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Tuesday, December 18, 2018

All I want for Christmas is... an HT-37 L25 Choke-- 9 Henries at 135 ma


Following my own advice to prepare for Straight Key Night, I tried to fire up my venerable Hallicrafters HT-37 transmitter. It didn't work.  I quickly determined that none of the oscillators were working, so my troubleshooting focused on the power supply.  Sure enough, the choke in the low voltage power supply is open.  That's bad.  

I briefly considered giving up on this old rig.  I don't really like working with tubes anymore.  And this thing is very heavy -- a real beast.  DX-100-like in its heaviness.  It can be hazardous to your health just moving this thing around.  Opening up the case is not easy. And there are nasty voltages in there....  

But I  have had this transmitter since 1973 or 1974.  I have fixed it many times, in several countries.  I got it from a member of the Crystal Radio Club when I was a kid.   There are parts given to me by Pericles, HI8P in the Dominican Republic. I used it to transmit through Russian satellites.   That transmitter is like an old friend.  I just can't give up on it. 

So I need to replace or repair the choke.  Is there anyone out there who has a junker out in the garage or some other source of L25?   Or does anyone know of a business that could rewind the choke.  Please let me know.  I have decided to leave the rig on the bench until I get this thing fixed (it is too heavy to move multiple times!)   

It it Hallicrafters Part Number 056-300259. L25 -- 9 Henries at 135 ma.  Help! 

15 comments:

  1. Is it one of the chokes attached to the tube in the photo ? http://www.k3msb.com/ht37_sx111/ht37_sign1.JPG

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    1. After reading your description its clear, not a RF choke.

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  2. Failing acquisition of an original part, the Electrical industry could be worth investigation. Motor rewinders customarily work with laminated formers and copper wires to exacting standards - and ought to be able to 'detail' the original (de-wind, determine gauge and # of turns). I had the power transformer of a Gunn 100W guitar amp rewound by a hobbyist/enthusiast in that industry (could call him an Electrical Ham?) ;). Sadly later mistreatment saw both it and the output tranny incinerated, but until then it was perfect!

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  3. Another source of a replacement may be the guitar amp repair part suppliers. One manufacturer is Mercury Magnetics. There are others. A customer transformer manufacturer may be able to make an exact replacement but that may be expensive.

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  4. Play Things of the Past has a 12H at 150ma that might be ok. $10

    Ed KC8SBV

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  5. When next you lift the Hallicrafters, think about replacing the supply choke with a Gyrator. Gyrators consist of transistors and capacitor circuit, which acts like a 9 Henry choke carrying 135 mA with ease.
    Your back will thank you for it!
    73
    John ZS6WL

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  6. What is the blue thing on the right? TB9BCD 73.

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    1. Those are four electrolytic capacitors - many of the replacement electrolytic capacitors now being used are blue in color.

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  7. What kind of transmitter is this? What is it used for?

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  8. Ahhh! Power filter, not RF choke. I may have something in the junque box. What are the dimensions? Especially hole spacing.
    Indeed, re-winding the choke is not as bad as one would assume. Messy but educational. Upon opening it you may find the fault is at one end of the winding and easily repairable (once you get past the tar).
    73,
    Don, ND6T

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    1. I'm wondering if he just happened to forget to use Micro or Milli henries in his description ? But yes, if he is correct, it isn't an RF choke typically. 175 ma is pointing towards a power supply choke isn't it … Hope your doing well, N6GRG

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    2. Its listed as filter , nine hy choke, 135 ma in this parts list. If this is the one he's talking about. 056-300259 Here https://www.ecse.rpi.edu/courses/CStudio/ham_radio_docs/ht37_opr_service%20manual.pdf I'll be glad when I get to listen to this, Probably tomorrow evening. Amazed at how much info is available. N6GRG

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  9. Bill, after further inspection of the service info I noticed that that choke does not seem to be potted. That will make it easier.
    Any possibility that it opened due to high current? Is that cap OK?
    73,
    Don, ND6T

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  10. Another possibility is the Hammond model 159P. It's available new from Digikey for $28. Before ordering, please check the clearance between L26 and the back of the set. The Hammond choke is 2-1/4 inches in that direction.

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  11. As Chip points out, Hammond still makes a line of chokes which includes the 159P. This has a slightly lower current capability then the 135 mA listed, but I don't think the circuit draws more then 50 mA overall. I did some calculations on the required voltage drops for the different power supply voltages and came up with 36mA to 40mA. Also, the R of the 159P is 155 ohm, much lower than the series resistors that are part of the filter anyway.

    Since the Inductor went bad, you might check the surrounding R's and C's to make sure they are within the required tolerances. No need to blow up a perfectly good new choke!

    Anyway, these chokes are readily available through commercial parts distributors, and also tubesandmore.com (where I buy tubes from for my Drake C line and R-390A). https://www.tubesandmore.com/products/filter-choke-hammond-open-bracket-7-h-150-ma#files_and_docs. They also have the datasheet for this line of chokes showing various options.

    73's Jim N8CAH

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