Simple DIY composite video mod

[TomelloSoulMan]

6 hours ago, simon.plata said:

Excellent, lowering the trimmer value should augmentate the gain until you get the desired image after the splitter.

Thank you so much, that did the trick! And the capture card is working fine! There's definitely still some brightness drop when i plug the splitter, but curiously it's only noticeable in the CRT TV, not in the capture... But i'm perfectly fine with that.

 

Now (well, not now, in the next days) i'll be doing another one of these and trying to install it in a undocumented 3-chip clone Kashiwa. I'll post my results here when done!

[TomelloSoulMan]

Well, somewhat I've fried my PAL Jr's TIA and/or 6507 by trying to install a pause mod, so this has speeded up the process of installing this AV mod in my Kashiwa clone. Reference here:

This is working mostly OK but I'm facing two small problems:

 

- TV shows an interference pattern that I can't get rid of, no matter what components I remove or disconnect. AFAICT I've isolated completely the RF module from the rest of the board, that shouldn't be receiving any power or signal by now. Maybe I'll have to assume that this thing is simply badly built with no interference shielding at all, and live with this.

 

- Video signal is kinda weak. The trimpot in the AV mod board doesn't have any effect on the output signal, so I can't manipulate it like I did in my Jr for compensating the brightness drop when I connect the splitter and the capture card. Input voltage in the AV mod board is somewhere between 4.80 - 4.90 V. ¿Is this ok? Maybe it's just the video input what's weak, but I've checked many times and it's totally disconnected from the RF modulator input - it's just unsoldering a resistor, no rocket science here unless I am missing something.

 

What I'd like to fix the most is the weak video signal. Is something I can check for finding out if something else in the board is draining the signal? Is there any modification I can do to the AV board in order to boost the signal a bit? Oh, how I miss my sturdy and reliable Junior

[simon.plata]

3 hours ago, TomelloSoulMan said:

Well, somewhat I've fried my PAL Jr's TIA and/or 6507 by trying to install a pause mod, so this has speeded up the process of installing this AV mod in my Kashiwa clone. Reference here:

This is working mostly OK but I'm facing two small problems:

 

- TV shows an interference pattern that I can't get rid of, no matter what components I remove or disconnect. AFAICT I've isolated completely the RF module from the rest of the board, that shouldn't be receiving any power or signal by now. Maybe I'll have to assume that this thing is simply badly built with no interference shielding at all, and live with this.

 

- Video signal is kinda weak. The trimpot in the AV mod board doesn't have any effect on the output signal, so I can't manipulate it like I did in my Jr for compensating the brightness drop when I connect the splitter and the capture card. Input voltage in the AV mod board is somewhere between 4.80 - 4.90 V. ¿Is this ok? Maybe it's just the video input what's weak, but I've checked many times and it's totally disconnected from the RF modulator input - it's just unsoldering a resistor, no rocket science here unless I am missing something.

 

What I'd like to fix the most is the weak video signal. Is something I can check for finding out if something else in the board is draining the signal? Is there any modification I can do to the AV board in order to boost the signal a bit? Oh, how I miss my sturdy and reliable Junior

Video interference could be produced by a noisy power rail, and this is very common in old equipment due to electrolytic capacitor aging. I suggest to check with an oscilloscope the 5V power rail and verify it does not have ripple. I do so, then replace the bypass capacitor.

[TomelloSoulMan]

On 3/3/2022 at 3:53 PM, simon.plata said:

Video interference could be produced by a noisy power rail, and this is very common in old equipment due to electrolytic capacitor aging. I suggest to check with an oscilloscope the 5V power rail and verify it does not have ripple. I do so, then replace the bypass capacitor.

