issues with bad pictures in some homebrews but I didn't think that issue would crop up in a composite modded unit. The only thing I can think of is to replace the big cap on the board. The voltage regulator was replaced and switches cleaned but I don't think any caps were swapped out

Edited by AtariLeaf
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That's very strange that you'd get any "RF" type interference with a composite modified system. I have around 20 such systems that I use at home and shows and I have never seen interference like this from any of the homebrew games I have run in them (which is pretty much all of them!) Most of the systems use Longhorn Engineer's mod and have composite and s-video output. Also have a bunch of systems with Chris Wilkson's "CyberTech S-Video Mod" (the first 2600 systems I modified in this fashion) and never saw interference issues with those, either. The entire point of a composite and/or s-video mod is to eliminate this type of interference.

 

Now, having said that, obviously you are seeing some type of interference from these games. Not sure what would be causing that, and I am not an electrical engineer so I will not begin to pose a theory. But hopefully others more knowledgeable about this subject and the 2600's hardware can chime in. I will say that 2600 homebrew games do not have the metal shielding that Atari's cartridges had, so they certainly emit more RF interference, and Melody boards seem to emit even more than average, in my experience. Something in your system seems to be picking up this interference.

 

Do you know what video mod was installed in your system?

 

..Al

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I will add that Omicron is unplayable on that same system if I remember correctly . The screen was I think mostly black or very dark and I almost couldn't see anything . I think the mod board was from Electronic Sentimentalities . Omicron was the only one that was dark and games like Toyshop Trouble and Falldown had a lot of static . Again , it has been quite a long time since I used that modded console since I use a modded 7800 most of the time . I could always try to pull the system out again and try other homebrews .

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I will add that Omicron is unplayable on that same system if I remember correctly . The screen was I think mostly black or very dark and I almost couldn't see anything . I think the mod board was from Electronic Sentimentalities . Omicron was the only one that was dark and games like Toyshop Trouble and Falldown had a lot of static . Again , it has been quite a long time since I used that modded console since I use a modded 7800 most of the time . I could always try to pull the system out again and try other homebrews .

 

That's strange, as Electronics Sentimalities modified a good number of the systems I have, and I'm pretty sure he's using the Longhorn Engineer mod. Again, it is strange that A/V-modified systems pick up this interference, since eliminating that noise is one of the biggest benefits in performing such a mod.

 

..Al

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Yeah it has been so long so I would hate to say my post is 100 percent accurate for all the issues being on a modded console . I know for a fact Toyshop Trouble and Falldown had static issues but can't 100 percent say it was on that exact modded console . But I also know that Omicron had issues on that modded console . I will pull it out tomorrow and try again .

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I'll ask the member who sold me the console what the mod was called but the RF is removed completely so there's no chance of stray interference coming from it. I'll try another tv or two tonight, this was one of my Commodore 1702's that I normally use for playing. When I do initial diagnosis for things like his I do usually try other televisions at first but with this only affecting homebrews I didn't think it was necessary but you never know. Could it be a bad cap that causes this? The big 2200uF main cap perhaps?

 

This brings me to my next question - is it possible that with all the new tech in these modern games like Draconian that they're putting some extra strain on the Atari and it affects it like this? I'm no expert so I'm just postulating at this point but other than lack of shielding it's the only difference between a new homebrew and something released in the 70's.

 

Another question - Juno First looks great while Draconian doesn't. What's the differences, board or tech wise, between those two?

Edited by AtariLeaf
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Colony 7 and Space Rocks are also fine BTW. Just Draconian which is the worst, a little bit in Fall Down and Lady Bug has that blocky rectangular interference look that I've had on RF machines in the past. I would have thought Space Rocks would be bad too as it seems similar to Draconian but it looks sharp. If it's not RF it's some kind of electrical interference masking as RF interference? I'm going to be gone until late tonight so I might not be able to try anything until after work tomorrow evening.

Edited by AtariLeaf
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I've seen this also on some mods. It's something to do with the cart board or some signal feeding back into the system, but no idea what, and the mod is not grounded or filtered properly. There is a long thread here somewhere where someone went through a bunch of testing with shielding, etc. to try to find it.

Edited by R.Cade
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I've seen this also on some mods. It's something to do with the cart board or some signal feeding back into the system, but no idea what, and the mod is not grounded or filtered properly. There is a long thread here somewhere where someone went through a bunch of testing with shielding, etc. to try to find it.

 

I tried to find that thread with no luck. To your recollection they never found a solution?

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  • 2 weeks later...

It also looks like you didn't do the other mods to improve the video output. Normally you would remove a couple of those resistors...

 

Interesting. I didn't mod the system though I bought it modded from another forum member. Would removal of said resistors help improve this homebrew interference issue?

Edited by AtariLeaf
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I did some research and found this pdf online and removed R209 (Q202 was already removed) but it had no effect. The only other thing I can think of is try to replace the video wire going from the perf board to the yellow composite connector. Hopefully a thicker wire, maybe copper, will improve things. Moving that wire around while the machine is on, especially if it's in contact with the ground wire, can clear the picture up so I'm guessing these wires aren't well insulated. Where does one find wire? I don't think I just have stray wire around the house. I can maybe find some at work that our electricians may have but what do I look for in a kind of wire for this job? Just insulated copper wire?

 

 

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Edited by AtariLeaf
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The pvc/plastic insulation on the wire shown does nothing for shielding the wire from stray signals. It's just there to prevent shorts. Going thicker won't help either. Thicker wire just means it can carry more electrical current. A video wire has minimal current.

 

One thing that might help, is to use shielded wire. And chances are, you have a bunch of it sitting around your house unused. Take any cheap RCA style cable, cut off the ends. When you strip it, you will find an inner conductor that is insulated from some kind of sheild. Most likely some braided wire. Make sure the braid and inner conductor are not touching, connect the shield braid to the ground in the atari and to the outer part of the jacks. And then connect the inner conductor to the center of the jack and wherever it connects on the atari/video PCB.

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Here is some video I took yesterday. As you can see touching it and moving it seems to change the picture and at one point with Draconian, clears it up. Grounding issue? The ground wire out on this mod just runs from the metal strip on the 2600 board to the 3 rca plugs but when I look at diagrams online the ground goes out from the first rf pin to the perf board and then out from the perf board to the rca jacks. You can see pics of my ground wire in post 16 and then to the rca jacks in post 21

 

Edited by AtariLeaf
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