Portable arcade test rig

In working with arcade PCBs and trying to debug, diagnose, or just determine what they are it’s nice to have an arcade cabinet to plug them into.  Sometimes you want to set them up where having an arcade machine is difficult, or putting the board on a bench is most helpful.  In that case, a JAMMA harness and arcade monitor would be the usual prescription.  That is a little cumbersome to have to carry around all the time, and also can be fragile as monitor neck is exposed.  It’s possible to get a 15khz to 31khz converter to allow arcade boards on a regular computer monitor, but where’s the fun in that.  To solve this problem I packaged all the contents of an arcade machine in to a mini-ITX case and made it as portable as possible.

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my dumb genesis controllers can be seen in their first application

To start the discussion of some choices, first I need to go over JAMMA.  JAMMA stands for Japan Amusement Machine and Marketing Association, Inc. and is the shorthand for a rough standard based around a 0.156″ pitch double sided 28 pin long card edge connector.  I say rough because I’ve found some variations and omissions that I will try to correct on this rig as time goes on.  I don’t intend to try to make this an everything tester initially, but if I need to test additional boards that use other unpopulated (or otherwise mapped) pins I’ll make revisions.  I compiled a ‘comprehensive’ set of pinouts for the JAMMA connector based on a lot of various incomplete or different variations (I almost said universal instead of comprehensive, but that’s a brand of arcade manufacturer with their own variation):

jamma-pinout

Here’s my explanation:

  • Some things are pretty constant, the top power rails, the video connections, and the  coin, joystick, and a few buttons
  • Some things have been omitted more and more like the lockout coils,  the coin counters, and whatever power rails a particular board doesn’t need
  • Other things get extended into unused or redundant pins, like buttons replacing grounds or test switches
  • Creating stereo sound seems to have been re-invented at least twice

MGD is the arcade incarnation of the dreamcast, and I’m not planning on hooking one of those up any time soon (but if I do I know it’ll mostly work.  I do like Neo-Geo, so having those buttons work would probably be nice (maybe I can get clever and mix the sound).  The rest of the issue is to try to populate things that are usually omitted and hope I don’t run into too many crazy things.  Right now I have populated everything the JAMMA connector I used had wires for, and ordered more crimps to populate the others.  I used the common L and 10 connections for mono sound out, even though that’s not supposed to need an amplifier.  As you can see this is pretty much all you would need for any arcade machine, not even just JAMMA compatible ones.  Using this interface and some adapter boards it is possible to connect just about anything to this tester, although it may require a special adapter to be made.  Some adapters may not be passive either, Nintendo had this thing that inverted the colors to their monitors, others may use different voltages, but all this can be made to work now that most of the items are in a nice compact case.

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Interior shot

This lets me show the various things I’ve done to it so far.  the JAMMA connector goes out the I/O backplane hole, the DE-9 ports I have used for controller ports go out the PCI slots (the brackets already had DE-9 knockouts in them).  The power supply is a regular arcade power supply riveted through the immobile side of the computer case, it is hooked up to the back of a PC power supply that I cut the back plate off of.  That plate has the fan, power switch, 110/220 switch, and power connector on it and they all work.

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The monitor is a Samtron 5″ monitor from here although their prices are not accurate any more.  The monitor chassis is mounted on the motherboard tray so I can access the tuning controls, and I cut away most of the 3.5″ bays to allow my hand to get back there.  The monitor came with a degauss coil, but no associated drive circuitry, I initially hooked it up with a button, but that blew up the first coil, apparently a “positor” or positive temperature coefficient device is used to open the circuit at the optimal time to fix the screen.  I pulled the device out of an old 15″ CRT monitor that came in with a Windows 95 era wood CNC.  The monitor has inputs for RGBI (although it appears no one has ever documented actually using one for that) and was converted to analog with help of a sync splitter and this guide.