Could well be the reason, but due to the apparently cheap build quality of this thing, I don't know if I will bother trying to fix that. It's like a wand that goes from bottom to top of the screen. Here's a video in which you can see the interference, image comes from a cheap USB EasyCap: https://clips.twitch.tv/OpenUninterestedAirGuitarTBTacoLeft-RLhF6Rs-SpSYkPdS

 

Image is much darker and fuzzier in the CRT than in the capture. If I unplug the EasyCap from the splitter, it gets pretty decent. So... no way to power it up a bit without getting an active splitter?

[simon.plata]

38 minutes ago, TomelloSoulMan said:

Could well be the reason, but due to the apparently cheap build quality of this thing, I don't know if I will bother trying to fix that. It's like a wand that goes from bottom to top of the screen. Here's a video in which you can see the interference, image comes from a cheap USB EasyCap: https://clips.twitch.tv/OpenUninterestedAirGuitarTBTacoLeft-RLhF6Rs-SpSYkPdS

 

Image is much darker and fuzzier in the CRT than in the capture. If I unplug the EasyCap from the splitter, it gets pretty decent. So... no way to power it up a bit without getting an active splitter?

I should give it a try. It's almost impossible to be cheaper built than the original Atari :-). A bad bypass capacitor could make also the image darker.

 

Also, Yes, it is possible to "power it up a bit" and use a passive splitter, it's a matter of impedance matching. You will need to figure out which is the input impedance of the splitter and then match the output resistor of the AV mod to this value. After that you can fine tune it adjusting the amplifier gain with the trimmer. 

[TomelloSoulMan]

On 3/5/2022 at 10:59 PM, simon.plata said:

Also, Yes, it is possible to "power it up a bit" and use a passive splitter, it's a matter of impedance matching. You will need to figure out which is the input impedance of the splitter and then match the output resistor of the AV mod to this value. After that you can fine tune it adjusting the amplifier gain with the trimmer. 

This is a great answer that put my brain to work. Being an electronics newbie, I first didn't know what this all meant, but I've been reading on the subject and that made things clear. So, at first, I assumed that both the TV and the capture card would have an input impedance of 75 ohm, following the norm, and that would give 37.5 ohm when used in parallel through the splitter. But I've tested this and I have:

 

- Only TV: ~77 ohm (within tolerance, I guess)

- Only capture card: ~57 ohm (!!!)

- Both: ~33.4 ohm

 

So, as I sometimes will be using this setup with only the TV connected and sometimes with both the TV and the capture card, I thought of a mod in the output resistor area in the circuit to let me select this "streaming" mode via a switch. Attached is a simple diagram where I add 68 ohm resistor (the closer value I have to that 57 ohm) and a switch to "enable" it (AKA connect it in parallel to the 75ohm resistor). Would something like this work or is it a really stupid idea? Resistance would equal 35.7 ohm so it would be closer to that calculated 33.4 ohm load.

 

Also, since it's a long way to the next electronic component show from where I live and shipping costs are expensive, I've done some calculations and the best match I can do with some resistors in series and parallel is 33.77 ohm : 75 || 150 || (68+18+18), being "||" parallel and "+" series. Should I bother with a setup like this or just take the 75 || 68 approach as good enough?

[TomelloSoulMan]

Oh, I've got a better calculation that would give me 31.4 ohm in parallel, just 2 ohm short from the calculated 33.4 at the splitter: 75 || (18+18+18)

[simon.plata]

2 hours ago, TomelloSoulMan said:

This is a great answer that put my brain to work. Being an electronics newbie, I first didn't know what this all meant, but I've been reading on the subject and that made things clear. So, at first, I assumed that both the TV and the capture card would have an input impedance of 75 ohm, following the norm, and that would give 37.5 ohm when used in parallel through the splitter. But I've tested this and I have:

 

- Only TV: ~77 ohm (within tolerance, I guess)

- Only capture card: ~57 ohm (!!!)

- Both: ~33.4 ohm

 

So, as I sometimes will be using this setup with only the TV connected and sometimes with both the TV and the capture card, I thought of a mod in the output resistor area in the circuit to let me select this "streaming" mode via a switch. Attached is a simple diagram where I add 68 ohm resistor (the closer value I have to that 57 ohm) and a switch to "enable" it (AKA connect it in parallel to the 75ohm resistor). Would something like this work or is it a really stupid idea? Resistance would equal 35.7 ohm so it would be closer to that calculated 33.4 ohm load.

