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 Post subject: THS7316/74 Series for Video Amping Arcade Monitor?
PostPosted: Sat Apr 07, 2018 5:07 pm 


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Location: San Jose, CA U.S.A.
Been following Bob's guide over here with the intent to connect my consoles with that circuit to an arcade monitor. Been pretty unsuccessful in getting stable image.

I know that I need DC-coupled output for connection to an arcade monitor but am not sure how to do this. TI's datasheet simply says that the THS7316/74 chips can drive output with DC-coupling.

Code:
Note that the THS7374 can drive the line with dc coupling regardless of the input mode of operation. The only requirement is to make sure the video line has proper termination in series with the output-typically 75 ohms.


I followed Bob's guide exactly thinking that his guide does what the data sheet explains. I know his instructions aren't meant for arcade monitor hookups. But is there a way to achieve DC-restoration, clamping, amping, DC-coupled output with these IC's? Is there another step I have to take? Perhaps grounding the 75 ohm resitors? Or should I go with another IC, like the MAX4386E the same one Ultimarc uses?


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 Post subject: Re: THS7316/74 Series for Video Amping Arcade Monitor?
PostPosted: Sat Apr 07, 2018 5:43 pm 


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Also, I am tapping into a Sync Strike for 5v to power the THS7316. For Csync, I am getting that right off the Sync Strike. Here is a pic of what I am doing (ignore rough solder job):

https://photos.app.goo.gl/MYKzAwvsReLLRNl63

And here is what the image looks like. This looks very much like I am piping the AC-coupled SCART RGB straight from the Sync Strike without even amping the signal through the THS7316:

https://photos.app.goo.gl/zZSmvmMA7V87rKUf1


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 Post subject: Re: THS7316/74 Series for Video Amping Arcade Monitor?
PostPosted: Sat Apr 07, 2018 7:24 pm 


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I still have to find a proper, nice way to collect consoles to my arcade monitor with a satisfying image quality.

I have tried all sort of things and they all came short.

I'll be following closely this thread hoping something good turns out (possibly documented with pictures of various consoles).


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 Post subject: Re: THS7316/74 Series for Video Amping Arcade Monitor?
PostPosted: Wed Apr 11, 2018 12:25 am 


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Joined: 22 Aug 2005
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In the picture I posted are THS7316 PCB's I made from chips I got from eBay somewhere in China. Might be counterfeit for all I know because there really is no difference from what I can tell running the console RGB directly to the JAMMA harness or running the "amped" RGB from the THS7316 to the JAMMA harness. I do have some legit THS7374 chips that were purchased from Digikey. Waiting on the PCB's to arrive, ironically enough from China, to see if chips from a known legit vendor makes a difference.

Also, the resistors I added per Bob's instructions go right into the RGB lines to feed the arcade monitor. I am thinking I should run those resistors to ground on the THS7316 PCB I built; is that what is meant by 75 ohm termination? Then tap into that at some point between the end of the resistor and the grounding joint to run a line to the arcade monitor. Hopefully that gets me the DC coupled output needed. Totally don't know anything about the electrical stuff. Keep you posted.


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 Post subject: Re: THS7316/74 Series for Video Amping Arcade Monitor?
PostPosted: Wed Apr 11, 2018 1:31 am 


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I use THS7316's to connect to my arcade monitors from a PC source. Without amplification the picture is only just visible, it's night and day for me :)

The RGB lines need to be terminated at 75R to ground on the way into the amp but do not want caps, and I just use 1K pots on the output lines and nothing else, into the chassis RGB lines. I don't bother amplifying sync, just combine it with some resistors and maybe a diode, into the chassis. Also, make sure not to use the caps across the power supply rail - or anyway they produced noise issues for me. Given the only difference between a PC and a SCART console is sync, maybe try simplifying the circuit like above? Also, what chassis are you using?


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 Post subject: Re: THS7316/74 Series for Video Amping Arcade Monitor?
PostPosted: Wed Apr 11, 2018 2:32 am 


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buttersoft wrote:
I use THS7316's to connect to my arcade monitors from a PC source. Without amplification the picture is only just visible, it's night and day for me :)


Part of this effort later down the line is to also amp my CRT_Emu based MAME setups so very interested in how you are achieving this. Saves the need to use a J-Pac for amping while also allowing for joystick control rather than keyboard emulation of the J-Pac. Perhaps you can post some pics of your PCB?

buttersoft wrote:
The RGB lines need to be terminated at 75R to ground on the way into the amp but do not want caps, and I just use 1K pots on the output lines and nothing else, into the chassis RGB lines.


