New 7800 S-Video Board available

[+-^CrossBow^-]

A mini update on my 7800 MK video install. I’ve swapped out the 10k trimmer for a 1k. It makes setting it much easier. FWIW, I have it set at around 780R, which comes out at around 700mv luma on a predominantly white screen. I’ve also swapped the two 68R resistors for 75R resistors. All on a PAL 7800. It’s all just a little bit better for it.

I did something similar recently but instead of changing out the two 68R I changed out the 1k6 on the end with a 2k. That gave me much more range of adjustment. I've got some 5k trimmers coming in the next day or two that I'd like to work with and see what more precision is possible.

[Magic Knight]

I used a 5k trimmer on my pal development PCB and it came in a lot finer than the 10k and easier to adjust. So I'm adopting that value. I also tried 3k and was very dark across the board (so to speak).

So 5k works best I think without the change in the 68R. But maybe better with 1k & 75R?

 

I changed 18k across the TIA audio and pokey to get both aspects and it made very little difference (on PAL). Im guessing that the NTSC version has this imbalance and was subsequently changed for PAL versions developed afterwards.

 

Still to try and balance the exit resistors for a better 2600/7800 brightness ratio.

 

Waiting on Heremes to deliver my 7800 NTSC console but any day now...

Edited March 27, 2019 by Magic Knight

[+Mitch]

I changed 18k across the TIA audio and pokey to get both aspects and it made very little difference (on PAL). Im guessing that the NTSC version has this imbalance and was subsequently changed for PAL versions developed afterwards.

 

 

 

What game did you test with for the audio check?

 

Mitch

[Magic Knight]

I check commando for pokey and xenophobe usually for TIA (similar sounds of guns and stuff) but any game that has a continuous sound works.

As my hearing is sensitive(hate loud TV) I put it at a level where one is just at a level that's annoying and adjust accordingly.

 

-^Cro§Bow^- has done the NTSC comparisons recently and found 18k /8k2 to work to balance sound (instead of a 8k2 twin resistor pair for TIA / Pokey.

 

I'm comparing pal with NTSC models when my NTSC pro system arrives, for improving component changes.

Edited March 28, 2019 by Magic Knight

[+-^CrossBow^-]

Yeap Commando is the main game I use as well but I also use the 7800 Utility Cart rom and the audio test there even more since each channel or voice of sound is done 1 at a time. I can listen to them and see if the TIA channels and the Pokey are at the same or darn near the same amplitude this way.

 

And yes I found that 18k from the TIA and 8k2 or 8k from the Pokey replicated off the south legs of R6 and R5 is the best way to get balanced audio and still retain full RF functionality. That is what I've been doing past few months now or in the case of your s-video board MK, I replaced out both of the resistors you had on it as I think mine had 6k8 in both places with the 18k and 8k instead.

[Magic Knight]

Ok - Some component changes -

 

Luma adjustment - Pot trimmer - instead of a 10k / 1 turn pot, im now using 5K pot with 23 turns - no issue with getting exactly the setting you want and drift is now negligible over long periods..

 

Sound - 18K resistor now in use instead of the 8K2 for audio - NTSC improved in balance. PAL appears to make no or little difference so to keep things harmonized, im going with this.

 

coming soon -

 

Composite built in - WIP at the moment, but soon there will be jumpers or a switch for selecting this during the install. Makes the board more universal for all TV's.

 

2600 compatible - it is at present compatible but going through the variations to establish and test the variants (Heavy Sixer, Light Sixer, Woody, JR etc with both versions (PAL/NTSC)

 

Latest batches..

 

 

 

 

 

[DJ Clae]

I have the 8bitdomain s-video mod kit installed in my 7800. Is this still considered to be a good mod? Is it the same as other mods still being done now?

[+-^CrossBow^-]

I have the 8bitdomain s-video mod kit installed in my 7800. Is this still considered to be a good mod? Is it the same as other mods still being done now?

I can't personally answer this as I've never tried the 8-bit domain kit. Not sure their kits were available anymore once I started to AV mod my systems back in 2012? If they were I wasn't aware of them then as the first 7800 AV mod I became aware of was the LHE.

 

I've looked but it appears the WBM sites doesn't have their 7800 page archived so I'm not able to look at it or see what was involved in installing it?

[ongikong]

I successfully modded my PAL 7800 with Magic Knights S-Video-PCB.

It gives me a nice coloured picture for MARIA & TIA on my Bang & Olufsen MX6000 CRT TV.

But unfortunately I have problems with 2 other devices:

My Commodore 1084S-D2 monitor and an older Mitsubishi projector (which both have a direct S-Video Input) only display a TIA picture in B&W, whereas the MARIA/7800-picture is nice and coloured.

 

Do you guys have any idea why this happens?

Should I try to amplify the chroma signal somehow?

