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Atari 2600 Power Noise vs Video output
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CkRtech:
Greetings EEVblog,

In the modern era, it is common practice among those interested in old video game consoles - the Atari 2600 in this case - to modify the video output to something more convenient. For the record, I still have a few 300 ohm RF CRT sets to use - but my issue and questions stem from observing composite video and s-video output. The Atari 2600 went through numerous revisions when it was manufactured, and my unit here is revision 14 (woodgrain, 4-switch).

TL;DR - I think the custom AV chip ("TIA") is rather sensitive to noise, and I am wondering if feeding it cleaner power would help. And I am wondering how I should best do that.

This story started with something I noticed in this unit with two different video mods - diagonal lines of interference in an otherwise fairly decent video output. The lines run from the top right to the bottom left. At once point I noticed I could rotate them and reduce/increase their thickness if I messed with the power switch. I thought, mistakenly, that my problem might be from the switch - and I then later noticed that I could also alter their orientation and thickness by pushing the cartridge around a bit.

An example of these lines can be seen in the purple at the bottom of the first screen in this video, and in the blue sky during the second part. They are much more pronounced in person -

There seemed to be no change after doing a bit of a refresh (new power jack, 7805, electrolytics, a film cap, etc. - some of which were part of an early 80s service bulletin) as well as trying a different video mod.

At some point between that and : doing a cartridge port reflow, adding a 220uf cap to the 7805 and a 220uf cap + .1uf ceramic to the TIA power->ground, the lines cleared up A LOT. They don't dominate the signal the way they used to. Sometimes, they are barely noticeable.

After talking to another Atari guy, we wondered what might be done with the power feed to the TIA chip (could an inductor in series with the power feed to the chip help, etc.)

So I reviewed "EEVblog #594 - How To Measure Power Supply Ripple & Noise" to double check my settings, did some measuring, and ended up here to get some thoughts.

I am ashamed to admit that I did still use the ground clip from a single probe to do the measurement.

I also noticed a change in the intensity of the power signal when going from one screen to another for the game I had hooked up at the time (used composite video to a PVM monitor - standard 75 Ohm termination)

Open to thoughts and opinions.

David Hess:

--- Quote from: CkRtech on April 16, 2018, 12:07:14 am ---At some point between that and : doing a cartridge port reflow, adding a 220uf cap to the 7805 and a 220uf cap + .1uf ceramic to the TIA power->ground, the lines cleared up A LOT. They don't dominate the signal the way they used to. Sometimes, they are barely noticeable.

After talking to another Atari guy, we wondered what might be done with the power feed to the TIA chip (could an inductor in series with the power feed to the chip help, etc.)
--- End quote ---

There is a good chance that adding LC decoupling to the TIA power would work well.  Adding LC decoupling to the source of the interference if you can find it should further improve the situation.  It might be necessary to use a low value resistor in parallel with the inductor to keep the Q down.

When modifying an existing design, I have sometimes added small solid tantalum capacitors directly across the ground and Vcc lines from pin to pin of the integrated circuits in question with the shortest possible leads.
CkRtech:
Thanks, David.

What about the value of the inductor?

Could I possibly pull one off a parts board (386 motherbord with some LCs at the keyboard port), lift the +5V pin from the socket, solder the LC + a resistor (10 ohm?) to the lifted +5V pin and get the other side of the two to components to ground as close as possible? (Been skimming this a bit - http://www.analog.com/media/en/training-seminars/tutorials/MT-101.pdf)

I wish I knew the source of the interference (or how to best find it) - I do think that it perhaps changes depending on the cartridges used in the system.

It seems like C204 should be closer to the power side than the ground side, or does it make a difference? The chip's +5V feed that I am trying to filter actually runs under the entire chip prior to via-ing to the other side and connecting to the power pin on the TIA. Is there anything "wrong" with this design? I attached a photo pointing out my questions - wondering if it could also have a noticeable effect on interference in the TIA.
David Hess:

--- Quote from: CkRtech on April 18, 2018, 07:30:23 pm ---What about the value of the inductor?
--- End quote ---

The self resonate frequency of the inductor limits performance and the inductor has to carry the DC current without saturation so the same types of inductors used for switching power supplies are appropriate.  I usually find whatever is in my junk box and make a single winding without close spacing using a wire large enough to make the winding stiff.  An inductance of 10s to 100s of microhenries is typical.


--- Quote ---Could I possibly pull one off a parts board (386 motherbord with some LCs at the keyboard port) ...
--- End quote ---

They might not support the DC current you need without saturating.


--- Quote ---It seems like C204 should be closer to the power side than the ground side, or does it make a difference? The chip's +5V feed that I am trying to filter actually runs under the entire chip prior to via-ing to the other side and connecting to the power pin on the TIA. Is there anything "wrong" with this design? I attached a photo pointing out my questions - wondering if it could also have a noticeable effect on interference in the TIA.
--- End quote ---

The schematic does not show the board interface so it is not really clear.  Usually small bulk decoupling capacitors are located where power enters the board and small ceramic capacitors are located at the loads which in this case are the ICs.

Going by the schematic, if 1 microfarad C200 is the bulk input decoupling capacitor, then it looks like they added 22 microfarad C204 because they were having problems with interference.  I think C204 *at the TIA chip* is where the LC decoupling network should be placed.

If C204 is worn out, then that would explain the problem.  They used such a large value to get a low enough ESR at high frequencies.  A sold tantalum capacitor would achieve the same performance with 1/4 the capacitance and not wear out.


CkRtech:

--- Quote from: David Hess on April 19, 2018, 03:58:35 pm ---
The self resonate frequency of the inductor limits performance and the inductor has to carry the DC current without saturation so the same types of inductors used for switching power supplies are appropriate.  I usually find whatever is in my junk box and make a single winding without close spacing using a wire large enough to make the winding stiff.  An inductance of 10s to 100s of microhenries is typical.


--- Quote ---Could I possibly pull one off a parts board (386 motherbord with some LCs at the keyboard port) ...
--- End quote ---

They might not support the DC current you need without saturating.
--- End quote ---

Ahhh. Well - I do have a few PC power supplies available for parts such as this one with a handful of various inductors. Seven in the included photo, for instance.


--- Quote ---then it looks like they added 22 microfarad C204 because they were having problems with interference.  I think C204 *at the TIA chip* is where the LC decoupling network should be placed.
--- End quote ---

So C204 was actually replaced prior to my starting this thread, and it seemed to not make a difference. Despite being next to the TIA chip itself, it is not close to the +5v input pin. When you emphasized "at the TIA chip" for C204 being where the LC decoupling network should be placed, would it be OK to:

1: Leave C204 where it is
2: Grab power from the VIA near TIA +5V pin and then run it into the LC decoupling network
3: Solder other side of LC net to lifted +5V pin? (TIA chip is socketed)

I've included a component side photo (stock photo from elsewhere) that shows C204 relative to the TIA chip as well as a crude mock-up.

If you think that will work with any of the inductors from the power supply, I can wire it up as illustrated.
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