easier 7800 Composite video mod

[Underball]

Look at the picture you posted.

 

You removed R4 & R5. R4 is part of the video circuit.

Edited September 25, 2010 by Underball

[ramdass]

Look at the picture you posted.

 

You removed R4 & R5. R4 is part of the video circuit.

 

I'm mortified. Must be creeping senility. Changed it to the correct resistors and the audio works fine.

[PacManPlus]

Just as a side note, this mod also works for the 'Stunt Cycle' stand alone SC-450. Specifically, I did this one: http://www.atariage.com/forums/topic/127926-easier-7800-composite-video-mod/page__view__findpost__p__1796143

 

No resistors to remove, and the audio already has it's own connector.

[Deteacher]

I can pretty much follow all those steps, except for the 7800 Amplifier schematic (I have never been able to read schematics.)

 

Would it be possible to post a picture of the assembled amplifier, labeling the parts? With a pic of the real thing, I could probably build it with little to no problem. Sorry. I'm just getting into this aspect of tinkering (I'm getting pretty good at soldering now) and I'd love to attempt this mod on my own before taking it to someone to do it for me.

 

I know...I'm a n00b!

[twoquickcapri]

Thanks to everyone who took time to work on this mod. I Had an 7800 that need a new power switch so I figure while I was in there in would A/V mod too. I used Underball's instructions.

 

I removed the RF unit and removed its guts. then I removed the channel switch to make room for the sound phono. I reused the case to mount the video phono and then used the bottom of the case to make a mount to house the sound phone.

 

 

Then remounted it to the board and did the A/V mod.

 

Was able to the mod without cutting the case

[ApolloBoy]

Very clever reuse of the RF modulator!

[iesposta]

I just finished this easier 7800 A/V mod on my lousy Serial #X201 7800.

I did Post #97 PacManPlus way, only with parts I had, the $0 mod

I didn't have a 75 ohm resistor (did anyone??), so I used a 82 ohm resistor instead.

I didn't find any RCA connectors, so I used a cord that ended in female RCA jacks.

Thanks for the tip that the Atari XM301 Modem has three of those value transistors!

I also had used the modem's box for an SIO2SD.

I'm quite pleased with the video and sound, however LittleJoe's 'Longhorn' 7800 A/V mod with S-Video is nicer in my opinion.

[AtariBrian]

will the xm work with modded systems?

[mimo]

will the xm work with modded systems?

yes

[RevEng]

Took my first whack at this mod, with generally good results. I didn't get any ghosting, but there is a fair bit of delay between brightness and color, which tends to soften edges. I believe this is as good as the mod will get without more intrusive changes to the console chroma and lum0 lines... might look into that later.

 

I used a 75ohm resistor in series with the video out. Together with the 75ohm resistor to ground, that should impedance match the line correctly for composite video, which will reduce the potential for ghosting. (and has the nice side-effect of reducing the hot signal people complain about) Also not a bad idea to throw a .1uf ceramic cap across ground and +v if you build this on a perfboard with long leads.

[phoenixdownita]

I simplified the mod even further.

 

Basically I used the 2222 in simple emitter follower without biasing the input.

 

Connect collector of 2222 to +5V, connect base to composite, connect emitter to 47 ohm resistor and that to the center of the RCA connector.

[obviously this still requires to cut the RF pin and to remove R3 and R5].

 

The results are pretty good on an old CRT, but on my LCD it is too bright with a very faint ghosting/bleeding, I will try a higher resistor on the emitter to see if it fixes it, maybe a 68 or 75 or even 100 could just work. If that doesn't work I will try the 47K on R11 to simply tone it down at the source.

 

To summarize the mod is the same without the 2.2K and 3.3K resistors on the base, without any resistor from emitter to ground, but with a resistor from the emitter to the RCA. [i found this kind of mod all over for all console types when buffering luma signals, I just tried it on my 7800 and it worked].

 

Also for now my audio has a 10K and it sounds OK, I guess in this condition the POKEY would be too low, but given there are only 2 or 3 games with it I may never find out ... I do have a set of resistors coming my way so I'll probably replace it with the suggested 47K once I receive them.

 

Anyway thought of sharing this as it makes the mod simpler. After all the 2222 is used as a simple voltage buffer and the biasing of the base does not look like it solves any particular problem. I am pretty sure it works the same with a 3904, all my luma buffers used the 3904 it just so happened I had a 2222 around.

 

On a side note, on my first attempt I did not remove R3 and R5 and on the CRT it made no difference, but on the LCD I could see the audio noise all over. I wonder why Atari decided to premix the signals rather than doing it on the RF modulator itself?

Edited August 19, 2013 by phoenixdownita

[phoenixdownita]

OK, replaced the 47 ohm with a 100 ohm and the signal is better, not so hot anymore and now the colors are fine on the LCD.

