easier 7800 Composite video mod

[jmetal88]

Glad you've pretty much sorted it out. Actually I think you're lucky it works as well as it does on your capture card.

My capture card has major "dot crawl" problems with my NES, and it couldn't sync to an svideo 7800 I was experimenting with.

 

The signal from old game consoles probably violates NTSC spec, and I think some modern devices don't tolerate it as well as an old CRT television. If something doesn't work on a capture card, don't worry too much until you've tried a TV.

 

Yeah, I've had five capture cards, so far (I think). The one old consoles seem to do best on is the WinFast TV2000 XP. They also do OK on the PCHDTV 5500 and the ATI TV Wonder HD 600 PCIe, but DScaler gets a little laggy from time to time with those two. The cards made by Adaptec don't even sync up with the video properly.

[Rex Dart]

With this mod, is it possible that I could cut the traces to the channel selector switch & instead have it route 5v either to the RF box on the circuitboard, or to my composite mod? I would have to leave the other three pins still connected to the RF box... would that be a problem?

[Underball]

With this mod, is it possible that I could cut the traces to the channel selector switch & instead have it route 5v either to the RF box on the circuitboard, or to my composite mod? I would have to leave the other three pins still connected to the RF box... would that be a problem?

I suppose you could do that. I don't see why you would want to though. Once you add teh A/V mod, there's really no need for RF anymore.

 

Also - part of what makes this mod work is disconnecting audio from the RF unit and feeding it straight to an A/V jack, to prevent interference. You would need to set up a separate switch to re-enable audio on the RF unit as well.

[Rex Dart]

Alright, thank you. I guess it wouldn't be worth the effort. I'm just trying to do the mod & retain/reuse as much as possible. What about removing the RF shielding from the RF modulator box & re-using that RCA jack for composite video? I tried prying the shielding apart, but it wouldn't budge and I'm afraid to force/break anything. Is the "lid" soldered through to the bottom of the PCB perhaps? Or would the interference be a problem again?

[Underball]

Alright, thank you. I guess it wouldn't be worth the effort. I'm just trying to do the mod & retain/reuse as much as possible. What about removing the RF shielding from the RF modulator box & re-using that RCA jack for composite video? I tried prying the shielding apart, but it wouldn't budge and I'm afraid to force/break anything. Is the "lid" soldered through to the bottom of the PCB perhaps? Or would the interference be a problem again?

Yeah, That RF box is harder to get into than Fort Knox. I gave up and just left it there.

 

And that jack is pretty old/probably rusty or corroded. I prefer to use nice, new gold plated color coordinated jacks, myself.

[Rex Dart]

Got a good source for those? Frys and Radio Shaq either don't carry or don't bother to stock them (gold-plated color-coded), and the only places I can find online want $2.50 apiece.

[Underball]

Got a good source for those? Frys and Radio Shaq either don't carry or don't bother to stock them (gold-plated color-coded), and the only places I can find online want $2.50 apiece.

I got 'em at Radio Shack.

 

They put all this kind of stuff in a confusing set of Drawers at the back of the store now.

[Rex Dart]

Yeah, I asked and they pointed me to the $20 video cables They don't have crap.

[CongoZombie]

I've got a PAL rev. C 7800 and I'm trying to do this mod. I built the amp (substituting a BC548 for the 2n2222) and connected it to the rf modulator pins (I have cut the pins running to the RF Modulator and connected the wires to these). I get a video display on most tvs (though strangely not on my Samsung LCD- I'm guessing the sync signal isn't strong enough), but it has quite a bit of interference. I have tried cutting R5 and R6 to get rid of these (The pal schematic says that these are the resistors coming from TIA audio), yet this does nothing to help. Also, as suggested in the first page, Cutting R1 (R3 on the NTSC schematics) blanks video completely. All I can guess is that the resistor numbers on the schematics do not tally up with the pcb revision that I have. Either that or I'm being an idiot

 

Does anyone have any ideas? (I'm tempted to just cut the audio pin on the TIA and make my own amp circuit for it).

