Modifying a CRT television for use as an arcade monitor

crt-arcade-monitor

Finding himself in need of an arcade monitor [Eric Wright] turned to this ancient CRT television. The problem is that arcade monitors and televisions didn’t operate in the same way, differing in both resolution and refresh rate. [Eric] modified the television to work like an arcade monitor, but only with limited success. He’s hoping a few more alterations will lead him to a complete solution.

The image above shows him testing a Pac-Man machine on the altered Sharp television. Those familiar with the game will immediately notice that there is something wrong. We see most of the tracks upon which Pac-Man and the ghosts travel, but he maze itself is completely missing. To get to this point [Eric] consulted the television and arcade schematics to figure out how to connect the composite sync and three color channels directly to the arcade machine. This way the CRT timing is forced to conform to the game standard. The problem is that there is no way to adjust the drive and cutoff of the individual color channels. This is something [Eric] hopes to fix in the next iteration of his experiments.

If you already have a working arcade monitor but no gaming cabinet why not use a Raspberry Pi?

Comments

  1. dan tan says:

    He Should be using a mutisync PC monitor or an LCD, 320×240 or 640×480 resolution. He can also use a scan converter as well.

  2. HowardC says:

    If you run a pc monitor or lcd on a real arcade pcb, the higher dot pitch makes the games look terrible. TV’s use the same tubes as arcade monitors, so the dot-pitch is the same.

    That being said, there are simplier ways to fix his issue. The most popular one is to simply buy an arcade chassis specifically designed for old tv tubes.

  3. Monitor Nerd says:

    It’s unlikely that he’s going to find a multisync PC monitor that can sync down to 15.75 kHz. If the jungle IC is outputting RGB…

    I’ve swapped tubes and although it works, it’s a PITA. You never know in advance what the impedance of the TV CRT’s yoke is going to be, and you therefore never know if you’re going to have a suitable (original arcade monitor or third-party generic RGB) chassis on hand to drive it. If the yoke matches, you’re probably fine, but if the yoke doesn’t match and is bonded to the tube, you’re stuck with trying to drive the yoke you’ve got. If the yoke doesn’t match and *isn’t* bonded to the tube, you’ve got to pull out the gloves and the mirror and try to adjust purity/convergence after swapping the CRT’s yoke with the arcade monitor’s yoke. Doable, but a PITA.

    Looking at this guy’s hack, if all the jungle IC does is turn composite into RGB suitable for driving the neck board, he’s indeed about halfway there: figure out what the neck board wants (which could be as easy as looking at what the TV is putting out and messing throwing a 500-1K pot between the emitters of Q852/Q854/Q856 and the cap, and a 10-20K bias pot between that and ground. (What’s Q881 for? Shuts off the beam if there’s no deflection?) Basically, what I’m getting at is that all he has to do is compare the schematic of the neck board he’s got, with the schematic of a neck board that actually has drive/bias adjustments, and do what comes naturally.

    15.75-kHz multisync monitors are going to be hard to find, and 19″ monitors in that sync range are practically nonexistent. Unburned arcade CRTs are getting harder to come by. Salvaging tubes from 19″ and 25″ televisions is as much an art as it is a science, and many people are reluctant to attempt a tube swap.

    If he’s come up with a technique that can be reduced to “find RGB outputs of jungle IC, and optionally add drive/bias pots to neck board”, he’s greatly simplified the process, because the conversion can be done with nothing more than an old TV and parts that most people already have in their junk drawers. That’s awesome!

  4. DanAdamKOF says:

    Terminated with wrong resistance?

  5. Robert says:

    I have an old Amstrad computer and I was considering modifying an old TV to use as a monitor.

    I am also a qualified electronics technician and did several years servicing TV’s etc.

    Can I suggest that no-one tries this unless they know what they are doing!

    TV’s have high voltages inside. The thick red wire that goes to the screen is normally 12,000 Volts to 26,000 Volts depending on the screen size. The screen is like a big capacitor and this voltage will remain for some time after power is disconnected, it has to be discharged. Some older TV’s have what is called a “hot chassis” in which “ground” all the metal parts and the reference “earth” that comes out the connectors you attach floats about 160 Volts to 320 Volts.

    On the circuit board the drive to the big black “fly-back” transformer is normally 800 Volts to 1200 Volts.

    Great idea, but not for the inexperienced.

