Vintage CRT Monitor Repair Thread

Discussion in 'Retro & Arcade Worklogs' started by sean0118, Nov 13, 2016.

  1. OP
    OP
    sean0118

    sean0118 Member

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    An update on the Topcon MDA monitor is coming soon.

    Until then, here's some pics of the Commodore CM-312, looked pretty good from the outside, however inside....


    [​IMG]



    Things were off to a bad start... corrosion and dust everywhere and a film capacitor had split open...


    [​IMG]

    [​IMG]



    The cause of all this decay is one of the 200V mains caps in the power supply had gone bad, rupturing its corrosion inducing electrolytic everywhere. I've never seen one that bad (or lumpy) :Paranoid: :Paranoid: :Paranoid: :Paranoid: :Paranoid:


    [​IMG]

    [​IMG]



    "We can rebuild him; we have the technology"
    :Pirate:
     
  2. callan

    callan Member

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    That is a thing of awful beauty: I, too have never seen a cap lose it's structural integrity like that. :Paranoid:

    Callan
     
  3. Flamin Joe

    Flamin Joe Member

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    I know RIFA caps can make a bloody awful mess when they blow but I think that wins hands down. :shock:
     
  4. davros123

    davros123 Member

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    Just watched that vid. Wow, that guy is a modern artisan ! Amazing.
     
  5. power

    power Member

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    CRT nerds lend me your expert knowledge.

    Is there an easy way to use a PC monitor with an Amiga 500, Atari ST or even something like a vintage console?

    I have access to a couple of nice old PC monitors (A 19" Black Dell flat, 19" Samsung flat that only have dsub cables no connectors of any kind) and even a 1084S (currently my go to for the Amiga) but I'm wondering if it's worth trying to go down this route or if it's even possible - now I haven't bothered searching anywhere I figured you lot would know more than the googs. It seems a waste to restrict these old monitors to PC's.

    So I guess bottom line is this, is it doable turning a PC monitor into say and RGB general use display easily?
     
  6. elvis

    elvis Old school old fool

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    Depends on the horizontal scan rate at either end.

    If you're going from a 15KHz (CGA, 240p, 480i) system to a 31KHz (VGA, 480p) display, there's a bunch of upscalers (basically just line doublers) you can get off eBay that are pretty good and cheap. They're designed mostly for arcade operators who want to put a VGA or higher res monitor/LCD in an old cabinet, and offer lag-free solutions, but I know quite a few retro-computing users who use this stuff to hook up a VGA PC CRT monitor to an older computer.

    For example:
    http://www.ebay.com/itm/Arcade-Game...onverter-HD-Jamma-Mame-and-More-/331317770835
    http://www.ebay.com/itm/Arcade-Game...rter-For-HD-Jamma-Mame-and-More-/192115020367

    You'll need to make a custom cable, but generally it's pretty easy, as many older computers spit out TV-standard RGBS. If they only spit out composite/s-video, then find a unit that takes that as an input. Should be easy enough to find one under $50 in most cases.

    Going the other way - from 31KHz to 15KHz, the cheap downscalers typically only offer interlaced output (480i), which sucks. Good quality expensive ones will do 240p and various de-interlacing stuff, but are expensive.
     
    Last edited: Jul 12, 2017
  7. power

    power Member

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    expensive eh.

    hmm I was hoping their might exist a magic boxen you can plug a VGA monitor into and you plug your shit in the other side. Like a receiver does or whatever. I always forget about scan rates and that monitors tend not to just cope very well with this.

    Although those ebay thingys just might be pretty close to that anyway.
     
  8. elvis

    elvis Old school old fool

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    Yeah, for old ~240p computer to new VGA/480p monitor, it's pretty easy. If your old computer can do s-video, one of those links I had there will take s-video in, VGA out, so it's all plug and play.

    If your old computer only offers RGBS (which is going to look much nicer than s-video anyway), then you'll need to make a cable. But often these are trivial. If the converter board can up the gain, then it's straight-connect. If not (say, the picture is way too dark), then a 75ohm resistor with 220uF cap combo on just the RGB lines is generally enough to get things working to the right levels.
     
  9. power

    power Member

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    that all sounds reasonably basic and I'd nearly bet that someone would sell that RGB cable you are referring to if it's different from the Commodore one. Must investigate the s-video and Atari ST side. The only Atari ST stuff I've seen is B&W hoping for colour.
     
  10. OP
    OP
    sean0118

    sean0118 Member

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    Need some help to test the MDA Topcon monitor. :D

    I've got an old 486dx box that had a VGA card in it. I removed the VGA card and replaced it with an IBM MDA card, which I already had but have never used (may be faulty).