 

Also, since it's a long way to the next electronic component show from where I live and shipping costs are expensive, I've done some calculations and the best match I can do with some resistors in series and parallel is 33.77 ohm : 75 || 150 || (68+18+18), being "||" parallel and "+" series. Should I bother with a setup like this or just take the 75 || 68 approach as good enough?

It should work, however it could be necessary to adjust the gain trimmer every time you switch (change) the resistance.

[TomelloSoulMan]

6 hours ago, simon.plata said:

It should work, however it could be necessary to adjust the gain trimmer every time you switch (change) the resistance.

Sorry for coming again with this... I've just tested the mod and it didn't work as expected. In fact, it does the opposite thing: when the switch is on, I get an even darker image. I've double and triple checked everything and all seems ok. When the switch is off, it shows 75 ohm resistance, and when it's on, it's a bit more than 31 ohm, as expected (I've built the second option, that is 75 || (3*18)). I'm surely missing something here, but in theory this should work...

[simon.plata]

1 minute ago, TomelloSoulMan said:

Sorry for coming again with this... I've just tested the mod and it didn't work as expected. In fact, it does the opposite thing: when the switch is on, I get an even darker image. I've double and triple checked everything and all seems ok. When the switch is off, it shows 75 ohm resistance, and when it's on, it's a bit more than 31 ohm, as expected (I've built the second option, that is 75 || (3*18)). I'm surely missing something here, but in theory this should work...

It is working as I expected. You need to adjust the gain trimmer after changing the output resistance value. Did you?

 

May be I need to recalculate the other resistors to avoid saturation of the amplifier.

[TomelloSoulMan]

18 hours ago, simon.plata said:

It is working as I expected. You need to adjust the gain trimmer after changing the output resistance value. Did you?

Forgot to mention that I tried that and the gain trimmer did nothing, it didn't cause any effect on the output. It didn't work either before this mod but it did work on my Atari 2600 Jr. I'm using the very same AV mod board in this clone.

[TomelloSoulMan]

Sorry, my fault. I tried the gain trimmer again and it works, but I can only lower the brightness with it. It was already at its maximum setting. I had to roll it down waaaay long to start to see the image dimming, until a point where it was so weak that made my TV lose sync.

 

I've been scratching my head to understand this, maybe the video input into the AV mod board is just weak. So I opened the console yet another time and draw a diagram of the video part, it's attached here. This arises a few questions:

 

- Why R27 and R28 (pull up resistors for SYNC and Lum1) are 4.7K instead of 3.3K as in the original specs?

- Isn't R35 (resistor for Lum0) a bit low at 100K? In the original specs, it's a 110K one, and logic says that it should be near the double of R34. Would it be better if I change this for a 110K one?

- What is the D4 diode doing at the Lum1 line? I tried bridging it and the image became a bit brighter, not enough but a bit better.

- As this worked ok on my Junior, which has totally different resistor values and a CD4050, would it be worth the hassle if I copy all the Junior circuit into a perfboard, taking the outputs directly from the TIA pins, and feed the AV mod with its output? I'd only do this if I can't repair my Junior, because it sounds like a lot of work

 

As a side thing, regarding the static interference, I checked the power rail capacitor and it's a 1000mF 16V one, instead of the regular 2200mF. Maybe I'll change it in the future.

 

As a last thing, I'm sorry because this issue is becoming so long. I really appreciate you spending your time with my little problems, and I would totally understand if you don't want to waste more time on this. I'm glad only by sharing my findings.

 

[ChildOfCv]

On 3/7/2022 at 5:42 AM, TomelloSoulMan said:

This amplifier doesn't match impedance this way.  The output impedance of this kind of transistor amp is 0 ohms.  Period.  Of course in the real world it's closer to 10 or so.  But the load resistor does not change output impedance.  You need to try different resistors on the path to VIDEO_OUT instead.

