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mtarallo

Atari 7800 and 2600 Composite Video Mod - Ghosting only on 2600

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Hi guys - just curious if you can address something for me:

 

I bought a few video mod chips (composite AV): from eBay

 

It worked well (crisp and clear audio and video) on my 7800 following instructions I found here: http://vintagegamingandmore.com/guides/mods/ - now the 2nd one I installed in an older  6 switch Atari 2600 following the 2600 instructions on the same site - and with this one - I get a "ghosting" faint double image effect which I am told may be because the signal is weak. Just to rule out other things I tried different composite cables, TVs (both LCD and Tube) and power supplies.

 

Still the same. 

 

Just curious if you ran into this and if there is a fix? - not sure if it is something else - but thought I'd check with you all.

 

I'll be installing another one in a newer 4 switch and I'll note the results - but was just curious.

 

Thank you.

 

w2krb9.jpg

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I'm having this exact same issue. Basic transistor/2 resistors mod composite mod - 7800 video looks absolutely perfect, 2600 has visible ghosting on games with solid-fille color backgrounds, i.e. Activistion games. Especially bad on Barnstorming, etc.

 

any ideas?

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Most ghosting is due to signal reflection due to impedance mismatch, but you may need to be more exact about what kind of kit you're using.  Even among the "basic 2 resistors and a transistor" mods, I've seen a couple variants.  Are you referring to this one?  http://atariage.com/forums/topic/279366-yet-another-2600-composite-mod-help-thread/

 

If so, you should try adding a potentiometer between the transistor and the composite plug for impedance matching, then adjust for best picture.  The expected resistance value will likely be around 50 ohms.    Note that the advice from that thread of adding the resistor to ground is unnecessary.  When the composite is hooked up, it becomes the path to ground.

Edited by ChildOfCv

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Most ghosting is due to signal reflection due to impedance mismatch, but you may need to be more exact about what kind of kit you're using.  Even among the "basic 2 resistors and a transistor" mods, I've seen a couple variants.  Are you referring to this one?  http://atariage.com/forums/topic/279366-yet-another-2600-composite-mod-help-thread/
 
If so, you should try adding a potentiometer between the transistor and the composite plug for impedance matching, then adjust for best picture.  The expected resistance value will likely be around 50 ohms.    Note that the advice from that thread of adding the resistor to ground is unnecessary.  When the composite is hooked up, it becomes the path to ground.


It's not a purchased mod. I put it together using a 2222 transistor and the expected 2.2 and 3.3k resistors. I left out the 75 to ground already.

I'll try the pot like you said and see what shakes out, thanks!

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Tried using a pot and various resistors in line to the video output. Ehh... it helps a bit, but it doesn't really eliminate the ghosting on 2600 games. it just makes it slightly better. I'm thinking there is some other component on the board that needs to be removed or replaced that is the culprit in the 2600 part of the circuit, since 7800 output is damn near perfect.

 

I've seen a few of the 2600 only video mod instructions telling people to remove C215 and leave it removed, or replace it with a different value cap. from the 2600 board near the power regulator.

 

Does the 7800 mother board have an equivalent to this "C215" from the 2600, that can be removed or replaced from the 7800 video output circuit? I'm shit at reading schematics.

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I guess that's revision-dependent.  On one schematic, C215 couples the color and sync signals into the luma, and you definitely don't want to remove that.  On another, it seems to be a filter just before the RF modulator.  Its equivalent on the 7800 is C7.

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One quick check for a more fundamental issue with 2600 mode is to check the voltage to R18-R23 from Q2.  They should all have a common connection side, and in 2600 mode it should be around 5V.

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I guess that's revision-dependent.  On one schematic, C215 couples the color and sync signals into the luma, and you definitely don't want to remove that.  On another, it seems to be a filter just before the RF modulator.  Its equivalent on the 7800 is C7.

Ok, so which one on the 7800 motherboard (NTSC) is C7? Looking at the schematic and then board layout - it looks like it's the light green one wedged in between the channel 2-3 switch and the the RF box?

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Not quite..it is located just to the right of the bottom section of the RF modulator box itself next to R2 and also just below left of the Q2 tranny. I can't recall where I got this pic from, but it has been massively helpful when trying to find where stuff actually is vs the schematics and BOM listings. I wish I had these for the 5200 and other consoles as well.

