1200VAC at 0.5mA Needed

Discussion in 'General Electronics Chat' started by Cavemaaan, Sep 13, 2013.

  1. Cavemaaan

    Thread Starter New Member

    Sep 12, 2013
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    Hello All :)

    I'm trying to repair my Wavetek-SSI 3000 radio service monitor for ham-radio use. I bought it used and dead back in 2001 and am now taking a big stab at getting it up and running.

    The original transformer was gone when I got it but through a lot of research, I was able to identify and contact Northlake Engineering who made the original transformer back in the early 80's and get the specifications for the transformer. They haven't made this one in years and it would cost over $1000 just to make one for me. It is a 115V primary with six individual secondaries of 6.3 (Filament), 7.7 (5Vdc), 14.9 (-15Vdc), 15 (+15Vdc), 200 (CRT 200Vdc), and 1200 (CRT 1200Vdc?). It is a very small transformer for what it contains. The 6.3, 200, and 1200 are all for running the internal 3" crt oscilloscope.

    The 1200vac at 0.5milliamp secondary voltage is proving near impossible to replicate in my research so far. Hammond makes a 1250Vac tranny, but it's well over $100 alone and is way too big to fit in the chassis along with five other trannies for the other voltages that I need.

    A friend recommended a neon-sign tranny but I'm not familiar with them and I can't find any on the web close in voltage.

    The 1200vac secondary goes to the high-voltage power supply board where it is immediately rectified through a bridge rectifier and conditioned for use by the CRT.

    I'm an electronics tech at a TV station and work with some nice colleagues, but they are all theoretical guys and I need help in the "re-engineering" realm :)

    I have attached a schematic of the circuitry if that helps.

    Thanks in advance,
    Dan G.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Go to to a local electronics recycler and pick up an old CRT monitor or small TV and salvage the flyback, (Horizontal ouptput) transformer.
    You just need to detect the primary and the H.V. overwind is obvious.
    You don't need the deflection circuits I assume?
    You may need to experiment with frequency and amplitude to get the necessary 1200v, a small TV CRT usually requires about 7-9Kv.
    Max.
     
  3. Cavemaaan

    Thread Starter New Member

    Sep 12, 2013
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    Hey Max :)

    Thanks for the reply :)

    Yeah I have old/small tv's coming out of my ears since working at a television station and being a repair guy for 20+ years :)

    Trouble is, even small 5" or bigger tv/monitor CRT's use at least 2kV for the anode voltage and of course the flybacks are designed to run at around 15.7kHz. I even looked at some of our old out-of-date vectorscopes and waveform monitors by Tektronix and those run 4kV on the anode and only 300 or so Vdc from the power supply for their deflection circuitry.

    I've also considered and then ruled out due to high cost, diode-capacitor voltage-doubler/tripler, series wired transformers, etcetera. I'm trying not to re-invent the wheel here and then have to mangle the high-voltage power supply in this radio service monitor. Also "winding my own" is a daunting and likely dangerous endeavor.

    I'm a good repair tech but I'm not an engineer or a super electronics hobbyist. I'm at the extent of my "what about" knowledge and hoping you guys have the key :)

    It's just buggin me because even though it's 1.2kV, it's low current and only half a milliamp or less current that's needed - dangit :)

    Dan G.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I would be tempted to experiment with H.O.T. and a power transistor, a simple oscillator into the base, take a look at some small TV schematics to get an idea.
    You already have the H.V. rectifier on it and the frequency should not matter if you are not using it for deflection, and I assume you would have an H.V. DC probe to test the output?
    Max.
     
  5. #12

    Expert

    Nov 30, 2010
    16,318
    6,818
    I would surrender to the idea of making several power supplies in a separate box and running a multi-connector cable to the back of the radio. You can buy test lead wire rated at 5000 volts to carry the 1200 volts.
     
  6. Cavemaaan

    Thread Starter New Member

    Sep 12, 2013
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    0
    Okay Max...you're starting to pique my interest...

    Since my horizontal-output-section theory really faded years ago, all I can think of is impedance matching, interstage coupling to the flyback, signal duty-cycle, B+ supplies, etcetera.

    Instead, I ask this...

    What about using say, a 115-to-12vac standard transformer direct-coupled to the flyback input? I can't remember the general amplitude of the signal driving the flyback primary. How about even using a 6.3vac transformer? I do have a small variac to test with possibly. I also do have a high-voltage probe available to use.

    My dilemma with trying to create an osc-transistor drive is getting specs for the flyback and such to be able to build something that doesn't keep burning up while trying to create it.

    Another concern is the space required...I still need to install individual transformers to obtain the 200Vac, two separate 15vac's, 7.7vac, and 6.3vac.

    Well, time to head to work, second-shift is calling and I still need the paycheck :)

    Dan
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You can always experiment, you already have the equipment, but the HOT is ferrite core, so the efficiency is not going to be there.
    Max.
     
  8. Cavemaaan

    Thread Starter New Member

    Sep 12, 2013
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    0
    Hey Max,

    "but the HOT is ferrite core, so the efficiency is not going to be there"

    as I'm clueless to what you mean, why do you see this as a problem? I know what a ferrite core is and what an iron core is, but I don't know otherwise...

    Dan
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Iron core: Low frequency transformer <1Khz
    Ferrite core: higher frequency, H.O.T. starts around 10Khz.
    Max.
     
  10. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    Voltage multipliers aren't at all hard or costly to make.

    Back in 1957 or thereabouts,a radio magazine in Australia published an article on a 5" TV,using a 5BP1 tube.
    They used a bunch of 6H6 dual diode tubes in a voltage multiplier to get around 2kV for the tube EHT.

    A few years later,I found the mag & was fascinated by it.
    I started picking up some of the parts to make this project,but realised I didn't know enough & put it on the back burner till the mid sixties.

    I never got the thing going properly,but the voltage multiplier,which I built up on perfboard,using OA210 silicon diodes instead of the 6H6s worked a treat!
    It wasn't that costly,even back in those days of expensive parts & lower wages.
     
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