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Simple DIY composite video mod


simon.plata

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Good points about the schematic.

Elsewhere in this thread you'll find a PCB image of the Mod and it's essentially what you're asking for. The PCB layout is what I referenced to build mine but I assembled mine on a proto board.

Edited by djmips
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The PCB image is this one, right?

 

post-62422-0-75815900-1511299317.png

 

This is even less clear to me than the scheme, and that is the reason I want to see the solder side of the board, justo to copy it.

 

Anyway, let´s try to follow the scheme. I take the video signal from 4, then to the resistor R7. Capacitor C1 is not neccesary as you stated? If neccesary, where to solder to the resistor? After? Behind?

 

Next, the video signal goes to the trimpot. Mine is like this one:

 

21622795860446_158_m.jpg

I guess leg 2 is the middle one, but which ones are 1 and 3? Next guess is the video signal goes to leg 1, and legs 1 and 2 are joined. Then, leg 3 joins to two resistors same time, R1 and R2. The other leg of R2 goes to ground, and other leg of R1 goes to +5V, originated in the pin 3 of the console board.

 

Then, the point where these two resistors and leg 3 of trimpot joins go to the leg 2 of transistor 2N3904. Leg 3 goes to R4 and leg 1 goes to R3, but before joining the resistor, the line joins the leg 2 of transistor 2N3906. Leg 3 joins to R5 and leg 1 joins R6, but before the resistor we can take the video out.

 

Is all that correct or I made mistakes? :-o

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Hello, very interresting topic ... I've installed TFW8Bit mod and the image is really too dark on a CRT and quite black on an LCD.

 

I will give à try to your mod, but as I'm a bit new to this game, could you make a list of the components to buy. It's easy for the resistors but I'm not sure of R8 (gain ?).

 

Thank you !

 

Mine was the same way. I used a VCR to amp the signal, and it looks... better. It's not dark anymore.

 

I will have to piece this thread together and give this mod a try. :)

 

Thank you for this, I hope it works well with my RetroTINK.

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  • 6 months later...

I made my own board today to replace tfw8b AV mod which was too dark for my liking. I used a 10k trim pot as I couldn’t get a 5k one. 

My Atari is a 4 switch Vader PAL model. On first boot I was pleasantly surprised to find that it worked but I had some smudging across the screen - adjusting my trimpot all the way down solved this.

Great mod I am very happy with this thanks@simon.plata also thanks to @Djoulz as I used your diagrams to build my board.

Edited by slaanesh
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1 hour ago, slaanesh said:

I made my own board today to replace tfw8b AV mod which was too dark for my liking. I used a 10k trim pot as I couldn’t get a 5k one. 

My Atari is a 4 switch Vader PAL model. On first boot I was pleasantly surprised to find that it worked but I had some smudging across the screen - adjusting my trimpot all the way down solved this.

Great mod I am very happy with this thanks@simon.plata also thanks to @Djoulz as I used your diagrams to build my board.

Hi @slaanesh, I'm pleased it worked for you. It was designed for NTSC so the optimum gain could be in the edge for PAL.

 

Also I want to thanks @Djoulz for making it easy and accessible for more people.

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12 hours ago, slaanesh said:

@simon.plata what would be required to modify your board so that it works more comfortably with the PAL spec?

 

I'd be willing to make another board :)

Hi @slaanesh,

 

PAL requires a higher video bandwidth (5.0MHz) than NTSC (4.2MHz), and this is the reason why you get the better results with your trim-pot all way down, which is the point of the highest gain for the circuit.

 

To optimize the circuit for PAL's signal, it’s needed to move the knee point or the roll-off frequency point to the right (Higher frequency), which requires a complete redesign of the circuit, because it its DC coupled. The roll-off frequency is the result of the superposition of the RC low pass filters formed by the transistor's parasitic capacitances and the input/output equivalent resistances.

 

You may try to change the C1 capacitor from 47pF to a higher value like 68pF or 100pF to increase the gain in the high frequency end. That trick could work but it’s not warranted.

 

Unfortunately I’m being too busy at work, otherwise I would try to redesign the circuit with PAL bandwidth in mind. I will try when I have a truce from work.