 

 

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BTW, did you really mean that the picture was bleeding, rather than ghosting?  Here are a couple of pictures pulled from AA to compare notes:

 

(bleeding)

http://atariage.com/forums/uploads/monthly_11_2014/post-37192-0-75793500-1415726256.jpg

 

 

(ghosting)

http://i65.tinypic.com/w2krb9.jpg

 

 

The distinction is important because they have 2 different causes.

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BTW, did you really mean that the picture was bleeding, rather than ghosting?  Here are a couple of pictures pulled from AA to compare notes:

 

(bleeding)

http://atariage.com/forums/uploads/monthly_11_2014/post-37192-0-75793500-1415726256.jpg

 

 

(ghosting)

http://i65.tinypic.com/w2krb9.jpg

 

 

The distinction is important because they have 2 different causes.

Ghosting, as evidenced by the second photo.

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 I can't recall where I got this pic from, but it has been massively helpful when trying to find where stuff actually is vs the schematics and BOM listings. I wish I had these for the 5200 and other consoles as well.

 

 

 

That would be me. I was in the process of converting it to a cleaner motherboard silkscreen image and making a second one for the other motherboard rev. Never got around to finishing it.

There should be similar motherboard silkscreens in the 2600 and 5200 service manuals.

 

Somewhat related, I don't think I ever posted a link to this one. http://www.atari7800.org/boardscans/7800explodedview.jpg

7800explodedview.jpg

 

Mitch

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That would be me. I was in the process of converting it to a cleaner motherboard silkscreen image and making a second one for the other motherboard rev. Never got around to finishing it.

There should be similar motherboard silkscreens in the 2600 and 5200 service manuals.

 

Somewhat related, I don't think I ever posted a link to this one. http://www.atari7800.org/boardscans/7800explodedview.jpg

 

 

Mitch

 

Mitch Thank you! Seriously this unpopulated 7800 board scan has been incredibly helpful and I have seen the drawings in the service manuals for the others, but they aren't as clear as having a scanned in board is.

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Ghosting, as evidenced by the second photo.

Update:

 

So yeah, removing C7 didn't help. Actually, it made the video jumpy and unstable. I put it back and it's where it was before.

 

I'm thinking the 7805 needs to be replaced, minimum. I'm going to start there as I have a spare.

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Hmmm, so which components do you currently have removed/modified?  Also, just to make sure all the bases are covered, are you using a 75 ohm video RCA cable, and not just an RCA cable?

 

 

 

Just some FYI about how the video output works:

 

TIA and MARIA both send black/white (luma) separately from the color.  There are some OR gates that combine the signals from both chips.  So if one of them holds a signal high, then the output is high.  The outputs of the OR gates go through varying strengths of resistors to convert the output into an analog 3.5V swing due to the R14 voltage divider.

 

MARIA feeds its OR gate inputs directly.  TIA requires pull-up resistors as referenced above (R18-R23).  In 7800 mode, these pull-up resistors are dead, so that MARIA can run the output system.  In 2600 mode, the pull-up resistors should pull pretty close to 5V, and apparently MARIA just holds its outputs low.

 

So if there is trouble with transitions of shading, it could be that the resistors aren't strongly driven by Q2.

 

For color it just has two resistors that feed from either TIA or MARIA into a common output.  This output then goes through a voltage divider to reduce its strength from a 5V swing to a 0.4V swing, then goes through a band-pass filter into the composite output.

 

It is also joined there by the 4.5MHz FM-modulated sound signal.  So point #1 in making a composite mod is to remove this connection, so that the video can be just video.

 

Now, composite video is defined to be 1V swing, and we see 3.5V at the composite gathering point.  I wonder if the Heck Hack adequately compensates for this with its base voltage divider?  Unfortunately, the only way to know for certain, is to put a scope on the composite output with the mod hooked up and connected to the TV.  But I wonder if it ought to be using smaller resistor values in order to reduce the drive strength?

Edited by ChildOfCv

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 But I wonder if it ought to be using smaller resistor values in order to reduce the drive strength?

 

This depends on if you are using it on a CRT vs modern flat panel display. On a CRT you add a 75r to the video output and ground to drop the brightness down and prevent it from looking washed out. But on a modern display you end up with a really dark picture and then have to remove the resistor to compensate.