  6. Vonskippy says:

    you’re stuck with trying to “drive the yoke you’ve got.”

    Aren’t those lyrics from an old Stones song?

    • localroger says:

      No, they’re the lyrics from an old Crosby, Stills, Nash, & Young song. You’re thinking of that Stones classic Under My Yoke.

    • Monitor Nerd says:

      LOL, I should probably explain a bit more.

      Impedance is magic, so standard practice when swapping tubes is to use the resistance of the X and Y yokes as a ballpark substitute. If the monitor chassis is built to drive a yoke whose X and Y coils have resistances of 2.5 and 10 ohms respectively, it’s not going to work if you try to hook it up to a tube with a yoke whose X and Y coils have resistances of 2.5 and 50 ohms. Either the picture will be distorted, the magic smoke will come out, or both. This factor is independent of the neck board. If the *yoke* of the TV happens to be close enough in impedance to match the design of the original monitor chassis, then the original monitor chassis will *probably* drive the yoke.

      The neck board drives the RGB guns and its socket to match the pinout of the tube. If the tube of the TV happens to be pin-compatible with the tube of the original arcade monitor, then you know that the monitor chassis can drive the tube.

      In the arcade, monitors with broken necks typically had the dead tube trashed, and although the yoke was reusable, arcade operators typically discarded the yoke along with the broken tube because it takes a while to swap yokes and bring purity/convergence back into alignment. The chassis was kept, however, because a working chassis can simply be dropped into a monitor with a failed chassis. The game keeps running (and earning money) on the spare chassis while you debug the failed chassis. Fast-forward 30 years and we have a surplus of dusty old monitor chassis in storage, a deficit of unburned tubes, and a deficit of yokes.

      Intercepting used TVs between the home and the recycler is the easiest way to find tubes these days. In the home, TVs get disposed of as a unit. We had a (temporary) surplus of 19-25″ CRTs when the DTV transition took hold. Fortunately for the environment, but unfortunately for collectors, many of these sets have been recycled (crushed) and tubes will eventually become scarce. It’s rare to find CRT TVs in thrift shops, people have learned that it’s illegal to dispose of them in dumpsters, illegal for recyclers to sell them back to you from the big pile of doomed electronics, and even sympathetic recyclers look at you funny when you rush up to some guy lugging an old 19″ Zenith trying to stop him *before* he drops off the set with a $5 bill in your hand. They do get a laugh when you show up next week with a 19″ TV that just happens to have heavy Pac-Man screen burn in it, though :)

      Back to the normal method of swapping tubes salvaged from old TVs. For instance, if all you have is a G07 chassis (a common early 80s arcade chassis), you need a donor TV with a tube that’s pin-compatible with a 19VTJP22 (a common tube of its day) and a high-impedance yoke (x=2ish ohms, Y=50ish ohms). If all you have is a Wells-Gardiner 19K4600 (another common chassis that used the same tube), you need a donor TV with a tube that’s pin-compatible with a 19VTJP22 tube and a low-impedance yoke (x=2ish ohms, y=10-12ish ohms). If you have *both* chassis sitting in your parts bin, great, you check the pinout on the donor TV, and if it matches, you ohm out the yoke on the donor TV and use that to determine which of the two old arcade chassis you swap in as a replacement for the TV’s chassis. Or you order a third-party chassis like HowardC suggests.

      Best-case scenario with a pin-compatible donor TV is not messing with the yoke because you have a suitable chassis handy. Meh-case scenario is the pin-compatible donor TV has a yoke with the wrong impedance, but it’s removable, and you’re lucky enough to have a spare yoke, so you spend hours dialing it in after swapping yokes. Worst-case scenario with a donor TV is the yoke is bonded to the tube and you have the wrong kind of chassis, so you can’t swap yokes. TV goes to the pile of “donor tubes” and you call it a day, or you have a TV with a tube with an incompatible pinout, and you can’t drive it with any of the chassis you have on hand. TV goes into the pile, everything gathers dust. In all cases you’re basically hoping (measuring with an HV probe) that the HV source from the original chassis doesn’t exceed the max rating for the donor tube, which should be printed prominently somewhere on the inside of the donor TV set.

      This hack offers an end-run around a lot of those variables: you know the tube and yoke are compatible with the chassis and tube because you’re using the TV’s chassis to drive X, Y, and HV. You’re trading a lot of messing around with yokes and chassis and dice-rolling on tube pinout compatibility for an hour or two of poking/cutting traces, and/or wiring pots to an existing neck board.