I should also note that 75 ohms is way too low for the load resistance.  Somewhat unintuitive, but this resistor affects the INPUT impedance.  So the signal going in to pin 2 is going to see different loads based on load resistor.  This could affect the signal on the input side and cause distortion if the circuit wasn't designed for it.  It's better to use values of at least 300 ohms.

Edited April 7, 2022 by ChildOfCv

[simon.plata]

6 hours ago, ChildOfCv said:

This amplifier doesn't match impedance this way.  The output impedance of this kind of transistor amp is 0 ohms.  Period.  Of course in the real world it's closer to 10 or so.  But the load resistor does not change output impedance.  You need to try different resistors on the path to VIDEO_OUT instead.

I should also note that 75 ohms is way too low for the load resistance.  Somewhat unintuitive, but this resistor affects the INPUT impedance.  So the signal going in to pin 2 is going to see different loads based on load resistor.  This could affect the signal on the input side and cause distortion if the circuit wasn't designed for it.  It's better to use values of at least 300 ohms.

I'm sorry to contradict you, but the output impedance of the common emitter BJT amplifier is equal to:

 

Zo = RL || ro

 

Given ro is very high, the output impedance is very close to RL.

 

Figure 9.2.4 Common emitter or source small signal models.

(Source: https://wiki.analog.com/university/courses/electronics/text/chapter-9)

[simon.plata]

On 3/9/2022 at 2:42 PM, TomelloSoulMan said:

Sorry, my fault. I tried the gain trimmer again and it works, but I can only lower the brightness with it. It was already at its maximum setting. I had to roll it down waaaay long to start to see the image dimming, until a point where it was so weak that made my TV lose sync.

 

I've been scratching my head to understand this, maybe the video input into the AV mod board is just weak. So I opened the console yet another time and draw a diagram of the video part, it's attached here. This arises a few questions:

 

- Why R27 and R28 (pull up resistors for SYNC and Lum1) are 4.7K instead of 3.3K as in the original specs?

- Isn't R35 (resistor for Lum0) a bit low at 100K? In the original specs, it's a 110K one, and logic says that it should be near the double of R34. Would it be better if I change this for a 110K one?

- What is the D4 diode doing at the Lum1 line? I tried bridging it and the image became a bit brighter, not enough but a bit better.

- As this worked ok on my Junior, which has totally different resistor values and a CD4050, would it be worth the hassle if I copy all the Junior circuit into a perfboard, taking the outputs directly from the TIA pins, and feed the AV mod with its output? I'd only do this if I can't repair my Junior, because it sounds like a lot of work

 

As a side thing, regarding the static interference, I checked the power rail capacitor and it's a 1000mF 16V one, instead of the regular 2200mF. Maybe I'll change it in the future.

 

As a last thing, I'm sorry because this issue is becoming so long. I really appreciate you spending your time with my little problems, and I would totally understand if you don't want to waste more time on this. I'm glad only by sharing my findings.

 

My apologies, I'm very glad helping you. However I have been very busy this days at work. I will come back when I have a breath. 

[ChildOfCv]

7 hours ago, simon.plata said:

I'm sorry to contradict you, but the output impedance of the common emitter BJT amplifier is equal to:

 

Zo = RL || ro

 

Given ro is very high, the output impedance is very close to RL.

 

Figure 9.2.4 Common emitter or source small signal models.

(Source: https://wiki.analog.com/university/courses/electronics/text/chapter-9)

You looked at the wrong amp.  The AV mod is a voltage follower (common collector) amp.  The amp in this picture is common emitter.  The output impedance of the common collector is RL || rE.  rE is extremely low, so in their example they conclude:
Referring back to our gain example in figure 9.4.2, we can also calculate the output resistance, which will be the parallel combination of the 1KΩ R_Land the 3Ω r_E or 2.99Ω.

