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DickGalactic

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Okay, so a little while ago I built my own SuperGun. It is RGB out with no encoder. The RGB lines go to 500 ohm pots and the sync goes through an LM1181 sync stripper and that's it.
I recently found out through a Twitter conversation about some other SuperGun that I am pushing too much juice into my Framemeister and that I need to bring down the signals to
the proper levels. I got my hands on a THS7374 and took a look at the datasheet, and I have some questions.

On the inputs, it looks like there are resistors all going to ground but there is no value written. What value would be appropriate?
On the outputs, I can see they are all going to 75 ohm resistors. That's fine but shouldn't I put some caps in there also? I know other systems use 220uf caps but would that work for
arcade stuff too?

As you can tell, I am in the "A danger to myself, others, and expensive equipment" phase of my modding and repair studies. Thanks for helping a nub out.
 

Mobiusstriptech

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The value for the resistors before the amp depend on the inputs you have. I was actually going to work on this tonight myself and have been sidetracked with trying to solve SSD3 problems.

On the output the resistors depend on what is needed to get you to 75ohm levels. It's not guaranteed to be 75ohm resistors that you need.
 

DickGalactic

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Thank you for the guidance. I guess for now I am stuck because while I have a rough idea of what I need to look at, I don't yet have the equipment to do it. I will look around some more. I am sure someone has to know roughly what arcade pcbs are spitting out.
 

Mobiusstriptech

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Yeah normally when you are building a video circuit you will want to use a scope. Generally the signals should be .7pp being sent from your circuit.

I can't say for certain but it wouldn't surprise me if it is not a 1 size fits all for arcade hardware. Simply because the manufacturer had the ability to choose each component for their device, including the screen, which obviously wouldn't be the case with home console hardware.
 

yavuzg

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You don't need that line buffer/driver chip on an arcade pcb video. That IC is generally used to drive long video cables (like security cameras) or "amplify" (not attenuate) weak video sources.

Arcade PCB's generally put out 3-4 Vpp video signal. So its a powerfull video signal. Ofcourse this is not a written-by-blood type of standard like NTSC/PAL or VGA video standards (0.7Vpp) but almost all arcade PCBs outputs video around this level.

So... Just use simple 220Ohm standard resistors (as skate323k137 said) or if you like more control, use 500Ohm pots (as you said) to simply attenuate the signal.

Also, you do not have to use LM1881 because I've never seen an arcade pcb putting out "composite video" on the sync line. It is pure sync nothing else there. So you do not have to use a LM1881 to strip anything. That is what the "console guys" do to strip the composite video component out of the sync line.

Yes, I know, people may say that "no thats a sync cleaner you better use it". Funny, if your sync line is not "clean" then fix your power supply or PCB :)
 

Mobiusstriptech

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The problem is if you are sending those signals to something like a framemeister, it is above the max specs and can cause damage over time.
 

DickGalactic

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I don't use the LM1881 for the stripping function but for the clamping function. Some PCBs, like my Tiger Road PCB, have a sync signal that oscillates too much for the Framemeister to grab a hold of. Without a sync stripper the screen just constantly judders, shakes, drops, and is completely unplayable.
As for the part about attenuation, I guess I should detail how this all started.
A guy posted a Youtube video about a Super Gun called the Retro Electronik. Voultar commented that it wasn't designed to properly attenuate the signals. Someone later asked what would be a good way to do it and he suggested the THS7374. The Twitter thread is here.
After reading that, I realized I was essentially doing the same thing with my SG, and that I needed to get my house in order. So now, I am just trying to figure out the best way. Thanks again to everyone for the advice and feedback.
 

ekorz

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Voultar also recommended RGB's HAS as preferable to that Retro Electronik device. I totally agree but that's just because I'm a happy customer, not an expert like the great Lord V. Maybe we can page @RGB and ask for his advice here?
 

plasia

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I have a retroelectronik supergun, but honestly I only use it for feeding my Capcom IO on my Naomi setup.

I have a HAS for my neo/CPS3 :) Really feel for the people that want a supergun though, nothing seems really available/good.

It would be great if there was a circuit I could show you!
 

DickGalactic

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Yeah. Thanks for reading and wanting to help. :)

I will keep poking around but I may just end up getting an encoder like a JROK and slapping it in there. I didn't go that route in the first place because RGB is all I need but I may have to in the
end.
 

RGB

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Hey, what you want to do is attenuate the RGB signals using a big potentiometer setup as voltage divider (500R at least, but 1-10K would be even better) and connect to some video amp with a gain of 2, for example the popular Texas Instruments amp (THS7314/16 or THS7374/76 series). For CSync just use a 470R resistor, or also buffer it. This thread will give you the idea - [Neo Geo] AES RGB-Bypass with THS7314 RGB-Amp

Another solution if you just want to build a one-off, would be using Tim's PCB called "AV DRIVER" - http://etim.net.au/av-driver/arcade/ It takes care of both the video and audio.
 
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