      (Oh yeah, and everything Robert said on safety. That 800-1200V bites way harder than the 20kV HV, one of the reasons I hate swapping yokes.)

  7. Enzo says:

    I live in Europe, Italy.
    I usually connect old (PAL) CRT TV as monitor arcade, using “scart” connector.
    I’m initially surprised about all these problems, but for sure you’re using NTSC TV, right?

    • Monitor Nerd says:

      Correct. SCART has R/G/B/composite-sync signals, but many North American TVs, particularly the low-end 19″ and 25″ sets, never had SCART inputs, just NTSC composite video — one single-pin RCA jack carried everything.

  8. is0-mick says:

    I was just going to say the same thing, the board should just hook up to the scart RGB and work.

    Years ago (late 80’s), I had an old space firebird board hooked up to an old 26″ TV in my bedroom via an amstrad CPC computer modulator (not many TV’s had scarts then). The modulator took in RGB + Composite Sync and converted them to RF.
    An Amiga modulator would also do the same job.

    Had a few boards wired up like pacman, crazy climber etc, and never had any problems like the above.

    Mick

  9. strider_mt2k says:

    I just knew when I read about this project that it would bring out the best of the hack-a-day crowd.
    A great read all around folks.

  10. biomed says:

    Gosh… this was low-level hacking back in the ’80s just out of NEED. Anyone remember how much a Green Screen or, heaven forbid, and Orange Screen monitor cost?

    The tricks back then, just out of memory, were to get the vid in but bypass the video filters that took out the higher frequencies so you could use all the bandwidth the tube and drivers were capable of. We started out with 40 character displays… and drooled over 80.

    As a previous poster did correctly state… impedance was magic!

    Resistance IS impedance. Impedance is not just resistance! Good clue to good clear vid.

    To echo something more important from another previous poster…. you should not be trying this unless you know what you are doing. 25KV can be fun or a funeral.

    Gawd… half the tv’s I did this too had tubes!

  11. Ryan says:

    A while back I had a Killer Instinct (maybe KI2) cabinet that I bought at an auction for $50. The monitor, board, and power supply were all dead. I removed the 27 or so inch monitor and happened to have an old CRT tv in the house that wasn’t being used. I decided to sacrifice it, and it fit nearly perfect into the cabinet. I had to make new mounts for it and had to modify the existing bezel, but it fit nearly perfectly. After that it was a snap. I had a VGA to Component converter and found that the Neo Geo MVS 4-slot board I was putting in was OK to combine Green and Sync (or the other way around, can’t remember now since it was 10 years ago). Anyway, soldered the JAMMA connector to be wired correctly from the MVS and the converter accepted the signal and would display it on a Component input on the TV. Turning the TV on and off was a bitch, and I had to make a new control panel for the Neo configuration, but I was the coolest adult male in my house for a while. I also eventually used the cabinet for the Neo Geo Hyper 64 and finally a JPN PS2 with all the fighting games I loved. I had to sell it when my whore wife decided we had to move. I divorced her too late, it seems.

  12. NewCommentor1283 says:

    i feel so bad for having smashed an (working) orange screen.

    can anyone explain why unbonding a yoke is bad???
    even techs i talked to didnt know
    they usually replaced the tube+yoke as one
    but still had the skills of realignment if they had to
    (i saw em do it)

    PS when people say green/orange screen they are almost always
    referring to a monitor accepting MDA/CGA/EGA and not VGA!
    9 pin connector, NOT 25
    ONE-BIT TTL video and NOT analouge. the EGA had two-bit TTL buuuut…
    also the frequencys could be anywhere from NTSC up to one half of VGA
    VGA will NOT take the signals from this “very” old system
    on a VGA card when using the modes of MDA/CGA/EGA,
    the frequencys are multiplied in the card itself

    it is possible to mod them for analog but NOT VGA.
    i did it with poor results
    (cutoff, bias, linear? ect nessesary for analog but not present)
    of course its non standard and no (computer video)card exists for it.
    … i was converting the two-bit EGA into (two-bit) analoge and sending it onto the drive circuits sans retrace-blanking ect
    so i could force a one bit monitor to display 2bit “grey-scale”

    • Monitor Nerd says:

      Unbonding a yoke isn’t bad, it’s just labor-intensive. (Heat gun + time + burnt fingers + even with gloves, how hard do you want to yank on something that’s glued to a very fragile part of the CRT?) Experienced techs could and can realign the yoke, and knew how to use little adjusting magnets that you sometimes find on the back of a tube. I never did enough CRTs to get good – screwed a couple up trying to get them dialed in, managed to get them back to where they were when I started, and left well enough alone.