 

When you think about it, this makes the most sense:  The Voltage Follower is named because the output voltage follows the input voltage regardless of load.  The only way that can happen is if it has no internal resistance to that load.  The amp is effectively half of a voltage divider and the load is the other half.  So to hold the voltage the same regardless of changing load, the amp must have the lowest resistance possible.

Edited April 8, 2022 by ChildOfCv

[TomelloSoulMan]

On 4/8/2022 at 3:56 AM, simon.plata said:

My apologies, I'm very glad helping you. However I have been very busy this days at work. I will come back when I have a breath. 

No problem Simon, in the meantime I managed to repair my original Jr and it's working as neatly as before. I can split the signal and it shows nicely both in my CRT and capture card simultaneously. Maybe I'll replicate the same output circuit the Jr has (with the CD4050 and same D/A resistor values) on one or both of my clones, but it's no more a priority.

 

On 4/7/2022 at 9:41 PM, ChildOfCv said:

This amplifier doesn't match impedance this way

I have to admit this conversation you're having flies straight over my head. I need to learn a lot on this subject, stop being just a blind circuit replicator and be able to understand and analyze what I'm doing. Maybe the wiki.analog.com link provided above will be a good starting point for me.

Edited April 11, 2022 by TomelloSoulMan
grammar

[TomelloSoulMan]

Sorry for necro-posting... Last weekend I was in a retro expo building one of this mods for a clone and a nice guy came to my side to see what I was doing. I explained it and talked him about the problem I had with this mod while splitting the output and he suggested what I think is a pretty good solution, only adding another 3906 and two resistors (18 and 75 ohm), to get a secondary video output. I built it into my clone and it works great! Image quality is not affected either I connect the video capture card or not, it's always great in my CRT. I made a diagram showing it, what do you think about this solution? My only slight concern was if it could overload the 7805 but I played for a long while and it didn't burn itself... It gets very hot, but it did that before, as it isn't mounted into any heat sink.

[TomelloSoulMan]

Ok, here's the actual diagram ?

[simon.plata]

On 4/8/2022 at 4:29 AM, ChildOfCv said:

You looked at the wrong amp.  The AV mod is a voltage follower (common collector) amp.  The amp in this picture is common emitter.  The output impedance of the common collector is RL || rE.  rE is extremely low, so in their example they conclude:
Referring back to our gain example in figure 9.4.2, we can also calculate the output resistance, which will be the parallel combination of the 1KΩ R_Land the 3Ω r_E or 2.99Ω.

 

When you think about it, this makes the most sense:  The Voltage Follower is named because the output voltage follows the input voltage regardless of load.  The only way that can happen is if it has no internal resistance to that load.  The amp is effectively half of a voltage divider and the load is the other half.  So to hold the voltage the same regardless of changing load, the amp must have the lowest resistance possible.

Not really, maybe you're confused because first stage is voltage follower, but second stage is a common emitter. Output impedance is defined by the last stage, in this case, a common emitter.

[simon.plata]

20 minutes ago, TomelloSoulMan said:

Ok, here's the actual diagram ?

Very clever solution, I think it will work without issues. The additional second stage should draw less than 70mA, so is not a problem for the 7805, as far the heat sink could dissipate the additional power. In the worst case the 2600 will shutdown because the thermal protection of the 7805, until it cools down. Normally a 7805 does not burn itself because is current limited and thermally protected.

[Capellão]

simple, organized and an excellent result!

 

CRT TV.

[ChildOfCv]

2 hours ago, simon.plata said:

Not really, maybe you're confused because first stage is voltage follower, but second stage is a common emitter. Output impedance is defined by the last stage, in this case, a common emitter.

Sorry, yeah the emitter is hanging from the power supply there.  This thread shows both a common-collector amp (which also has the 75-ohm emitter resistor and no series output resistor, which is wrong), and a 2-stage common-emitter amp, which is correct.