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REALLY bad RF video output from 600XL


llopis

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Hello,

 

I'm new to the forum and pretty new to Atari 8 bit computers (although I'm very familiar with other 8 bit systems like Spectrum, Amstrad, or C64 and I often repair them).

 

I recently acquired a lot of Atari 600XL computers and, surprisingly (to me at least) they all work perfectly! They only have a glitch: Really bad RF video output.

 

When I try the monitor connector (composite), it looks pretty good.

post-67384-0-82026400-1548692229_thumb.jpg

 

But when I try the RF output, it's horrible. And I don't mean "slightly grainy RF video quality is bad". I mean HORRIBLE, barely visible.

post-67384-0-49018900-1548692270_thumb.jpg

 

The odd thing is that this happens to all 4 Ataris I have. The TV and cable work great with other systems, so that's not it. Are there some capacitors that are going bad that are likely to cause that? Since the composite video out is good, is this problem happening inside the RF modulator box?

 

Thanks!

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Are you using the same RF cable to test all 4 Ataris? If so, you might just have a bad cable there...

Yeah, but a cable that works with 20 other different retro systems? Unlikely unless there's something about the Atari video I don't know about.

 

These are PAL systems BTW, in case that matters.

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It looks like you are on the wrong channel. All the shielding of off in the one pictured. You should put it all back.

 

Bob

I searched all over the UHF spectrum and that's the only (and best) sync it found.

 

It was exactly the same without the RF shielding too, so that doesn't seem to matter in this case.

 

The RF cable with a magnet is interesting. I had never heard of that. I'll look into it.

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UHF? Not familiar with PAL, but an NTSC modulator would be low VHS - channel 2 or 3. 60 or 70 MHZ, somewhere.

Oh! Now we're getting somewhere! Every other computer system I'm familiar with syncs in the mid area of the UHF. I didn't even think to look beyond that!

 

So I did a full search and I got a much better sync in what the TV called V-L range. It's bad, but waaaaay better than the other one. At least there's color here! :-)

post-67384-0-29618900-1548697635_thumb.jpg

 

I tried putting the shield back on and it didn't make all those speckles go away. Maybe this is where I need the ferrite core in the RF cable. Still, this is waaaay better! And that explains why all of them had the same result :-)

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That same (bad) quality is what I would expect from the RF. I never got any better from my 600xl or 130xe. The good thing is that you got a PAL 600xl with composite and svideo included. I needed to add an UAV, solder the Din 5 pin and make a new opening in the case to have svideo signal connection.

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That same (bad) quality is what I would expect from the RF. I never got any better from my 600xl or 130xe. The good thing is that you got a PAL 600xl with composite and svideo included. I needed to add an UAV, solder the Din 5 pin and make a new opening in the case to have svideo signal connection.

Really? That's weird. I mean, the Atari 2600 has a much better picture than that using RF. I'm surprised to hear they went backwards in quality there.

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Are the RF modulators ok, here in the UK its around the 32 mark for the channel but an overall scan would have found that. That is unless the output channel has been played with and its near a strong channel causing interference on the signal. Mind you I've never seen an Atari or C64 with that bad an RF signal, its not the best picture in the world but nothing like seen, the pic in post #10 smacks of a strong signal over riding the Atari's..

 

EDIT: Just realised that unless there was an aerial Y adaptor being used with an aerial that its unlikely you would get that much outside RF interference.....Man flu scrambled what little brain there is ...D'oh..

Edited by Mclaneinc
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Oh! Now we're getting somewhere! Every other computer system I'm familiar with syncs in the mid area of the UHF. I didn't even think to look beyond that!