[CongoZombie]

Forget the part about it not working on my LCD. I was evidently using a dodgy cable. Still got audio interference and diagonal lines though.

[CongoZombie]

Ok, I've pretty much sorted it out now, with some fiddling and the helpful info from alex_79's post

[Madaracs]

Glad you were able to fix the POKEY issue. I was holding off on doing this mod until the POKEY issue was resolved. Are you going to update the instructions with the fix?

 

Good Idea. I just updated the instructions (Post #14) to include the POKEY audio fix and the resistor change.

 

 

 

Has anyone seen these?

 

eBay Auction -- Item Number: 300444248112

 

I'm wondering if they take into account the Pokey chip sound and if anyone has actually tried this kit?

[Rex Dart]

 

Has anyone seen these?

 

eBay Auction -- Item Number: 300444248112

 

I'm wondering if they take into account the Pokey chip sound and if anyone has actually tried this kit?

 

 

Considering that it looks exactly like what's described in this very thread, it probably could be made to work with POKEY sound if the packaged instructions don't account for it already.

[Underball]

Just got a new 7800 this week, and after having experimented the living crap out of this mod on my old one, I'v posted some updates to Puppetmark's instructions for this mod that make it both EVEN EASIER, and much clearer video:

 

 

*UPDATE*

7-8-2008; I have updated the instructions to include the POKEY audio fix and a resistor change that will bring down the brightness so light collored objects are not over-driven. Here is the info for the mod:

 

Parts List:

------------------------------------

2.2k 1/4 watt resistor

3.3k 1/4 watt resistor

75 ohm 1/4 watt resistor No longer needed.

47K to 68K 1/4 watt resistor

47K resistor (optional) No longer needed.

2n2222 or 2n3904 transistor

1uf capacitor.

2 RCA jacks

hook up wire

-------------------------------------

 

Procedure:

 

1. Open up the 7800 and remove the metal shield.

2. Locate R3 and remove it. This is a good time to drill two holes and install the RCA Jacks. I installed mine facing down on the edge of the case.

3. Locate R11 and replace it with a 47K resistor. This is optional but it cuts down the brightness a bit. I didn't have this problem on my magnavox monitor but on my TV some light collored characters were over-driven. I found that this was counterproductive, and made tme mod more susceptible to minor interference. Leave the original 38k Resistor at R11 where it is.

4. Cut the pins that connect the Video modulator to the board. (don't do this if you ever think you will use the RF out in the future) De-solder the pins from the RF unit's connecting board and bend them back using pliers. leave them intact. Then, de-solder the RF unit from the bottom of the board and remove it. (again, less interference is the goal, plus more room to work.)

5. Build the circuit in the schematic.* Edit: DO NOT include the 75ohm resistor between the ground and the video output from the transistor. This causes severe ghosting on certain games with solid, light color-filled backgrounds like the grey roadway in Pole Position II and the blue water in Frostbite. I just soldered the parts together then soldered the transistor's collector to the 5V connection on the expansion bus connector. The connection is the third point from the bottom on the right side. I made my ground connection by soldering the resistors to the edge of the board. I then ran a small wire to the video connection on the expansion bus connector. It is much cleaner and easier to get +5v and video input from the middle two pins on the RF unit. Pin 2 (closer to where the RF unit was) is video input. Pin 3 is +5v. As PacManPlus pointed out, using an old floppy connector is great here, since you won't have to solder to the pins, just plug it in. Makes for easier removal of the motherboard later on too, for cleaning or more mods/work. Finally I ran a yellow wire to the RCA jack. this worked well for me but it may be easier for some to build the circuit on a small piece of PC board such as Almost Rice did. (I built mine on a board as well.)