 

So the mod is as follows:

 

Connect collector of 2222 to +5V, connect base to composite signal, connect emitter to 100 ohm resistor (probably 82 or 75 will work too) and the other end of the resistor to the center of the RCA connector. Connect ground to the RCA rim.

[obviously the mod still requires to cut all the RF pins from the connector and to remove R3 and R5 to stop audio interference].

All of the signals for video [+5V, GND, and composite] are right there on the RF connector, I soldered the transistor directly on the pins as +5 and composite are adjacent.

 

Not sure how the quality fares with the other mods, but it is simple and it works reasonably well.

[elefas]

Hello fellow atarians,

 

Is this the best I can get from my pal 7800 ? I did the a/v mod described in this thread, only differences are that I used a C548C transistor I had spare and R1, R2 are approximated values (I combined some spare resistors in series to achieve the approximated value). Tried both in lcd monitor via tv tuner and 14 inch crt, in my case the lcd monitor displays better than the crt. Should I be satisfied with the result or can I do something to make it better?

[Magic Knight]

My... - A lot of Ghosting going on there....

Edited October 7, 2013 by Magic Knight

[elefas]

My... - A lot of Ghosting going on there....

hehe I count 4!

[mimo]

Hello fellow atarians,

 

Is this the best I can get from my pal 7800 ? I did the a/v mod described in this thread, only differences are that I used a C548C transistor I had spare and R1, R2 are approximated values (I combined some spare resistors in series to achieve the approximated value). Tried both in lcd monitor via tv tuner and 14 inch crt, in my case the lcd monitor displays better than the crt. Should I be satisfied with the result or can I do something to make it better?

I did this mod for someone on the Retro computer museum forum and the video output looks way better than your picture, even on an LCD screen which normally has problems with old out of spec signals

[mimo]

[keilbaca]

Just as a side note, this mod also works for the 'Stunt Cycle' stand alone SC-450. Specifically, I did this one: http://www.atariage.com/forums/topic/127926-easier-7800-composite-video-mod/page__view__findpost__p__1796143

 

No resistors to remove, and the audio already has it's own connector.

 

 

Looks like I'm modding mine!

[elefas]

so any suggestions to improve the ghosting effect? should i change some resistors (in other threads I read about replacing the R3 75 ohm resistor of the amplifier with a 47 ohm one)?

[phoenixdownita]

so any suggestions to improve the ghosting effect? should i change some resistors (in other threads I read about replacing the R3 75 ohm resistor of the amplifier with a 47 ohm one)?

 

I would suggest you take away R1 and R2 from the mod, and then R3 instead of going to GND you connect it to the RCA center pin.

 

So you end up with the transistor having the composite signal in base [without any other components], having +5V on the collector, and out of the emitter a 75/100 ohm resistor that IS your composite video output.

 

Tell us if it helps. Also you may need a different transistor but I doubt it.

[elefas]

Ok I tried what phoenixdownita suggested, got rid all resistors and just kept R3. 75 ohm R3 still had a lot of ghosting so I started trying bigger resistors...till I settled down with 1K! Taken the opportunity to open my 7800 I did some other things also. Firstly I desoldered the RF modulator and placed there a headphones jack (ground, audio-single channel, video). With a Y rca cable I get the composite and audio rca. Turned out a very compact solution to provide output for a/v. Plus I also did what PacManPlus connected a plug in video, 5V, ground connectors. The result is better than before but ghosting still exists. I think I can live with it. Thanks helping out!

[phoenixdownita]

It looks like in line with what I have, when I connect to a CRT I can tell you the ghosting subsides a lot, so I thought it may have to do also with the LCD TV I am using.

 

BTW, another way to try could be to put back R1 and R2 AND use a small capacitor in front of the whole mod on the composite signal. Ideally something like 47uF.

 

What the R1+R2 do is a voltage splitter attempting to keep the transistor working close to the ideal point. That is important for small signals, to achieve maximum amplification. But because we're taking the signal from the emitter it is always going to be Vbase - 0.7 [the voltage drop between B and E in a transistor is pretty much a constant]. For AC signals that can go negative it is important to bias the base up [with the splitter] and to keep it there you filter the DC component on the input signal by a small capacitor that doesn't filter out too much of the AC signal.

 

So if you want to do more experimenting you could try it.

Finally most composite outputs are AC coupled only, so normally there's a 220uF capacitor in series to the resistor, at the output, you could try that too. Modern TVs do not seem to care too much if you forget the out cap, but it may help your case. Again I have not tried it myself but if you have the time and energy it's worth a try.

Edited October 12, 2013 by phoenixdownita

[elefas]

Thanks for the tips my friend. For the time being I am satisfied with the result. If it happens to open my 7800 again will try to follow your instructions, probably fit a capacitor.

[reverendshoebox]

Bumping an old post, but I just wanted to throw a hearty thanks in here - I made this mod to my 7800 and I'm getting gorgeous video now.

 

-=ShoEboX=-

[Lynxpro]

I wish I had the skillz and the courage to mod my consoles like you guys...