blowing the breaker...

Discussion in 'Amplitude Modulation' started by N2DTS, Feb 7, 2020.

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  1. N2DTS

    N2DTS Ham Member QRZ Page

    Plate modulation is tough on these things.
    This was a big one so I put in a MUCH bigger one.
    I thought this stuff was supposed to insulate.
    Does it wear out?

    The 4-125 rig would run fine then pop the breaker, quick reset and it would run fine for a while.
    HV wire can insulate very high voltages, you would think ceramic and air space would do better....


    [​IMG]
     
  2. W1BR

    W1BR Premium Subscriber QRZ Page

    Wow
     
  3. W2VW

    W2VW Ham Member QRZ Page

    Been there. Sometimes all it takes is a dust trail on the surface. Other times chassis feedthroughs develop tiny defects when torqued and used without the cork or paper gaskets.

    Troubleshooting is painful at the least and disastrous at the worst when expensive components are lost.

    My newer friend Teflon is much more forgiving and I don't miss the antique look one bit.
     
  4. W7TFO

    W7TFO Premium Subscriber QRZ Page

    Funny, I've had many more failures with Teflon insulators than porcelain in BC transmitters.

    Not that porcelain doesn't fail, just not as often in my time.

    73DG
     
  5. K4KYV

    K4KYV Subscriber QRZ Page

    Those thin ceramic sleeve insulators are a failure waiting to happen. Better to use bowl insulators - they come in reasonably small sizes as well as the familiar Dolly Parton variety. Or else use the solid cone-shaped type of feedthroughs that take a large hole. The factory specification for the one shown in the photo is likely for no more than a few hundred volts. I always aim for a 100% safety margin. For example, the plate voltage is +1000 volts. At 100% modulation the peak voltage is 2000. 125% positive peak headroom brings it up to 2125. For 100% safety margin, it needs to be rated at minimum at 4250 volts. If it's carrying modulated RF plus the modulated B+, for example at the lead to the plate of the tube, double again to 8500 volts. See where this is headed?

    This is the reason why the plate tuning capacitor should never carry the full modulated DC between plates. Either use a plate choke and blocking capacitor, or mount the variable capacitor on insulators and feed modulated B+ voltage to the frame/rotor (which is RF-grounded through a mica or ceramic HV by-pass capacitor), so that the variable is handling only modulated RF.

    It seems that in the old days hams took a while to figure this out. Look at some of the circuits shown in late 20s through mid-1930s publications. They usually series-fed the tank circuit and mounted the tuning capacitor right on the metal chassis (if there was one) or else ran a wire to ground it directly. This required a capacitor with tremendous plate spacing; breakdown voltage does not increase 100% in direct proportion to spacing. If it ever arced over, unless there was some kind of overload protection, it would just keep on arcing until it melted a blob on the plates where the arc occurred. It wasn't until the mid to late 30s that circuits began to show up in handbooks and magazines that took the +HVDC off the plates.
     
  6. K5UJ

    K5UJ Ham Member QRZ Page

    One technique which was used by the builder of my 3-400 rig was to punch a square hole for the feed through. A plexiglass square is mounted over the hole using a pair of nuts and bolts, one on each side, and a round hole for the feed through is sawed in the plexiglass and it is mounted on it. Of course the square hole is sized as needed. This adds an extra level of insulation. See photo. 3-400FinalHVfeedthrough - 2.jpg

    In one of my old QSTs I came across an article, it might have been in Hints and Kinks, on how to make your own clear glass bowl insulators. You have to go out and purchase an appropriately sized Pyrex glass bowl. What I recall is that you make a wood holder to keep the bowl from moving around or spinning and use grinding compound and a small piece of copper tubing that's the diameter of the needed hole and put that in a drill press chuck and slowly grind through the glass with it. A search of QST is probably in order.
     
    K4KYV likes this.
  7. W2VW

    W2VW Ham Member QRZ Page

    The most annoying part of these failures is they are often not a hard failure. The TX will work for a time and fault when it's good and ready.
     
  8. WA3VJB

    WA3VJB Ham Member QRZ Page

    FUNNY I thought you put in a much bigger breaker, at first. Kinda like dropping a coin in the bottom of the old screw-in fuse boxes. Saves on replacements.

    My T-368 had several of these ceramics fail on the inside where you couldn't see any problem from the outside. The cork, intended to take up the mechanical difference as the chassis heated up, eventually remained compressed. The ceramic, with nowhere to go as things expanded, would then crack at the metal stud, and eventually arc internally (blowing the breaker).

    As Dave points out, it was never a hard failure. Only on voice peaks or initial key up. Eventually went to dual-density HV wire simply fed through the hole.
     
    KA0HCP likes this.
  9. K4KYV

    K4KYV Subscriber QRZ Page

    I did the same at my dawg-house at the base of the tower, with the OWL feed-throughs. Made a small rectangular window with the feed throughs mounted on 1/4" plexiglass sheet. My concern was rf losses in the wood between the feeders if I had simply mounted the feed-throughs directly on the plywood enclosure. Plus, any arc-over between feeders could conceivably ignite a fire. Feed-throughs instead of simply poking the wires through holes in the plexiglass makes it easier to seal out rainwater, plus that makes it more convenient to disconnect the OWL if necessary.

    It might be better to use a round hole instead of a square one as the builder of Rob's rig did. A hole saw and drill should be easier than blasting out a square hole with drill, saw and file. Plus the edge of the metal panel would be equidistant from the conductor all round the perimeter of the hole.

    I have seen the same technique used to mount a SO-239, when it was desired to isolate the outer conductor of the coax from the panel for whatever reason.
     
  10. N2DTS

    N2DTS Ham Member QRZ Page

    I have had a number of these fail over the years, but often differently.
    This one was fine till I put the RF deck in service a few days ago.
    It would make a noise then pop the breaker on the power supply.
    I could reset it and it would come right back up, for a while.
    I turned the voltage down and it would run for a while, then pop again.
    Only during a long transmission.

    I adjusted the spark gap on the mod trans, no difference.
    So I pulled the RF deck out and noticed the plate tuning cap looked like its flashed over more then once, its a bc610 tuning cap I think.
    Knowing these insulators often fail inside, I decided to check it.
    I made the hole much larger and used big insulators out of a heathkit antenna tuner (for the owl), no ceramic inside, a 1 inch hole through
    the chassis.
    Ran it up to 2500 volts with plenty of modulation without issue.

    A pair of 4-125's modulated by a pair, T368 mod transformer, does 400 watts carrier without working hard.
    I may have to look at the feed through to the RF choke, that is another feed through weak point...

    Its as Don says, there are some huge spikes in voltage, not just 2x the plate voltage...
     

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