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Along the top in the last two pictures is a 5.25″ bay set of speakers, this is what really led me to this project, just throw it all in a PC case.  I do also remember an old website where someone crammed several small black and white portable TVs into a case and fed them with a modified video card to display at the right frequency.  He managed to drive three monitors from one signal, one color for each monochrome monitor.  Originally that set of speakers had a cord hardwired in the back, I took it upon myself to eliminate the cord and put a jack so the harness could plug into it (there was a footprint on the pcb for a jack I had).  After opening it I realized that the speakers were just grills and the real speakers were regular big 8R drivers sit at an angle and pointed up, not even any isolation between them.  There’s just this big cavity and the sound will eventually make it out the front.  Well.  If they can do that so can I.  We were looking for a design to fit the blankout plate that is to go over the USB and sound holes when I pointed out that the Vault Tec logo is about the right proportions.  That plus blue meant that it now needs yellow instead of white paint.  At the insistence of Mike who pointed out that it looks great, except for that crappy speaker grill, I decided to just bandsaw off the front and he’s make another acrylic piece that went all the way up.  Along the way I decided to eliminate the power indicator and the aux-in jack and switch to a more appropriate knob instead of a slider.

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As you can see we borrowed a speaker grill off a thing and a knob off a guitar (it’s mono now, but it drives both speakers) to make the top audio section.  The text around the knob was cribbed from an HM audio generator.  The arcade coin buttons happen to perfectly fit in the floppy drive section and you can see the degauss button that’s actually bolted through the steel so it sits flush.  I bought real coin counters and added diodes to them to protect the arcade board, they are apparently low side drive so I hooked mine to the 12v line since they’re 12 coils.  The lockout coils I knew less about, I just assumed they’d go high with 5v and wired the other side of the LED to ground, but we’ll see once the terminals get here because that section wasn’t wired on this harness.  I added Tilt, Test, and Service switches even though Test wasn’t populated on my harness either.

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I gave this thing a new coat of paint in almost the same colors it was already, just to cover up the scuffing that had gotten done to it over the years.  I still have handles to mount, additional pins to wire, and maybe a circuit to sit in the harness and talk to buttons 4, 5, and 6 after decoding real 6-button genesis controllers.  I also blew up the audio amp chip on the tetris board, oops.  Did you know that tetris by Atari has two power amp footprints on the PCB and only one populated? The schematic seems to imply that they are in parallel but I assure you the footprints are mutually exclusive.

EDIT: the tetris problem was because that mono output was supposed to be for an isolated speaker and I’ve now put a 1:1 audio transformer in the line to isolate my amp ground from speaker-.  Also some boards assume all the power and ground pins are hooked together in the harness, this isn’t true in mine yet but I added one bodge so far to bring up one board.  I may use vampire taps to join the remaining wires.

We picked up a jamma extension harness from china for super cheap, so far I’ve had to fix a bunch of stuff:

  • 12v was run with one wire and only connected to one side of the fingers, but both sides of the card edge female
  • 5v was run with one wire and only connected to one side of the fingers (two fingers), but both sides of the card edge female (all four pins)
  • -5v was not run
  • coin counters and lockout coils were not run
  • no key in female card edge
  • 7th pin not cutout of fingerboard

I can understand cheaping out by running fewer wires, and I can understand not using the coin counters and lockout coils.  The lack of -5v really shows that this was designed for a small target market.  The lack of joining the finger sides together also shows that it’s only really meant to work on ‘most’ machines.  The lack of a key and cutout, I don’t know any boards this would work on out of the box… That all being said, The cost of this harness was under $5 and well worth it after repairing it.

Further Edit: I drilled and tapped some holes and bolted some feet to it, these come from some old test gear and really help the viewing angle of the CRT if it’s sitting on a desk.

One Response to “Portable arcade test rig”

  1. Neo Geo compatibility for my portable arcade test rig | Evan's Techie-Blog Says:

    […] figured I’d go over some mods done to make my arcade test rig Neo Geo compatible. The first thing to do was make a JAMMA fingerboard adapter. I have a 2-slot […]

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