Ahh, so there is the key. So if I understand what you are saying, I have the 75 ohm resistors on the output without any termination to ground. I should instead have the 75 ohm resistors grounded just before the input?

I suspect you have the 1k pots to control either brightness or contrast on each individual RGB line. Is this necessary given the chassis has brightness and contrast controls already and RGB cutoff and bias on the neckboard? Also, do you struggle tuning the individual pots on the RGB lines? Or do you use a triple gang pot to adjust the three RGB lines all at once?

buttersoft wrote:
I don't bother amplifying sync, just combine it with some resistors and maybe a diode, into the chassis. Also, make sure not to use the caps across the power supply rail - or anyway they produced noise issues for me. Given the only difference between a PC and a SCART console is sync, maybe try simplifying the circuit like above? Also, what chassis are you using?


I don't have any problems with sync on my CRT_Emu setups or with the console hookups. My console SCART cables have Csync built in. Plus I am using the Sync Strke to strip sync. Other than that, I don't alter the sync at all. I just run that into my chassis on my console and CRT_Emu setups.

So that cap I have bridging ground and 5v should be removed? Weird, even the TI's documents say that it should be there. But I'll give it a shot.

I'm using a variety of chassis. But mostly Wells Gardner K7000 and Nanao MS8. On the K7000, don't have much issue other than needing to recalibrate the settings on the chassis since it can accept an AC-coupled video signal. But on the Nanao MS8, there is no adjustment I can make to make it look right.


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 Post subject: Re: THS7316/74 Series for Video Amping Arcade Monitor?
PostPosted: Wed Apr 11, 2018 5:25 am 


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Ok. So I stripped back some insulation from the RGB input wires. Wound the lead from the 75 ohm resistor around the bare wire and soldered that to the wire. Here is a photo of where this is at at this point:

https://photos.app.goo.gl/hMkTpPOZeOLJJU9s2

Twisted the other ends of the resistors together and soldered the resistors to ground. Then removed the resistors on the output. In the place of the removed resistors, I just soldered in the RGB output which will then plug right into the JAMMA harness. Here is that pic.

https://photos.app.goo.gl/CyAJUa7TiNil8w8m1

Then I plugged that into my cab. And here is what I got:

https://photos.app.goo.gl/T6pCRNPJGZz4TUiO2
https://photos.app.goo.gl/b3CDqC7MSqv1zud72
https://photos.app.goo.gl/NvpUTKuRVTc5X1002

And here is some 240p test suite action going on:

https://photos.app.goo.gl/ud8J7Y9bxfwxbQid2
https://photos.app.goo.gl/1v6M9XraXadWKUqH2
https://photos.app.goo.gl/wMBvsRBCeYx7oObl2

Wooohooo!! The images in the photos are much nicer than the actual display. The DC boot screen is barely visible. But, the image is stable. No idea if the IC is doing it's job beyond producing a stable image because it is so dim. Game screens are legit. Just the boot screen looks terrible. I'll keep at it.

1st EDIT: Probably the next step is to remove that cap bridging 5v and ground as buttersoft suggested. I do get a lot of artifacts as shown in the images. I'll add heat shrink to the resistors terminating to ground . Try some other consoles too and post snaps. Also, these tests were using an El Cheapo Dreamcast SCART cable. My cables from thefoo83 just arrived in the mail. Will connect those and see if I get a better image.
2nd EDIT: Will experiment with El Cheapo Dreamcast SCART cable and remove the 220uf caps from the signal lines and see if that makes a difference on the brightness of my DC boot screen.

FYI, I still plan picking up viletim's next SCART2JAMMA 3.0. But since I have more than one cab kind of need a cheap solution between now and then.


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 Post subject: Re: THS7316/74 Series for Video Amping Arcade Monitor?
PostPosted: Thu Apr 12, 2018 2:56 am 


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Removing the cap between the 5V and ground rail won't help brightness, it's only to remove some noise problems. If you're not having any noise problems you don't have to try removing it :)

As for a circuit, cable your RGB lines straight out of your source directly into the THS7316 chip (with a 75R termination to ground). If your SCART cables have caps or other resistors those could be your problem. For the output of the THS7316 remove the 75R resistors and just use the pots - you're right in that they're not strictly needed, so you can go a triple gang if you want. They're nice to have sometimes, but you should be setting the chassis pots as well. Totally ghetto circuit, but the TI chips have a limit on the output current, so they don't strictly need to be loaded, and you can guarantee the input stage of your chassis is anyway. Arcade video wiring wasn't 75ohm, there was no standard, so arcade chassis normally don't require input termination.