 

I tried putting the shield back on and it didn't make all those speckles go away. Maybe this is where I need the ferrite core in the RF cable. Still, this is waaaay better! And that explains why all of them had the same result :-)

 

Prior to my switching to XE's, I used 600XL's and what worked best for me on RF was a phono to coax adaptor. I'd then run a heavier coax cable, like from a VCR, to my TV and it worked great. :)

 

https://www.zoro.com/monoprice-cable-adapter-rca-m-to-f-type-f-4130/i/G2994625/feature-product?gclid=Cj0KCQiA7briBRD7ARIsABhX8aANCDfftkUiZvaE3z1vRMnsmVVB51iucU2vEgYtritOegfw6GDfb4YaAnrlEALw_wcB

Edited by zylon
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I've seen bad video like that only when some one was using an autoswitch box like on some classic game consoles or jaguar.... go direct with the cable if you have a switch box... also the rf modulator can section is usually sealed with a lid, I don't see it so it's possible some one tinkered in there...

Edited by _The Doctor__
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Maybe channel is mismatched? There's a channel 2/3 switch by the RF output on the Atari.

 

The PAL computers predominantly set to use UHF with channel 36 and have no channel switch. Most PAL cases didn´t have the hole for a switch, some models have a dummy plug at the switch´s hole. Some very very rare XL models have a VHF based RF modulator using channel 3/4 - like all PAL Atari 400 or 800 (they use also only channel 3 and 4 in PAL).

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I tried putting the shield back on and it didn't make all those speckles go away. Maybe this is where I need the ferrite core in the RF cable. Still, this is waaaay better! And that explains why all of them had the same result :-)

 

At least you should install back the modulator´s metal top cover. The TV screens looks like somebody changes the setting of the HF coils in the modulator circuit.

 

Before you re-assemble the 600XL, remove the capacitor C109 located at the upper left corner of the 4050 chip. Most times it´s a disk capacitor, brown. Removal of this scrim diffuser will significantly enhance the picture quality overall.

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The PAL computers predominantly set to use UHF with channel 36 and have no channel switch. Most PAL cases didn´t have the hole for a switch, some models have a dummy plug at the switch´s hole. Some very very rare XL models have a VHF based RF modulator using channel 3/4 - like all PAL Atari 400 or 800 (they use also only channel 3 and 4 in PAL).

Ah, interesting! So the correct tuning should be in the UHF area, like most other computers then (that would be the first picture, barely visible).

 

 

At least you should install back the modulator´s metal top cover. The TV screens looks like somebody changes the setting of the HF coils in the modulator circuit.

 

Before you re-assemble the 600XL, remove the capacitor C109 located at the upper left corner of the 4050 chip. Most times it´s a disk capacitor, brown. Removal of this scrim diffuser will significantly enhance the picture quality overall.

I did try it with the cover and the shield back on, but it didn't make any difference. I'll try it again today when I put it back together. I was the one who removed the cover of the RF modulator to see if there was anything obviously wrong inside.

 

What do the two adjustable coils do in the modulator circuit? Do they just vary the frequency of the modulation, or is there something more to it? I didn't touch them yet, but maybe I should.

 

And I'll try removing the C109. Thanks for the tip!

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What do the two adjustable coils do in the modulator circuit? Do they just vary the frequency of the modulation, or is there something more to it? I didn't touch them yet, but maybe I should.

 

Right, these coils are for setting up the base frequency for TV channel(s). First I would try to use "fine-tuning" option of your T.V. - if this won´t help, you can try to change the coils´ settings. Use non-metal tools for this!

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OK, final update on this: I tested the rest of the Atari 600XLs that I had and they all sync reasonably well in that V-L range (instead of the normal UHF range). The image quality in all the other one was much better than this one I was testing.

 

So I put this one back together, put all the shielding, screwed it back up, put the keyboard back and... it actually looks pretty much like the others now!

 

So I guess there were two things going on:

- Using the right frequency to tune to (although it was weird that it was tuning enough to see it and for the TV to detect it in the UHF range)

- Not just putting the shielding back on, but screwing it and putting the keyboard back on.

 

I learned a bunch of new things about this system already :-) Thanks for everybody's help.

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