9 replies to this topic

[FABombjoy]

Hey all,

I recently put together a CD4050 mod, and installed on my light 6 switch. Everything basically works, except it just doesn't look as good as the screenshots at the mod comparison sites.

For example, the opening screen on Pitfall 2 is nice and sharp, but there is no distinction between the 2 shades of green on the treetops & the forest area. I changed the final chroma/luma resistors to variable & messed with the values, but that didn't have much of an effect. I also tweaked the color delay wheel, and while I was able to get distinct colors with a radically different palette, no amount of subtle adjustments within the correct color settings worked.

With the chroma disabled, I still can't distinguish between the 2 areas. I checked over the component values on the luma section, and most everything seems to be within spec. I was thinking that I used an incorrect value on the luminance 'resistor ladder', but they're pretty close to spec. Could the component values be that critical?

Anyway, to make a long story short: Any ideas?

[cwilkson]

FABombjoy said:

Hey all,

I recently put together a CD4050 mod, and installed on my light 6 switch.  Everything basically works, except it just doesn't look as good as the screenshots at the mod comparison sites.

They almost never do.

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For example, the opening screen on Pitfall 2 is nice and sharp, but there is no distinction between the 2 shades of green on the treetops & the forest area.

This is helpful. The 2 luma values should be 2 (treetops) and 3 (underbrush). So check to see if LUM0 (the signal from pin 8 of the TIA) is getting through to your resistor ladder. If you're test equipment limited, just make sure that all your solder joints are good and that you're picking up the LUM0 signal from the right point.

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Could the component values be that critical?

Yes! You are building a digital to analog converter afterall! Very precision stuff! Ideally, the resistors should be related by a factor of 2. Of course you're probably limited to the standard 5% values. And I don't know which mod you performed, so I don't know what the suggested values are. Pots aren't the best idea unless you measure their values before installing them, but they'll work for debugging and test purposes.

-Chris

[FABombjoy]

Quote

This is helpful.  The 2 luma values should be 2 (treetops) and 3 (underbrush).  So check to see if LUM0 (the signal from pin 8 of the TIA) is getting through to your resistor ladder.  If you're test equipment limited, just make sure that all your solder joints are good and that you're picking up the LUM0 signal from the right point.

I'll double check this tonight, but I'm pretty sure that all the connections are good.

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Could the component values be that critical?

Yes! You are building a digital to analog converter afterall! Very precision stuff! Ideally, the resistors should be related by a factor of 2.

I'm using the CD4050 mod listed in the 2600 FAQ, the only difference being that I have pots installed in place of the 75ohm resistors on the outputs. I tried to match the resistance values on the ladder as well as possible, so they're pretty close, but they're not SUPER close (otherwise, I would have had to use a giant chain of resistors; I have a selection of value-correct resistors on the way right now).

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Pots aren't the best idea unless you measure their values before installing them, but they'll work for debugging and test purposes.

The primary reason that I installed them is because I was getting nasty vertical lines from the chroma signal, and needed to turn up the resistance a bit to smooth things over. I figured I'd toss one on the luma side too and see if I could get the colors to turn up with enough fiddling between the two. Obviously, this didn't work.

Thanks for your help! I'll look things over tonight and see what I can come up with.

[jsoper]

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is because I was getting nasty vertical lines from the chroma signal,

That chroma/luma mod should work just fine the way it it,
no adjustments needed. Did you clip the
base lead of the audio transistor? It will cause some interference if you don't. It's
the one marked as 201 on the AA schematics, or just follow it from Tia pins 12,13

[FABombjoy]

jsoper said:

Did you clip the
base lead of the audio transistor? It will cause some interference if you don't.  It's
the one marked as 201 on the AA schematics, or just follow it from Tia pins 12,13

The FAQ never mentioned that, so I didn't do it. I can give that a shot, but I've seen this sort of vertical pattern when doing other s-video mods (consoles using the CXA1145). In those cases, a minor adjustment to the chroma value was sometimes required, but not always.

[cwilkson]

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The primary reason that I installed them is because I was getting nasty vertical lines from the chroma signal

There are 2 possible causes for these. Either you have crosstalk (the chroma signal is affecting your luma levels), or you have poor grounding in your circuit. If it is a composite mod, you'll probably see a significant crosstalk issue...you're directly coupling the two signals together. there's not much you can do about that. Trying to lessen the amplitude of the chroma is a good start (is that what you're attempting to do with the pots?) Make sure you use good grounding techniques for all of your analog signals. Use twisted pair or coax for routing and make sure you have a local ground point. Especially for the chroma.

-Chris

[Bryan]

I'm the author of that part of the FAQ. It's installed in my 2600 and looks very good. The reason for the 4050 is that is totally isolates the output from noise from within TIA. That's why it's used the 800/XL/XE.

-Bry

[Bryan]

Here's what the mod looks like in my 2600, and an example of the picture on a Chroma/Luma monitor.

-Bry

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[FABombjoy]

Success!

For reasons I have yet to determine, the LUM0 line was AFU. I clipped the 18K resistor to the CD4050, and jumpered a test 18K resistor straight from the LUM0 to the ladder, and the trees lit right up.

Thanks for the handy suggestions. Now it's time to figure out what went wrong. Next time, I won't pack the entire circuit into a 1" square. Should make troubleshooting a bit easier

[Bryan]

Glad to hear it!

I think the upgrade would look even better if TIA's output pins (COL, LUM0,1,2) were removed from the circuit entirely (and pulled up with 1K resistors), but not all TIA's are socketed.

-Bry