This guy here puts a trimmer pot in the chroma lines while there is only a trimmer in the luma line on Magic Knights PCB.

Or is it possible to simply change resistor values to get a stronger chroma signal?

 

I am no expert in building circuits and will be thankful for any hint...

[ongikong]

.... For those of you who are interested:

I finally managed to get a coloured picture on my monitor and projector in 2600-mode.

 

I swapped the 2 resistors in the CHROMA lines with two 10K trimmer pots and tweaked a little bit until the colour popped up.

With the three trimmers in total there are much more capabilities now for finetuning the picture of your desired display device. - I'm happy now and can finally reassemble my console!

[Magic Knight]

Thanks for the update. Chroma adjustment looks like the way forward for displays that don't want to play ball.

[killersquirel]

.... For those of you who are interested:

I finally managed to get a coloured picture on my monitor and projector in 2600-mode.

 

I swapped the 2 resistors in the CHROMA lines with two 10K trimmer pots and tweaked a little bit until the colour popped up.

With the three trimmers in total there are much more capabilities now for finetuning the picture of your desired display device. - I'm happy now and can finally reassemble my console!

 

Do you have any pictures of what you did?

[ongikong]

 

Do you have any pictures of what you did?

 

Yes, won't win a beauty contest but it works...

I simply extended the legs of the trimmer pots and soldered them in place....

[+-^CrossBow^-]

My only issue with adding in more pots is that this becomes a project strictly for those willing to tackle it themselves. It doesn't work well for those of us that offer to AV mod other gamers systems because we have no way to dial everything in for a consistent look. I'll make all the adjustments on my AV equipment and get it looking good, then send it off to the client who ends up with a scrambled picture, or no color..etc..because it requires being dialed in for their setup. Most of my clients aren't willing to crack their system open to play with these adjustments because they are the type that doesn't like to mess with this stuff. That is why they sent their systems to me in the first place.

 

I really like the results the Magic Knight board gives, but the dialing in makes it tricky to offer as an option for clients. I have mentioned all of this to Magic Knight and he is aware of my thoughts and concerns in this regard. For the DIY type these are an excellent option and I have on that looks great in one of my 7800 test systems.

[+JAC!]

I've put the board into my 4 switcher 2600 PALbut now it seem to have issue with the luminances. Looks like only 2 of 8 exist.

 

This is how it should look like: 

 

I've taken some pictures and a short video: https://www.wudsn.com/tmp/vcs.zip

This is how it looks.  The text player on the left/right just go black/white instead of fading.

 

Sound, sync, color and power seem to be OK (or at least present).

 

I've unsoldered the 3 luminace wires (WHI/VIO/ORG) from the board and attached them test wise manually.

Looks like the luminances are cancelling each other out.

Any hints?

- Peter.

 



 

EDIT: I have also created a test ROM that display the classic rainbow effect to show the problem.

The left half of the picture is the TV, the right half is from Stella. The color sequence is not a rainbow because of PAL, that's normal.

 

[+JAC!]

I've tried all 6 permuations for soldering the 3 luminances and the result always looks wrong. Can someboy please post the values for R1 to R8?

[ChildOfCv]

Are you sure you followed the instructions?  Even if the resistors are wrong, you should still see shades.  From your description of the problem, it sounds more like the luminance outputs are shorted.  Check with an ohm meter.  If you have solder blobs, remove them.

[+JAC!]

12 hours ago, ChildOfCv said:

Are you sure you followed the instructions?  Even if the resistors are wrong, you should still see shades.  From your description of the problem, it sounds more like the luminance outputs are shorted.  Check with an ohm meter.  If you have solder blobs, remove them.

By trimming the potentionmeter I can make more shades show up about 3 or four. But the sequence is wrong, no matter which order I use. In fact it looks like one of the bits (B2 or B1) is inverted. On the other hand that is not possible, because black is displayed as black and white as white...

 

EDIT: I've traced the lines from the TIA and verfied these connections:

 

Wire / TIA / Board

WHI = LUM2 = TIA 6 = R1 = 470 Ohm
VIO = LUM1 = TIA 5 = R2 = 160 Ohm
ORG = LUM0 = TIA 7 = R3 = 912 Ohm

 

Here R2 looks totally wrong for me. It should be tween R1 and R3.

[ChildOfCv]

30 minutes ago, JAC! said:

By trimming the potentionmeter I can make more shades show up about 3 or four. But the sequence is wrong, no matter which order I use. In fact it looks like one of the bits (B2 or B1) is inverted. On the other hand that is not possible, because black is displayed as black and white as white...

Uh... what potentiometer?  The color delay?  Because that has nothing to do with shading.  The LUM0, LUM1, and LUM2 outputs of TIA give you shading while the COL output gives you the S-video chroma.

 

Make sure you're using the PAL pinout and not the NTSC one.

