CRT monochrome monitor fix

Discussion in 'General Electronics Chat' started by programmer6502, Sep 27, 2014.

  1. programmer6502

    Thread Starter Member

    Feb 1, 2014
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    Hi,

    I know CRT's aren't really the thing anymore but hopefully there's still some TV repair men or somebody that has experience with on here.

    So I recently picked up a vintage monochrome monitor from 1984 that I want to use for one of my vintage computers. This monitor is a Sperry branded monitor and I got it free because it didn't work. I tested this monitor and sure enough nothing happened. After playing around with it I finally took the case off to see if the back of the tube would light up to indicate that it's working. When I powered on the monitor, the back of the tube lit up but powered off within seconds. Isn't that light supposed to stay on when the CRT's in use? If so, what would be the common cause for this not to stay on? I already fixed all the cracked solder joints that I could see after discharging it (obviously).

    Thanks!
     
  2. ISB123

    Well-Known Member

    May 21, 2014
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    Does it make any kind of noise when it shut downs?
     
  3. programmer6502

    Thread Starter Member

    Feb 1, 2014
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    It makes a short static sound. That's it.

    Edit: and sometimes the tube will do nothing at all.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    On some early monitors (some not so early) the crt filament was fed of off the H.V. flyback transformer, so if it is the horiz starting up and something collapsing it soon after it starts, the CRT filament will collapse also.
    Just a guess!
    If you have the right means, test for crt H.V.
    Max.
     
  5. #12

    Expert

    Nov 30, 2010
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    Shutting down the filaments in the CRT is a symptom, not the cause.
    Max is on the right track in looking for a failure in the supply for the filament.
    I can't guess whether Sperry brand heats the filament from the horizontal section or from a low voltage supply in the early stages, right off the power cord. This is where a schematic is a very handy thing to have!

    You can tape a neon bulb to a plastic stick to see if the horizontal drive section is working, but with no experience, you'd still be fumbling around and guessing.

    Is it worth the price of a schematic?
     
  6. programmer6502

    Thread Starter Member

    Feb 1, 2014
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    So that light on the back of the tube is called the filament, good to know!

    I attached a photo of the neck board with the tube socket. One of these connections is where I would test for the horizontal drive correct? And answer to your question #12, it's probably not worth buying a schematic. I doubt I could even find one in the first place, I can't even find this monitor online.
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Or cathode heater!
    There would only be tube drive voltages on the socket.
    We are talking the 6kv - 9Kv on the side of the picture tube. this is generated by the 'usually' black transformer on the cct board.
    OWK as the H.O.T. or flyback.
    This does a few things, provides CRT H.V., Horiz scan, and in some cases the crt filament.
    The CTR H.V. can remain for some time even with the set off for a period, so take care in this area.
    Max.
     
  8. #12

    Expert

    Nov 30, 2010
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    What I meant by, "horizontal section" was the big, high voltage transformer and its associated circuits. Back when I was good at this, I could tell if the transformer was working, if the damper was working, if the yoke was shorted, and if the HV rectifier was working, just by holding an NE-2 bulb near the parts.

    And...this is a perfect example of guessing. You guessed that the horizontal drive was connected to the CRT socket, but it isn't. The horizontal drive creates the high voltage on the anode of the CRT and it drives the yoke (around the neck of the CRT), but there is no horizontal signal on the socket unless you consider the part where the cathode is turned on and off at the horizontal scan rate, but that's only "signal" level voltage...a long way from several thousand volts needed to run the high voltage end.

    See? A schematic is pretty much necessary to get this traced out.
    If you were good at this, you might guess lucky, but with little to no experience, you don't have a hope.

    and remember, it's only my opinion. There are some pretty magical people on this site. Maybe one of them can help you.
     
  9. alfacliff

    Well-Known Member

    Dec 13, 2013
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    monitors aound 84 had a circuit for shutting down the horizontal stage if the high voltage was to high for "xray protect". also, the supply voltages for the monitors usually was generated off the horizontal output stage. the main input supply operated around 150 volts rectified from the ac supply and regulated down to 135 to 150 volts. anything not drawing curent caused the voltage to go up and the xry protect to shut down the highvoltage.
    even a problem in the vertical stage could shutdown the monitor since the vertical stage drew the most current other than the horizontal stage. try to get a service manual on the monitor.
     
    #12 likes this.
  10. programmer6502

    Thread Starter Member

    Feb 1, 2014
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    There's always a first time for everything! That's the reason why I'm here. I wouldn't attempt anything without being totally sure about it (especially with CRT tubes!).

    From what you guys are saying, it's probably not worth fixing this thing though I would really like too. This monitor is uncommon and would probably waste tons of time trying to track down the service manual. But I do agree that I really need it.
     
  11. #12

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  12. programmer6502

    Thread Starter Member

    Feb 1, 2014
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    Very true. Though it is pretty rewarding to fix stuff like that. Plus I wanted that monitor to give my IBM 5150 the "vintage feel" that I want.
     
  13. #12

    Expert

    Nov 30, 2010
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    I fixed my 31 inch CRT TV by re-soldering the vertical drive chip. Getting a TV fixed for 15 cents worth of solder is a very rewarding feeling!
     
  14. programmer6502

    Thread Starter Member

    Feb 1, 2014
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    Nice! Yeah, I'll still hold on to the monitor and attempt repairs again down the road. In the mean time I can do some studying. At least you guys have me pointed in the right direction!
     
  15. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Is that Mitsubishi symbol at the model number?
     
  16. programmer6502

    Thread Starter Member

    Feb 1, 2014
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    Yes it is. As a madder of fact, the internals have Mitsubishi written all over them. Why?
     
  17. programmer6502

    Thread Starter Member

    Feb 1, 2014
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    By the way, this is the monitor I have. I just found this PDF online.
    But mine doesn't have the power indicator on the front.
     
  18. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Well, maybe if you spin a good story they will give you schematic for free. You know.. something along the lines of: " I got this really old piece that you guys made back in X, it is really really good. But it broke and I am trying to fix it. Help me please!" Something like that. Probably best to send it to the home office, not your local mitsu rep.
     
  19. programmer6502

    Thread Starter Member

    Feb 1, 2014
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    Hmm, I guess it's worth a try :D
     
  20. takao21203

    Distinguished Member

    Apr 28, 2012
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    they dont make them anymore as a matter of fact.
    Mostly you only get sales representatives for large corporations.
     
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