6. I connected a resistor to the bottom of C14. This is the audio out. Any resistor value between 47K and 68K should be fine. The other end of the resistor goes to the RCA connector Now clip the top connection (towards the back of the unit/cart cup) of R5 and R6, but leave the bottom connected to the motherboard. Solder the two clipped end together, and then attach a 1uf capacitor between the two soldered together ends and the center pole of the audio RCA jack. This will give you both standard TIA audio and Pokey audio for games that have that.

7. Run a wire from the top of C10 to the audio jack. This is the POKEY audio. I actually made the connection by running a wire from C10 to where the audio jack connects to the resistor I added for the audio output.

8. I connected the ground wires to the RCA jacks and connected the other end to the ground trace on the edge of the board.

9. Put the 7800 back together and enjoy the clean video and audio!

 

Here are some pics that will help explain:

 

Here is the upgrade. You can also see where I cut the RF modulator pins:

 

Here is a close up of the resistors to be removed:

 

 

Here is the Amplifier Schematic:

 

Here is the amplifier installed in the 7800. The only solder points to the board are the 5V, ground, and the video in signal:

 

Here is the audio connection:

 

Here is where to make the POKEY connections:

 

Here is a close up of the C10 connection:

 

Here is a close up of the connection point to the RCA jack:

 

Here is a closeup of the RCA connectors:

 

Here is another view of the RCA connectors:

 

 

Enjoy!

[CongoZombie]

Just got a new 7800 this week, and after having experimented the living crap out of this mod on my old one, I'v posted some updates to Puppetmark's instructions for this mod that make it both EVEN EASIER, and much clearer video:

 

snip

 

Did you get any kind of chroma/luma cross talk on 7800 games? I built the original amp, but the crosstalk is pretty bad on maria games (2600/tia games look fine, though there is a little ghosting). I'm still to try out the fix mentioned, but I was wondering if there was a way to do it with passives

Edited August 4, 2010 by CongoZombie

[Underball]

Just got a new 7800 this week, and after having experimented the living crap out of this mod on my old one, I'v posted some updates to Puppetmark's instructions for this mod that make it both EVEN EASIER, and much clearer video:

 

snip

 

Did you get any kind of chroma/luma cross talk on 7800 games? I built the original amp, but the crosstalk is pretty bad on maria games (2600/tia games look fine, though there is a little ghosting). I'm still to try out the fix mentioned, but I was wondering if there was a way to do it with passives

Not putting the 75ohm resistor between the ground and video output, and not removing R11 negates almost all of the crosstalk. It's clean as a whistle.

 

Depending on the TV set, some people might find the brightness too high, but in that case, putting a 20 ohm resistor (or thereabouts) between the transistor's video output and the video RCA jack brings it down to a duller level, without re-introducing the crosstalk.

[CongoZombie]

Not putting the 75ohm resistor between the ground and video output, and not removing R11 negates almost all of the crosstalk. It's clean as a whistle.

 

Depending on the TV set, some people might find the brightness too high, but in that case, putting a 20 ohm resistor (or thereabouts) between the transistor's video output and the video RCA jack brings it down to a duller level, without re-introducing the crosstalk.

 

Thanks, I tried it and it has indeed reduced the ghosting slightly, however, crosstalk is as bad as ever. I am using a PAL 7800, so it could be that this is due to hardware differences.

[Underball]

Not putting the 75ohm resistor between the ground and video output, and not removing R11 negates almost all of the crosstalk. It's clean as a whistle.

 

Depending on the TV set, some people might find the brightness too high, but in that case, putting a 20 ohm resistor (or thereabouts) between the transistor's video output and the video RCA jack brings it down to a duller level, without re-introducing the crosstalk.

 

Thanks, I tried it and it has indeed reduced the ghosting slightly, however, crosstalk is as bad as ever. I am using a PAL 7800, so it could be that this is due to hardware differences.

when you say crosstalk, do you mean ghosting/haloing or chroma/luma sync problems?