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6 hours ago, simon.plata said:

Hi @slaanesh,

 

PAL requires a higher video bandwidth (5.0MHz) than NTSC (4.2MHz), and this is the reason why you get the better results with your trim-pot all way down, which is the point of the highest gain for the circuit.

 

To optimize the circuit for PAL's signal, it’s needed to move the knee point or the roll-off frequency point to the right (Higher frequency), which requires a complete redesign of the circuit, because it its DC coupled. The roll-off frequency is the result of the superposition of the RC low pass filters formed by the transistor's parasitic capacitances and the input/output equivalent resistances.

 

You may try to change the C1 capacitor from 47pF to a higher value like 68pF or 100pF to increase the gain in the high frequency end. That trick could work but it’s not warranted.

 

Unfortunately I’m being too busy at work, otherwise I would try to redesign the circuit with PAL bandwidth in mind. I will try when I have a truce from work.

It might be faster and easier to design for an op-amp that's intended for video amplification though.  Something like this:

 

https://www.maximintegrated.com/en/products/analog/amplifiers/MAX4395.html

 

It sounds like you have better than cookbook-level experience with electronics, so I'm also curious why you went with the common-emitter approach, when the Atari seems to have enough voltage swing out of the gate, so no gain needed.  Is it just because a common collector didn't have enough input impedance?  (though the Colecovision Atari module mitigated that problem with a Darlington pair transistor).

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On 11/15/2019 at 1:40 PM, ChildOfCv said:

It might be faster and easier to design for an op-amp that's intended for video amplification though.  Something like this:

 

https://www.maximintegrated.com/en/products/analog/amplifiers/MAX4395.html

 

It sounds like you have better than cookbook-level experience with electronics, so I'm also curious why you went with the common-emitter approach, when the Atari seems to have enough voltage swing out of the gate, so no gain needed.  Is it just because a common collector didn't have enough input impedance?  (though the Colecovision Atari module mitigated that problem with a Darlington pair transistor).

Hi ChildOfCv,

 

In respect your first comment, you´re absolutely right . It's really easy (nothing to do really) to design the mod using an integrated video buffer in place of a discrete one. Indeed I ordered some video buffers to do another AV-mod for my VCS consoles. In this case I selected the TSH122. https://www.st.com/en/amplifiers-and-comparators/tsh122.html

 

However the objective of this post, was to give an easy to built AV-mod which really works and was affordable for almost everybody in the world. In some places it's not easy or affordable to order uncommon electronic parts. Also I wanted it to be possible to assembly in an universal PCB or even a bread-board.

 

Regarding your second comment, I might don't really understand your question. However I will try to respond. In the first post of the tread, I exposed two options:

 

The first one is a one stage amplifier in common-collector configuration, which is the best approach as you said.

 

post-62422-0-47928700-1509676726_thumb.png

 

However, given the high output impedance of the resistor network DAC (about 10.9KOhm), the current gain needs to be into the hundreds, which is in the edge of the capabilities of a general purpose transistor at 4.2MHz.

 

The second one is a modified common-collector configuration like the first one, plus a preamplifier stage in common-emitter. I choose the common-emitter configuration for the preamplifier to take advantage of the high input impedance and the convenience of embedded biasing of the common-collector second stage.

 

post-62422-0-87052200-1509677361.png

 

I hope I have responded to your question appropiately, otherwise please let me know your comments and I will try to do my best.

 

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15 hours ago, simon.plata said:

However, given the high output impedance of the resistor network DAC (about 10.9KOhm), the current gain needs to be into the hundreds, which is in the edge of the capabilities of a general purpose transistor at 4.2MHz.

Yeah that fits my hypothesis then.  The input impedance of a common collector amp is going to be less than gain*load resistance, around 7.5K in this case, at best.  That's a pretty large load for the resistor ladder, and explains why you typically remove the RF modulator in that case.

 

The Atari module that I mentioned replaces the 2N3906 with a MPSA13.  This is a Darlington transistor which has the same pin package and configuration but gain is advertised at 10,000.  So the input impedance should be closer to 750K.  That transistor also seems to be fairly cheap and common.  So perhaps it could be used in a cheap and simple common-collector solution.