 

This is why the UAV is designed the way it is. From I can gather looking at it, it is handling the Maria and TIA colors through different buffers/encoders on the UAV and then there are different circuits in place on the outputs between the composite and s-video lines. The difference here is that the picture brightness and quality is the same between CRT and modern displays with the pixels being sharper on modern displays of course. But overall brightness and saturation are the same.

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I followed the same instructions for all of these mods. Removed R3 to keep the audio signal out of composite. Disconnected all 4 pins of the RF box, and took video, 5v, and ground from the holes left behind where the RF connects to then mother board.

When you say a 75 ohm rca cable do you mean like the RF cable that originally came with the system? In that case, no. I'm just using a standard set of red white yellow rca cables, since a composite mod should only need those.

I also too the audio from the north pins of the tia and pokey line audio.

Again the problem is only on solid filled background 2600 games like Enduro, Barnstorming, etc. Black background 2600 games and all 7800 games look beautiful. In fact - it really only seems to be a problem on green solids. Chopper command's beige is fine, Dolphin's blue is ok.

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This depends on if you are using it on a CRT vs modern flat panel display. On a CRT you add a 75r to the video output and ground to drop the brightness down and prevent it from looking washed out. But on a modern display you end up with a really dark picture and then have to remove the resistor to compensate.

 

This is why the UAV is designed the way it is. From I can gather looking at it, it is handling the Maria and TIA colors through different buffers/encoders on the UAV and then there are different circuits in place on the outputs between the composite and s-video lines. The difference here is that the picture brightness and quality is the same between CRT and modern displays with the pixels being sharper on modern displays of course. But overall brightness and saturation are the same.

Hmmm... that shouldn't happen.  If you have a 75 ohm output impedance and 1Vp-p on output (and an undistorted signal), you should be golden.  Now, one thing that hardly anybody ever does, but ought to do for perfect output is to add an output decoupling capacitor.  If there's a DC offset, a good picture probably relies on the input's circuitry.  Of course now we're getting into something a bit more complex than the transistor and two resistors.

 

However, what I'm reading about the UAV shows that the creator has done a lot of engineering on this stuff.  (Of course it's also a lot more complex than the transistor and resistors too).

Edited by ChildOfCv

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Well, I had some fun today.

 

First of all, got a baseline reading on the unmodified 7800.  The composite meeting point has a DC offset of 3V and 1Vp-p signal.  Interesting, but the 2600 RF modules did indicate that there would be a pull-up.  And apparently it keeps a 1V signal just like the composite signal does.

 

Then I removed the RF module and mapped it out.

 

[attachment=638233:Screen Shot 2019-05-25 at 9.48.45 PM.png]

 

As expected, it has a 5V pull-up.

 

Reading the signal without RF (and with R6 disconnected), it runs between 2.5V and 0V.  But you can also see how distorted it is when it's that stretched out.  Look at the typical composite video signal, then imagine it was drawn by Salvadore Dali.

 

Anyway, I attached a potentiometer between the signal output and +5V, then adjusted it to look like it does with the RF module in.  Turns out to be around 1.5K is the sweet spot.

 

Then I built a refinement of the simple composite mod using what I had laying around.

 

[attachment=638235:Screen Shot 2019-05-25 at 10.16.40 PM.png]

 

The potentiometer is to adjust the brightness.

 

Seemed to work (with Defender, the only working game I have at the moment), and did not give any ghosting or bleeding (beyond what the RF already does).

 

Of course if you use the UAV that's a moot point :)  But it was still kind of a productive day.

 

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So yeah - I ordered a UAV. I know they are great as I had one in a 7800 previously that I sold.

Also - runs out this was all just a bad 7800. I had another 7800 mobo which I wasn't using, connected this same exact simple transistor mod to it, and the picture is pristine, no ghosting at all. But there is some RF interference/jailbars, so I'll use the UAV to eliminate that.

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So yeah - I ordered a UAV. I know they are great as I had one in a 7800 previously that I sold.

Also - runs out this was all just a bad 7800. I had another 7800 mobo which I wasn't using, connected this same exact simple transistor mod to it, and the picture is pristine, no ghosting at all. But there is some RF interference/jailbars, so I'll use the UAV to eliminate that.

 

I suspect that most of the jail bars or interference have to do with where the mod board is mounted.  After all, the RF module is encased in shielding to protect its innards from interference.  Your average mod board is not.  I think that if I were to do a permanent 7800 mod, I'd pull the guts out of the RF box and then build a board that fits inside to make sure stray signals don't reach it.

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