[CongoZombie]

Not putting the 75ohm resistor between the ground and video output, and not removing R11 negates almost all of the crosstalk. It's clean as a whistle.

 

Depending on the TV set, some people might find the brightness too high, but in that case, putting a 20 ohm resistor (or thereabouts) between the transistor's video output and the video RCA jack brings it down to a duller level, without re-introducing the crosstalk.

 

Thanks, I tried it and it has indeed reduced the ghosting slightly, however, crosstalk is as bad as ever. I am using a PAL 7800, so it could be that this is due to hardware differences.

when you say crosstalk, do you mean ghosting/haloing or chroma/luma sync problems?

 

Vertical banding and very bad dot crawl. From reading up in some other threads, seems that the clock line between TIA and MARIA is at fault, and the problem can be solved by buffering it with an and gate.

[Underball]

Not putting the 75ohm resistor between the ground and video output, and not removing R11 negates almost all of the crosstalk. It's clean as a whistle.

 

Depending on the TV set, some people might find the brightness too high, but in that case, putting a 20 ohm resistor (or thereabouts) between the transistor's video output and the video RCA jack brings it down to a duller level, without re-introducing the crosstalk.

 

Thanks, I tried it and it has indeed reduced the ghosting slightly, however, crosstalk is as bad as ever. I am using a PAL 7800, so it could be that this is due to hardware differences.

when you say crosstalk, do you mean ghosting/haloing or chroma/luma sync problems?

 

Vertical banding and very bad dot crawl. From reading up in some other threads, seems that the clock line between TIA and MARIA is at fault, and the problem can be solved by buffering it with an and gate.

Also - this seems like a problem more ont eh PAL consoles than NTSC. I haven't seen any of that.

[Rex Dart]

Any tips for those of us who CANNOT get that stupid RF modulator box desoldered & removed? I'm using a 30w with an old fat tip and that thing just won't budge.

[Underball]

Any tips for those of us who CANNOT get that stupid RF modulator box desoldered & removed? I'm using a 30w with an old fat tip and that thing just won't budge.

You don't NEED to remove it, as long as you've disconnected all 4 pins from it. It really depends on the motherboard revision and what kind/amount of solder they used to attach it.

[CongoZombie]

Also - this seems like a problem more ont eh PAL consoles than NTSC. I haven't seen any of that.

 

Ok, I had a 4 x 2 input NAND chip lying around, so I made a buffer out of that. The vertical banding has gone, now just the dot crawl remains. This might be due to my LCD tv (though I've not experienced it on other consoles to the same extent).

Edited August 4, 2010 by CongoZombie

[puppetmark]

Just got a new 7800 this week, and after having experimented the living crap out of this mod on my old one, I'v posted some updates to Puppetmark's instructions for this mod that make it both EVEN EASIER, and much clearer video:

 

 

Awesome job with this Underball! I have been so busy with other projects, I haven't been able to get back to this but I am glad to see it is still alive! I am going to try out the changes myself. It's funny, 75ohms is the correct impedience for an NTSC video amp but I guess something must be even more "unstandard" the way the chroma and luma are mixed than I thought. - oh Atari, didn't anyone understand video circuits in Sunnyvale??

 

I am also very interested in the buffer between Maria and TIA. I will probably give that a try too.

 

again, thanks for continuing to improve this!

[CongoZombie]

oh Atari, didn't anyone understand video circuits in Sunnyvale??

 

At least with the 7800 they didn't seem to- the RF out on my 7800 ghosted about 6 times. The RF out on my 800xl and XE GS doesn't ghost at all really...

 

The buffer made a huge amount of difference for me, eliminating the crosstalk, although a lot of dot crawl does remain (Xevious is particularly bad, since there seems to be a lot of contrasting colours in that game). This could just be due to the nature of composite video, but I'm pretty sure that none of my other consoles exhibit dot crawl to quite the same extent...

Edited August 19, 2010 by CongoZombie