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On 11/15/2019 at 10:53 PM, simon.plata said:

You may try to change the C1 capacitor from 47pF to a higher value like 68pF or 100pF to increase the gain in the high frequency end. That trick could work but it’s not warranted.

@simon.plata I will try a higher value cap as you suggest and report back my findings.

 

If anyone else has any other suggests to change the mod I'm willing to try on my PAL Atari(s). 

 

However I would need a circuit diagram (or mods to the above ones) so that I can build the board :)

 

The current mod I am using has a pretty good vibrant display. It's not perfect and there is still some ghosting but it's better than RF cable and of course more convenient to connect to TVs and monitors. I using a Samsung 940MG 19" LCD- which is good because it has loads of inputs including RF, composite, s-video, component and RGB SCART.

 

D7A91C59-0F69-4CB8-87CB-DEC8B7494BC1.jpeg
HERO PAL-60 version on my 4-switch PAL Atari.

You can see the Activision logo looks fine however there is ghosting. My 10K trim pot is turned right down. This is with the 47pF Cap in place.

Edited by slaanesh
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On 11/17/2019 at 2:42 PM, ChildOfCv said:

Yeah that fits my hypothesis then.  The input impedance of a common collector amp is going to be less than gain*load resistance, around 7.5K in this case, at best.  That's a pretty large load for the resistor ladder, and explains why you typically remove the RF modulator in that case.

 

The Atari module that I mentioned replaces the 2N3906 with a MPSA13.  This is a Darlington transistor which has the same pin package and configuration but gain is advertised at 10,000.  So the input impedance should be closer to 750K.  That transistor also seems to be fairly cheap and common.  So perhaps it could be used in a cheap and simple common-collector solution.

It sounds like a good idea. My concern is about the bandwidth and roll-off frequency. Darlingtons tends to have multiply the base-emitter capacitance of the hidden base, but should give a try.

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On 11/17/2019 at 5:56 PM, slaanesh said:

@simon.plata I will try a higher value cap as you suggest and report back my findings.

 

If anyone else has any other suggests to change the mod I'm willing to try on my PAL Atari(s). 

 

However I would need a circuit diagram (or mods to the above ones) so that I can build the board :)

 

The current mod I am using has a pretty good vibrant display. It's not perfect and there is still some ghosting but it's better than RF cable and of course more convenient to connect to TVs and monitors. I using a Samsung 940MG 19" LCD- which is good because it has loads of inputs including RF, composite, s-video, component and RGB SCART.

 

D7A91C59-0F69-4CB8-87CB-DEC8B7494BC1.jpeg
HERO PAL-60 version on my 4-switch PAL Atari.

You can see the Activision logo looks fine however there is ghosting. My 10K trim pot is turned right down. This is with the 47pF Cap in place.

Ghosting could be produced by overdrive of the video signal, try to reduce the gain an see if the ghosting persist.

 

If it persist then could be a mismatch of coupling impedance. Try a different video cable and different lengths the shortest the better.

 

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  • 4 months later...
On 4/14/2019 at 2:48 AM, simon.plata said:

Hi and thanks to everybody who tryed this mod.

 

May be I forgot to suggest to adjust the trimmer until you obtain the better picture in your set-up.

 

The calculated value is for 0dB gain and 1Vpp output which is the standar composite video signal span. However due to the tolerance of the components in the VCS board and the in the mod, it could be lower or higher than that. The trimmer was included to correct that deviation.

 

Best regards and enjoy!

Thanks for including the trimmer in your design, David! I used your composite video mod on my Atari 7800 PAL, and by turning the trimmer pot anti-clockwise I was able to improve the output quality quite a bit. Thanks for sharing this mod with the community!

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  • 6 months later...
On 11/21/2017 at 7:24 PM, simon.plata said:

Hi, Here is a schematic and PCB design for the mod made in Eagle CAD. I will fabricate it to replace my generic PCB.

 

post-62422-0-00139200-1511299299_thumb.png

 

post-62422-0-75815900-1511299317.png

 

If someone want to fabricate your own PCB, just let me know. I can share the PDF or the Eagle files.

Hi friend!!! Great post!! I will do that!! Very thanks for sharing with us!!
Please tell me. What is the way to get the BEST trimmer adjust ??
Thanks again!!!