      Now that you mention it, every time I swap tubes, I always remove and keep the arcade monitor’s yoke. The next time I swap a clean (TV Tube+TV Yoke) for a burned-out arcade tube, *before* I do the swap, I’m going to spend a few rounds removing/replacing the arcade monitor’s yoke against the arcade monitor’s tube. Given that the first thing I’m going to try is the perfectly precalibrated tube/yoke combo from the TV, there’s zero cost of failure and plenty of experience to be gained by removing/replacing the arcade yoke/arcade tube combo before doing the swap. Any future use of the old arcade yoke is going to involve putting it onto a tube anyways – why not get free experience while taking it apart?

      Oh yeah, one more thing. Most TV CRTs are 90-degree tubes. There’s a 90-degree angle on the back of the tube. Some TV CRTs were 100-degree tubes – flatter than the 90-degree tubes. These are of great interest to the vector game crowd. Tempest, Major Havoc, Star Wars, etc. used 100-degree tubes, and because the yoke for a vector monitor is special (vertical and horizontal deflection isn’t at any particular frequency, unlike a raster monitor that scans much faster horizontally and relatively slowly vertically), it’s hard to use the X/Y monitor’s yoke (which was designed to fit a 100-degree tube) on a 90-degree CRT.

  13. fajensen says:

    The transistors driving the electron guns on the neck board are probably Current Driven, not Voltage Driven. Driving them directly from a voltage amplifier will saturate the drivers because the colour levels are set by a Current level, not a Voltage. The “+BV” adjusts the bias for the drivers and blanks the tube.

    An oscilloscope will show what is going on: RGB not varying much – a few mV at some MHz, the “BV” a slowly varying DC voltage overlayed with a pulse train.

  14. sad_asd says:

    Why not use a raspberry pi… Yeah, why not? How else are you going to play Third Strike?

  15. really_anon says:

    Any idea what can cause a CRT monitor to power on and have a dead screen without even an on screen display? It still makes the same normal static charge-up sound on power up, as it did before; I can even hear it change resolutions and respond accordingly. It appears to work perfectly fine except the screen is dead (like it’s shut off except I can clearly hear it’s on). Monitor is like new and it’s from 1994 (brightness and contrast are really strong).

    Prior to this problem, my computer went into standby mode. I heard this can damage older monitors (early to mid 90’s) since they don’t have proper standby logic and sync signals stay on. But the monitor did appear to go into standby properly (led changed from green to orange).

    Horizontal size flickered overly wide (no noticeable color or brightness distortion) making a slight audible sine too; thought it was losing resistance somewhere.

    Degauss made only a quick click and twitch of screen after several stalled seconds when performed, so I don’t even know if that was normal; (screen didn’t tremble and shake for several seconds like newer CRT’s).

    I’m guessing blown fuse or cold solder joints. Bad degauss? Should I check deflector circuit first?

    I really want this monitor back. I was stunned when I first used it seeing it’s a 17 iincher from 1994. Was expecting like 800×600 at 60hz max resolution or something like that, but it actually does 75hz (or 80 maybe) at 1280×1024, 90hz at 1024×768, 160hz at 640×480. Blows the socks off the newest 17 inchers I’d seen up to 2004 or so (never seen one do over 60hz at 1280×1024 or 100hz at 640×480), and the colors are fabulous (blacks are pure black too and don’t have annoying red from overbrightness like the newer Sony Trinitron tubes); sure beats the hell out of LCD’s and beyond for my wants. Looking at some retail info I found this monitor used to cost around 800-900 dollars (more or less). Who the heck used such high standard displays back then anyway (actually does have a sticker to some engineering corporation)?

    Man you guys are jerks (just kidding); for not having a website dedicated to CRT’s. Seriously though, there should be a website or forum with you guys on there. CRT’s are so much better than LCD’s; unless you’re an LCD guy :). Higher refresh rates, no input lag for games, full range color production, virtually any resolution within sync range (none of that blurry scaling involved), don’t get knocked over by pets and kids, no chemical smells or nasty looking liquid.

    Regards

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