[RevEng]

Assuming he means the pot on the mod itself, not that I have one of these. It sounds like the signal was too hot previous to trimming the pot, overpowering luma even when just the lower bits were enabled.

 

@JAC!, seems all you need to do now is swap the wires going to TIA 5 and 6.

 

 

[ChildOfCv]

Yeah I finally noticed the third pot other than the 2 that the previous conversation's poster had added.

 

Good point about adjustments though.  If you have a scope, you can adjust the pot until you achieve a 1V output.  If you don't, then start at the lowest value and crank it until the video at least syncs, then keep going until it appears at a good brightness.

[+-^CrossBow^-]

46 minutes ago, ChildOfCv said:

Yeah I finally noticed the third pot other than the 2 that the previous conversation's poster had added.

 

Good point about adjustments though.  If you have a scope, you can adjust the pot until you achieve a 1V output.  If you don't, then start at the lowest value and crank it until the video at least syncs, then keep going until it appears at a good brightness.

This is how the luma trimmer that comes already on the board is adjusted. You adjust until you get sync and then increase until you have the brightness you want. Great! Then you try it on a different TV and find out it is now too bright or is out of sync again. That was the issue with one I added and still have in place as a test 7800.

[ChildOfCv]

21 minutes ago, -^Cro§Bow^- said:

This is how the luma trimmer that comes already on the board is adjusted. You adjust until you get sync and then increase until you have the brightness you want. Great! Then you try it on a different TV and find out it is now too bright or is out of sync again. That was the issue with one I added and still have in place as a test 7800.

There must be a design flaw then.  If you have the 75-ohm series output impedance with 1Vp-p, then you are outputting an industry-standard signal that any working TV should show in the same way.  The biggest mistake too many people make (haven't seen the MK board in person, so can't comment on this specific case) is that either they don't add an output coupling capacitor or they assume that the emitter resistor on the voltage follower sets the output impedance.  It doesn't.

[+-^CrossBow^-]

I don't disagree with this, I can only tell you that the results will vary depending on the display being used. I installed a Magic Knight into a client's 7800 sometime back. The luma trimmer was adjusted for best results on both 2600 and 7800 games here on my AV setup. I send it back and the client states they only get squiggly lines on 2600 and no color on 7800. I have them send it back for me to work on it. I plug it up to my AV setup and it is working just fine?! I then finally connected it to my CRT in another room and saw the same as the client. Squiggly out of sync lines on 2600 games and washed out to no color on 7800 games. I then adjusted the luma trimmer and had it looking great for both 2600 and 7800 games on that CRT. I then plugged it into my LCD AV setup and the colors were now washed out and too bright. 

 

So yeah, it is a great S-video solution, but it does require to be adjusted to the particular TV it will be used with as it just doesn't seem to be consistent between displays and types.

 

BTW...this s-video board is essentially the Saundby mod with some values tweaked and the audio portion added (Recently corrected with proper resistor values for NTSC consoles).

 

http://saundby.com/atari7800mod/7800_vidmod_theory.html

 

The LUMA trimmer on MK's board is being used in the same place as R5 marked on Saundby's schematic and even on that webpage it is noted that due to differences in the components used on both the console side and AV board side, plus display types, that a trimmer is needed there.

 

It was because of these inconsistencies that Bryan would eventually create the UAV. Initially for the 8-bit line of computers, but then designed/tweaked for use on the 7800 as well. Bryan's solution was to design a completely new video circuit to provide consistent results. Again, I find no real fault with MK's board or the initial circuit for a quick and easy s-video output solution. But it does require that such a board be installed into a 7800 whose user is willing to make adjustments as needed.

 

 

[ChildOfCv]

44 minutes ago, -^Cro§Bow^- said:

BTW...this s-video board is essentially the Saundby mod with some values tweaked and the audio portion added (Recently corrected with proper resistor values for NTSC consoles). 

 

The LUMA trimmer on MK's board is being used in the same place as R5 marked on Saundby's schematic and even on that webpage it is noted that due to differences in the components used on both the console side and AV board side, plus display types, that a trimmer is needed there.

Yeah you definitely need to set the proper P-P voltage level, and that requires some trimming.

 

I do have concern that the diode could confuse the TV's automatic gain control if the voltage divider doesn't bias the luma signal correctly though.  But maybe MK already tweaked those values.  An extreme silly example would be if the voltage divider biases to 4.5V and the luma has +/-0.5V swing.  Then you'd have sync pulling to 0 while the video sits at mostly white, once the TV scales it down to expected magnitudes.

 

One experiment would be to place a 100uF capacitor between the MK luma output and the S-video pin.  Polarity should be facing the MK board.  See if that improves consistency between TVs.

 

And yeah the UAV appears to be the best solution, at least by my understanding that it's a fully digital solution.