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58 minutes ago, pcrock said:

Hi friend!!! Great post!! I will do that!! Very thanks for sharing with us!!
Please tell me. What is the way to get the BEST trimmer adjust ??
Thanks again!!!

Hi,

 

I’m pleased you find it useful.
 

First of all the trimmer only adjust the gain of the amplifier. That means it varies the brightness of the image only. Ghosting and other distortions are not related to the trimmer adjust, but to impedance matching.


The best trimmer adjust could be seen from different standpoints. 

 

One of them is obviously according to the NTSC or PAL standard. Which is 714mV Or 700mV between the black and white level correspondingly. You will need an oscilloscope and a test cart to generate a color bar test signal.

 

My favorite adjust point is which “Looks” better in your TV set. I use to set the TV at factory settings for video. Then I adjust the trimmer until I get the more pleasant image. Lower than optimal gain will give a dark image but well defined colors. Higher than optimal gain will give an overdrive signal. The brightness isn’t increased but the yellow will change to white. Optimum gain should be near maximum brightness but having well defined yellow.

 

Hope it helps!

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4 minutes ago, simon.plata said:

Hi,

 

I’m pleased you find it useful.
 

First of all the trimmer only adjust the gain of the amplifier. That means it varies the brightness of the image only. Ghosting and other distortions are not related to the trimmer adjust, but to impedance matching.


The best trimmer adjust could be seen from different standpoints. 

 

One of them is obviously according to the NTSC or PAL standard. Which is 714mV Or 700mV between the black and white level correspondingly. You will need an oscilloscope and a test cart to generate a color bar test signal.

 

My favorite adjust point is which “Looks” better in your TV set. I use to set the TV at factory settings for video. Then I adjust the trimmer until I get the more pleasant image. Lower than optimal gain will give a dark image but well defined colors. Higher than optimal gain will give an overdrive signal. The brightness isn’t increased but the yellow will change to white. Optimum gain should be near maximum brightness but having well defined yellow.

 

Hope it helps!

Wow!!! Thanks a lot again friend!! I will try it!!!

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21 hours ago, simon.plata said:

Hi,

 

I’m pleased you find it useful.
 

First of all the trimmer only adjust the gain of the amplifier. That means it varies the brightness of the image only. Ghosting and other distortions are not related to the trimmer adjust, but to impedance matching.


The best trimmer adjust could be seen from different standpoints. 

 

One of them is obviously according to the NTSC or PAL standard. Which is 714mV Or 700mV between the black and white level correspondingly. You will need an oscilloscope and a test cart to generate a color bar test signal.

 

My favorite adjust point is which “Looks” better in your TV set. I use to set the TV at factory settings for video. Then I adjust the trimmer until I get the more pleasant image. Lower than optimal gain will give a dark image but well defined colors. Higher than optimal gain will give an overdrive signal. The brightness isn’t increased but the yellow will change to white. Optimum gain should be near maximum brightness but having well defined yellow.

 

Hope it helps!

Please tell me one more thing.
Is it correct 10K for R9 (audio) ???
It appears too much high value to me.
thanks again

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4 hours ago, pcrock said:

Please tell me one more thing.
Is it correct 10K for R9 (audio) ???
It appears too much high value to me.
thanks again

Typical audio line input impedance is 10K and line output impedance is 100 or 600 ohms. Due to short distance of the cable in comparison to the wavelength, impedance matching is not required.
For maximum audio signal level you may choose 100 to 600 ohms for R9. However I prefer to use 10KOhms (same as line input) for short circuit protection and to avoid input saturation.
 

Please let us know how it works for you and the video results.

 

bests!

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4 hours ago, simon.plata said:

Typical audio line input impedance is 10K and line output impedance is 100 or 600 ohms. Due to short distance of the cable in comparison to the wavelength, impedance matching is not required.
For maximum audio signal level you may choose 100 to 600 ohms for R9. However I prefer to use 10KOhms (same as line input) for short circuit protection and to avoid input saturation.
 

Please let us know how it works for you and the video results.

 

Hi friend.  I did the mod and all ran ok on building the circuit  but unfortunately the results was not good. 

