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Sunday, December 30, 2018

HT-37 Choke Failure -- Why did this happen? What is your diagnosis?


Look, it has been 60 years, so I'm not looking for my money back or anything, but having just repaired the LV power supply choke on my very venerable Hallicrafters HT-37, I started thinking about how and why it failed. 

Clues: 

-- I found it with four of the windings broken, with the eight broken leads kind of sticking out of the winding wrap. 

-- The four broken leads were on the outside of the winding (thank God!) an were at the part of the winding closest to the chassis and the back of the cabinet.  (See picture below.) 

-- There was evidence of burning on at least two of the leads. 

-- The choke is located in the extreme back corner of the chassis, near the back of the cabinet. 

-- The paper and cellophane wrapping around the windings was a bit deteriorated.  

So,  what is your diagnosis?   What happened to cause the choke to go open? 


9 comments:

  1. Are the four windings in series or parallel? Or some of each?

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  2. When I was a young lad, I did a small stint in a TV workshop. The majority of faults were caused by thermal stress and or corrosion.
    Steve M0KOV

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  3. I've come to believe that, when we build and use new rigs, our old rigs sometimes harm themselves to get attention. You might want to have a frank discussion with your 2B.

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  4. A picture is worth a thousand words, Bill: a duplicate of the 'tag strip' shot showing the choke in place, please, to tell where and how it was mounted. Possible weakening damage from the 'new caps' repair/mod, the 'bottom' cover/grille/base-plate's proximity or aperture for foreign objects (considering treatment/environment in storage between last use and recent discovery? Not being familiar with the rig, basic mechanical/environmental considerations would be my first line of investigation. And I agree with N8NM - a case of 'the sulks'? :)
    73

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  5. Impermanence is a NATURAL component of the universe: Everything fails eventually. A bit of forensics can be educational though.
    Inspection of the conductor ends under magnification will often reveal clues. If the ends are rounded blobs then assume a plasma event (heavy current). Thin tapered ends are often the result of corrosion or wear.
    In the early days of consumer electronics ('20s through 40's) the insulating papers often had high acidic content. This made the paper brittle and flakey (like the pulp publications of those days). This slowly attacked the copper.
    If the ends are jagged then assume mechanical failure. Flexing embrittles the copper, strains from moving or heat cycling will eventually separate the conductor.
    By "windings" I assume that you mean "turns"? Having never worked on an HT37 I would be surprised that a filter choke would have more than one winding.
    Failures in chokes, transformers, and voice coils are often at their weakest points, in this case where it leaves the coil and attaches to the pins. Usually I just unwind a turn for some fresh wire. Seldom is the effect even measurable.
    73,
    Don, ND6T

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  6. Bill, you mentioned difficulty measuring high inductance values. Despite having a fancy HP LCR bridge, my everyday instrument is a cheap-and-dirty "GM328 transistor tester" from Amazon for under $25. I have had 2 of them for over a year now. Great for big capacitors and inductors as well as transistor sorting. Yes, they are rife with limitations, but if you remain aware of them then they are priceless. Great for ESR tests, too.
    73,
    Don, ND6T

    ReplyDelete
  7. Bill

    If a part of the choke's winding shorted to another part of the choke's winding (why?), it would effectively cause a shorted 1 or 2 turn winding which would cause lots of current in that segment and very high localized heat.

    Joe
    W3JDR

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