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pacoarcade

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This is a video buffer to connect arcade boards to CRT tv sets. It's based on a THS7374 with DC coupled output for RGB, color calibration and a 74HC14 sync buffer outputting 75Ω ~400mV for SCART or TTL CSYNC. It's based on THS7374 documentation, RGB's sync message, viletim's sync message, AV-Driver and my own experimentation.

For audio just connect speakers to the JAMMA edge to get the best experience. This board doesn't have mounting holes to keep a small footprint; you can attach it using mounting tape.

Features

  • Small footprint, no vias for video signals
  • AC Coupled inputs and DC coupled outputs
  • Regulable Red, Green and Blue colors
  • Buffered sync output TTL or 75Ω ~350mV
  • Dedicated ground plane to minimize interferences
  • LPF filter on/off selectable by a jumper
  • Minimalist design so you can fix a bad supergun or consolizations

Notes

Published under Public Domain. Although it has been extensively tested, I take no responsability of the damage done by this design in any case.
 

Attachments

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Awesome. perfect for using RGB modded consumer TVs in cabinets where you don't need anything else than the video circuit!

Thanks for sharing, I am going to print some and have a go.

One thing that could be useful is some mounting holes. how are you planning on mounting yours?
 
Hi, you can use mounting tape. Mounting holes would be nice but the PCB would be bigger. I made this PCB to be small in order to attach it to fix superguns with bad video processing.
 
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So you choose not using 330uf caps in parallel with 0.1uf on RGB line?
Why are you using a buffer on csync? What the advantage?
 
After reading these documents I moved from AC-Coupled outputs to DC-Coupled outputs:
THS7374 allows DC coupled output. If you need AC-Coupled outputs you can always add the capacitors to the SCART connector.

With this CSYNC buffer you always get the same signal amplitude and you are able to select 75Ω ~400mV or TTL using a jumper. Apart from that the Schmitt trigger turns the signal into clean pulses.
 
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  • With me
After reading these documents I moved from AC-Coupled outputs to DC-Coupled outputs:

To AC-Couple or not to AC-Couple? That is the question!
Get a grip on clamps, bias and AC-Coupled video signals

If you need AC-Coupled outputs you can always add the capacitors to the SCART connector.

With that CSYNC buffer you always get the same signal amplitude (~400mV) and you are able to select 75Ω or TTL using a jumper. Apart from that it seems that the Schmitt trigger turns the signal into clean pulses.
1. You are supposed to use clear cables with superguns, without any mods, all the signal clean came from the Supergun, and 330uf won't be bad in anyway.


2. The buffer on csync won't be necessary, most of recent Superguns don't use it, just a potentiometer. For me, I only had problems using it, with Extron RGB equipament there's alot of interference.
 
This PCB is designed to be used with clear cables, that's why I also included the Blanking pad so you can get rid of the 180Ω resistor in the SCART connector. According to that documents, the AC coupling electrolytic capacitors can alter the video signal and they are not really needed, so that saves space and components.

I've tested this setup with several CRT TVs (Grundig, Panasonic, Sony, Philips and Blaupunkt) and apparently it works fine. The second document says that there's some standard where manufacturers put AC coupling capacitors in the inputs, using DC coupled outputs:

Consequently, video-equipment makers have a tacit agreement to AC-couple the input of their equipment, and DC-couple the output—requiring the next stage to re-establish the DC component (see EN 50049-1 for PAL/DVB [SCART] and SMPTE 253M section 9.5 for NTSC, which allow a DC output level)
Just using a voltage divider causes an impedance mismatch when connecting Arcade boards to SCART equipment. Arcade monitors have an input impedance of 1-10kΩ and SCART equipment 75Ω. More information on this here: http://etim.net.au/av-driver/arcade/

Also, you'd have to check the output signal is below 1V. With this circuit with JP2 closed you always get buffered ~400mV SYNC amplitude, that's safe for SCART.

I dunno about Extron. I'd check if it needs TTL CSYNC.
 
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Nice work! Do you sell these units? Or can I buy the pcb anywhere? :)
 
Hey, sorry but dunno why I missed this message. If there is interest I could order pcbs and assemble some units at 20€ + shipping (to Europe). Payment via SEPA transfer.

picogun-326.jpg
Hey :) you didn’t miss it m8, I bought one unit from you 1 year ago approx.
 
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