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 Post subject: Re: THS7316/74 Series for Video Amping Arcade Monitor?
PostPosted: Thu Apr 12, 2018 3:25 am 


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Joined: 22 Aug 2005
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Location: San Jose, CA U.S.A.
buttersoft wrote:
Removing the cap between the 5V and ground rail won't help brightness, it's only to remove some noise problems. If you're not having any noise problems you don't have to try removing it :)


I am definitely getting noise brutha. So I'll remove that cap. Weird that the datasheet recommends it, even Bob and borti says it is needed. But ok, this is for the unintended purpose of arcade monitor interfacing so it's a different application than what the datasheet is explaining and what Bob and borti are solving for.

I'll do that and post before/after pics.

buttersoft wrote:
As for a circuit, cable your RGB lines straight out of your source directly into the THS7316 chip (with a 75R termination to ground). If your SCART cables have caps or other resistors those could be your problem.


I am reading over here that the 220uf caps in the SCART cables are to remove the DC current. I will remove them from my SCART cables and post before/after pics too.

buttersoft wrote:
For the output of the THS7316 remove the 75R resistors and just use the pots - you're right in that they're not strictly needed, so you can go a triple gang if you want. They're nice to have sometimes, but you should be setting the chassis pots as well. Totally ghetto circuit, but the TI chips have a limit on the output current, so they don't strictly need to be loaded, and you can guarantee the input stage of your chassis is anyway. Arcade video wiring wasn't 75ohm, there was no standard, so arcade chassis normally don't require input termination.


Would have been nice to have a standard back in those days huh? Seems everytime you plug a JAMMA board into a cab you have to recalibrate. Let me see where I am at after removing the caps and adjusting the chassis pots. If everytime I switch from connecting consoles to JAMMA boards, I have to recalibrate, then 1k pots it is. As for ghetto, nah. This is going to be inside a cab, maybe even in a project box, and invisible for the most part.

I appreciate your help brutha. Thanks!


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 Post subject: Re: THS7316/74 Series for Video Amping Arcade Monitor?
PostPosted: Fri Apr 13, 2018 5:37 am 


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I hope it is actually a help! Works for me as described. And i was certainly getting interference depending on the chassis and source, even from some consoles.


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 Post subject: Re: THS7316/74 Series for Video Amping Arcade Monitor?
PostPosted: Fri Apr 13, 2018 7:29 am 


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Location: San Jose, CA U.S.A.
Ok, here is the screen with the 220uf caps still installed in the DC RGB cable:

https://photos.app.goo.gl/uan2HEJV9YOZoSg72

And here are a couple of shots of the screen with the 220uf caps removed:

https://photos.app.goo.gl/VvaRPHBXDIxc18Xh2
https://photos.app.goo.gl/cSXNZ5zBfXlFpryr2

Wooohooo! Contrast issues solved. This was with the El Cheapo Dreamcast cable. I'll remove those caps from thefoo83's cable that I picked up as well; that is such a nice cable. Great quality. I have had various DC VGA boxes over the years, and thefoo83's cable is the best solution I have had yet.

Also, you'll notice the noise on those screens. I plugged in the PS2 as well and aside from the noise, it looks fantastic as well. I was scared I would get a dim image because the PS2 has the caps installed internally. Seems that isn't a problem.

https://photos.app.goo.gl/aujxUMselbqR2JBL2
https://photos.app.goo.gl/FAiQUZqbTyaETuoC2
https://photos.app.goo.gl/tQ5F0ISaoFIxsbft1

I still have noise however. I'll remove that cap tomorrow and see where that is at.

Thanks again man. I have been at this for a while. Video was the last problem to solve for. I have started putting together a tutorial for this over at Arcade-Projects.com giving credit where credit is due.

EDIT:
Here is a shot of Macross M3's title screen with the cap bridging the 5v rail and ground:
https://photos.app.goo.gl/LptXsf0NIHPYt6sF2

And here is a nice shot of Macross M3's title screen with that cap snipped off:
https://photos.app.goo.gl/QAmBNqoqdlE2CQzj2

Final verdict...............for interfacing to an arcade monitor, the cap does introduce noise to the resulting image. Don't install it for this application. I'll get to building another one of these circuits for my MAME setup over the weekend. Should end up having the same or similar results!


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 Post subject: Re: THS7316/74 Series for Video Amping Arcade Monitor?
PostPosted: Mon Apr 16, 2018 2:44 am 


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For me the cap across the power supply rails of the THS7314 or 7316 was only an issue on some chassis. I have two ancient chassis from the early 80's, an MC-2000s and another Nanao, and neither one is affected by the cap. But those are the Rolls Royce of their age. All the TV chassis I've hacked, and the Wei-Ya universal i bought, get bad noise with the cap in place.


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