About the transistors I used BC548 and BC558 instead 3904 and 3906 that are unusual ones here in Brazil. 

I attached pics to show you the image issues to ask if you could give me some idea. 

For the trimmer I tried from 0K to 10k but had no bright change. The image is dark and full of interferences and contours blurry. 

The console is in NTSC mode X200 (xtal) is 3.579545 MHZ

I have great interest on this mod. 

 

Once more special thanks friend!

 

IMG_20201009_041216987_PORTRAIT.jpg

IMG_20201009_041210726_PORTRAIT.jpg

IMG_20201009_041006802.jpg

IMG_20201009_040724821.jpg

IMG_20201009_040714917.jpg

IMG_20201009_040658817.jpg

IMG_20201009_035824263.jpg

IMG_20201009_035820260.jpg

IMG_20201009_035818007.jpg

IMG_20201009_035750067.jpg

IMG_20201009_035742442.jpg

IMG_20201009_035417919.jpg

IMG_20201009_035353482.jpg

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8 hours ago, pcrock said:

Hi friend.  I did the mod and all ran ok on building the circuit  but unfortunately the results was not good. 

About the transistors I used BC548 and BC558 instead 3904 and 3906 that are unusual ones here in Brazil. 

I attached pics to show you the image issues to ask if you could give me some idea. 

For the trimmer I tried from 0K to 10k but had no bright change. The image is dark and full of interferences and contours blurry. 

The console is in NTSC mode X200 (xtal) is 3.579545 MHZ

I have great interest on this mod. 

 

Once more special thanks friend!

 

IMG_20201009_041216987_PORTRAIT.jpg

IMG_20201009_041210726_PORTRAIT.jpg

IMG_20201009_041006802.jpg

IMG_20201009_040724821.jpg

IMG_20201009_040714917.jpg

IMG_20201009_040658817.jpg

IMG_20201009_035824263.jpg

IMG_20201009_035820260.jpg

IMG_20201009_035818007.jpg

IMG_20201009_035750067.jpg

IMG_20201009_035742442.jpg

IMG_20201009_035417919.jpg

IMG_20201009_035353482.jpg

Ok, Colors and saturation looks good, then I can say the mod is working properly. The issues with the image are related to gosthing and image post processing by the TV DSP.

 

BC548B/C and BC558B/C are good replacements. Be sure there are BC5x8B or C, the last letter defines de hfe of the transistor. Letter A or no letter are low gain devices.

 

I suggest the following:

 

The 75 Ohm resistor is very important. Be sure it's 75 Ohm +/- 5%

The video cable is the responsible of most of the problems. Please ensure you're using a good video cable and connectors. Check cable resistance, should be less than 1.5 Ohms. Check ground.

Try to find a CRT TV set (old TV) and check the video image on it. Several new LCD/LED TV sets have several image processing algorithms that can distort the video. If the gain change didn't affect the brightness, it's an indication that the TV DSP is normalizing the video and further post processing is being done.

 

After checking the image in a CRT and tuning the gain, go to your LCD/LED TV set and disable ALL the image processing or enhancing features. This should improve the image.

 

Let me know how it works for you.

 

Bests,

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Again, thanks for answering friend.
About the transistors, I used BC548C and BC558B. They are original ones (not Chinese crap). I have two other consoles with other mods that work VERY WELL both in CRT and in this same led tv that I tested here on this post.

One of then is a very very simple thing and works very well. The other uses a CD4050 and transistors (the best I've seen so far till now) but it takes a lot of work to build so I was very interested in this one that you did because the idea looks great. Here are the schematics of the ones that I have used successfully so far in my both CRT and LED TV.
About the cables I tested, both are resistance less than 1 ohm. I even tested your mod with the original 75 ohm RF cable as video cable and also with a good AV cable but the defects are identical in both. In CRT the image is much better but still far from desirable. At CRT, the Trimmer works but does very little. The colors are correct in both Led TV and CRT but are weak, ugly and dark and have vertical interferences. This same led TV that I used in the photos works well in the mods I send attached to this post. The 75 ohm resistor is exactly 75 ohms in the digital multimeter.
Any idea?

Thanks friend again.

atariav-esquema.GIF

SVHS CD4050 e transistores.jpg

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