    When I attach the Topcon monitor I only get a blinking cursor in the top left corner. I'm guessing I need to install drivers for the MDA card? How do I do that? :Paranoid:
     
  11. nimmers

    nimmers Member

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    The OSSC seems to be getting better and better with every revision. I've been waiting until they are more readily available and refined but will definitely get one one day.
     
    Last edited: Jul 19, 2017
  12. Asteroid

    Asteroid Member

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    I've had this same dilemma. Got an old Cybelec control at work with a display and/ or control fault. CRT has a raster and sort of responds, but only displays a blank green screen or random characters. Some hard sleuthing has identified the signal as MDA with a 10 pin edge connector interface... but wtf can be used to source or display MDA these days?

    I've got an ancient DSE monitor with both composite and TTL input, but the TTL input is a weird 8 pin rectangular connector. (It was used with a Vic20 lol)
    |....|
    |....|
     
    Last edited: Jul 18, 2017
  13. DonutKing

    DonutKing Member

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    There might be a jumper or a BIOS setting to enable MDA video. EGA/VGA cards had an onboard BIOS so they could be detected automatically, but you had to tell the system to look for a MDA or EGA adapter.
     
  14. callan

    callan Member

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    (assuming your're running DOS) - have you tried just typing in "mode mono". Some cards were smart enough to emulate mono and CGA, and could switch between them. If you've got a cursor it sounds like you're most of the way there.

    Useful DOS MODE commands:
    mode mono ; switches DOS output to monochrome screen
    mode co80 ; switches to 80 column on CGA/EGA/VGA
    mode con lines=40 cols=132 ; switches EGA/VGA into 132 column mode. (works in a Windows DOS prompt, too!)

    Callan
     
  15. OP
    OP
    sean0118

    sean0118 Member

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    Thanks for the replies everyone, time for an update. ;)


    Stepping back to a few weeks ago, I didn't want to waste time rewiring the Topcon MDA monitor without confirming that it worked first. The power led came on, but the screen didn't, no test pattern or anything.

    So I set about reverse engineering and testing the horizontal deflection system (more info here). This circuit has extremely high voltages (even possible while off), so I did most of the testing while off and after checking that it was discharged.

    Everything seemed okay, power rails were there, the horizontal output transistor (HOT) and other components tested okay once unsoldered and removed from the circuit.

    Next, I started looking at the driver circuit which turns the HOT on and off (which turns the flyback transformer on/off). Turns out that it's almost connected directly to the H-Sync pin of the MDA output, which I hadn't rewired yet! As far as I can tell theres normally a complicated circuit in there that adds the test pattern when no computer is attached. This doesn't do that, if no computer is attached, the CRTs high voltage doesn't turn on at all!

    I only worked this out after a couple of hours of debugging and reverse engineering, took me less than an hour to add a new MDA cable on. :wired: :lol:


    [​IMG]


    Slight overkill, but I made up a chart that matches the pins of the monitors circuit board connector to the 9 pin d sub I soldered on. Mainly just so I wouldn't confuse myself:
    https://www.dropbox.com/s/i8ga9da7luh0rb5/Topcon_MDA_Pinout.pdf?dl=0

    This is the cable I used (cut one connector off):
    http://au.rs-online.com/web/p/parallel-cable-assemblies/1284259/


    The result:

    [​IMG]


    Was hoping for amber or green phosphor, but at least it works. ;)



    Thanks, I think this is what I had to do, found a jumper that switches the motherboard to MDA mode. I think the MDA card I was using has an issue as well, but the second one I had worked fine. :thumbup:
     
  16. Flamin Joe

    Flamin Joe Member

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    I watched these videos last night in preparation for my second monitor repair. I successfully repaired an IBM 5151 the other week (sorry no pics, bad habit of forgetting to take them). I need to replace the flyback + HOT on my 1084S and I'll limit the recapping to just the power supply section and any other caps which look suspect.

    Anyway here's the videos. The guy rambles on a bit but he has some good info and plans to do a part 3 shortly.



     
  17. Discojango

    Discojango New Member

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    picking up a sony trinitron flat screen for free from gumtree this weekend :D
     
  18. Matthew kane

    Matthew kane Member

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    Some of you guys are really going hardcore on restoring old cheap crt's.
     
  19. callan

    callan Member

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    Flamin' Joe. Where did you get the flyback for the 1084s?

    Callan
     
  20. elvis

    elvis Old school old fool

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    It's a good skill to learn. These things are being destroyed in huge numbers. A bit like retro consoles that are all now more expensive than when they were new, the mindset that "they're cheap and readily available" is a temporary one. These things are no longer in production, and only getting rarer. Now's the time to learn to repair them while it's easy to do so, and mistakes aren't so costly.
     

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