Need help repairing a 1950s 20kV power supply

Discussion in 'The Projects Forum' started by Lambda, Mar 24, 2012.

  1. Lambda

    Thread Starter New Member

    Feb 28, 2011
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    I'm wondering if anybody could help me out here. I recently acquired an old US Army infrared sniperscope, manufacture date 1951. It's a neat collectible piece, and I'm working on restoring it to working order.
    The image tube that converts infrared to visible light runs on 20,000 volts. I've determined that the power supply is not putting out any power.
    To achieve this voltage from a 6-volt battery, a vibrator relay pulses at 115hz. That is then fed into a step-up transformer, and then, in the original, would be half-wave rectified by vacuum tube and I assume filtered with a capacitor before being sent to the tube.
    Problem is, somewhere along the line somebody replaced the original circuitry with a single device. It's a potted plastic box that is marked "RCC 212-136 7208". It has 4 connections: INPUT, which appears to connect to one side of the transformer secondary, leaving the other wire disconnected; REF, which is connected to ground; FOCUS, which is not connected; and the HV output. Whatever this device is, it isn't working. I find it odd that it's connected to only one side of the transformer secondary, this shouldn't work at all. But apparently it was working the last time it was used.
    My goal is to eventually restore it to original condition, but for now, I'd just like to have it working.
    So, what do you think this device could be? By the focus wire, I'm guessing it had something to do with a CRT or iconoscope tube. It definitely looks newer than the 50s, I'd believe 70s or later.

    The transformer itself is odd, too. It's original, and it's all potted. Its inputs are 6v, gnd, and two wires from the vibrator relay. I really wish I had the original manual, which would have included schematics. But apparently it's pretty rare.

    [​IMG]
    Interior of the power pack. At left is the transformer. The can at upper right is the vibrator, the plastic tube at lower right contains the HV output.

    [​IMG]
    The RCC device.

    So, any ideas?
     
  2. debe

    Well-Known Member

    Sep 21, 2010
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    184
    It posibly is an EHT Tripler, used in coloured TVs that require 25KV for picture tube. They step 8Kv ac to 25Kv DC.
     
  3. Lambda

    Thread Starter New Member

    Feb 28, 2011
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    Hmm, that raises further questions. Where's the 8KV coming from? Maybe that resistor on top of the transformer was added at the same time as the tripler and serves to drop the secondary voltage down to 8KV?
    And how can it be connected to only one side of the secondary?

    I think what I'm going to end up doing is just designing my own circuit around the original rectifier tube.
     
  4. debe

    Well-Known Member

    Sep 21, 2010
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    I would be checking what the transformer puts out? It was probably 8KV then a tripler was used, its a common way of geting 25KV.
     
  5. CDRIVE

    Senior Member

    Jul 1, 2008
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    Probably not a bad idea. After all, those old vibrator circuits were just a cut above the efficiency of a Dynamotor. ;)

    Chris

    Edit: I just made note of the itsy bitsy vibrator. That really is miniature! My first car's auto radio used a vibrator but it was the size of a octal (metal can) vacuum tube. They had to supply power though, while your HV is a static potential like the HV anode of a CRT.
     
    Last edited: Mar 25, 2012
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    It's a tripler for sure. I don't know that brand but have seen many other types.

    That one looks like the smaller style found in small B/W CRT devices like some oscilloscopes.
     
  7. strantor

    AAC Fanatic!

    Oct 3, 2010
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    I'm sorry I don't have any valuable input, other than that is AWESOME! Even more awesome once you get it working again.
     
  8. studiot

    AAC Fanatic!

    Nov 9, 2007
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  9. Lambda

    Thread Starter New Member

    Feb 28, 2011
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    One issue I'm having is that all I can measure is 10kv DC. The only reason I knew it was producing no output is that I finally decided to connect my 10kv meter and, well, if it pegged than it was good.
    The 10kv meter uses a 200M resistor in series with the galvanometer. If I connect a 200M resistor in series with my oscilloscope probe, will that allow me to measure the AC output of the transformer?
     
  10. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    How much is the test meter loading.
     
  11. CDRIVE

    Senior Member

    Jul 1, 2008
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    If I were doing this I'd do it this way. The divider will give you close to 1000:1 voltage reduction. So 10KV will produce 10V at VF1.
     
    Last edited: Mar 26, 2012
  12. JB59

    New Member

    Dec 18, 2011
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    If your vibrator and transformer are ok then you should be able to see the high voltage pulse just by bringing your scope probe close to the transformer. It will vary in amplitude as you move the probe. Another thing we used to use was a miniature neon lamp taped to stick. They will glow in the presence on high voltage ac.
    The device at the output is a high voltage rectifier and focus divider. I don't remember any triplers (doublers, quadruplers) on tube tv's but they werre common on early solid state.

    There are solid state replacements for tube hivoltage rectifiers if you know what the original number was. Google NTE508 for one example.
     
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