100 OHM 100W resistor in Harris HT35FM PA Cavity

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Nathaniel Steele
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100 OHM 100W resistor in Harris HT35FM PA Cavity

Post by Nathaniel Steele » Tue Dec 04, 2018 2:20 pm

Need to replace this resistor and the two fingerstock rings that hold it in place. I cant get Gates air to reply to my parts request (been a week now) so is there anything I need to know about the reistor? I suspect the rings are proprietary, any suggestions for fabricating a replacement?
HT-35 PA Cavity.jpg
PA Cavity Drawing.pdf - Foxit Reader.jpg

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RGORJANCE
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Re: 100 OHM 100W resistor in Harris HT35FM PA Cavity

Post by RGORJANCE » Tue Dec 04, 2018 2:46 pm

I don't have any experience on that xmttr, but I am a bit curious about those rings. This is a real swag.....could those rings possibly be a part of a tube socket? You might try to get some dimensional detail on the ring, and diameter of the circle of finger stock the resister slips into and try to fab something locally.

I believe you can purchase bulk finger stock from a manu, or perhaps Eimac.

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Kelly
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Re: 100 OHM 100W resistor in Harris HT35FM PA Cavity

Post by Kelly » Tue Dec 04, 2018 4:42 pm

I believe that resistor is used as a corona swamping resistor. That's why the RF finger stock is around each end of the resistor.

Harris and RCA 1/4 wave cavity transmitters used various forms of these resistors to keep the PA for going into business for itself at startup.
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Steve Brown
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Re: 100 OHM 100W resistor in Harris HT35FM PA Cavity

Post by Steve Brown » Tue Dec 04, 2018 8:20 pm

And some of us remember that with a Collins/Continental FM transmitter when you killed the exciter the PA current and the power output dropped to zero, but not necessarily so with an RCA BTF-20. Sometimes when you killed the drive to those beasts the PA current DID drop somewhat but were still showing current and output power on SOME indeterminate frequency. Exciting times! I'll bet that the 'other' Steve Brown in Wisconsin could tell us stories about that....

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RGORJANCE
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Re: 100 OHM 100W resistor in Harris HT35FM PA Cavity

Post by RGORJANCE » Wed Dec 05, 2018 7:17 am

As I recall, the early Harris FM-20G/H/H2/H3 & K (starting sometime after 1965) series rigs had that resistor in the PA cavity. It had a short piece of tubing (antenna) at each end of the resistor and if memory serves, the purpose was to attenuate "circulating currents" in the cavity. It worked, and I recall hearing that Harris "borrowed" that idea from the RCA 20 which had the same problem.

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Kelly
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Re: 100 OHM 100W resistor in Harris HT35FM PA Cavity

Post by Kelly » Wed Dec 05, 2018 2:30 pm

There was a station across town that had an old BTF20 used as backup which we called The Blue Flame Special. Sometimes when you tune it or the exciter muted, it used to shoot a blue corona out the top. One day we were digging into it and discovered a similar swamping resistor was completely missing.
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Nathaniel Steele
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Re: 100 OHM 100W resistor in Harris HT35FM PA Cavity

Post by Nathaniel Steele » Wed Jan 16, 2019 10:45 am

So I pulled this thing apart a couple weeks ago and found a part number on the resistor.
20190116_101714.jpg
20190116_101558.jpg
The original manufacturer is currently owned by ohmite it seems, they are working on finding me a replacement part.
Kelly wrote:
Tue Dec 04, 2018 4:42 pm
I believe that resistor is used as a corona swamping resistor. That's why the RF finger stock is around each end of the resistor.

Harris and RCA 1/4 wave cavity transmitters used various forms of these resistors to keep the PA for going into business for itself at startup.
IS there any criticality to the location of the resistor in the cavity? I may end up with slightly different holders, or a slightly different length of resistor. is that of any importance?

Thanks guys!

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RGORJANCE
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Re: 100 OHM 100W resistor in Harris HT35FM PA Cavity

Post by RGORJANCE » Wed Jan 16, 2019 11:03 am

I am gonna SWAG this one. I don't think location is super critical. I have a gut feeling the resistor was placed in a "convenient and inexpensive" location.

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Nathaniel Steele
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Re: 100 OHM 100W resistor in Harris HT35FM PA Cavity

Post by Nathaniel Steele » Wed Jan 16, 2019 11:10 am

Thanks, I just got off the phone with another engineer who basically said the same thing. :D

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Re: 100 OHM 100W resistor in Harris HT35FM PA Cavity

Post by ChuckG » Thu Jan 17, 2019 12:31 am

That's a non-inductive resistor. You should be able to change it to a tab-connector (also non-inductive) replacement without issues if you can't find the circular fingerstock connectors. Surplus Sales of Nebraska has them from time to time too.
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Nathaniel Steele
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Re: 100 OHM 100W resistor in Harris HT35FM PA Cavity

Post by Nathaniel Steele » Fri Jan 18, 2019 10:09 am

Parts ordered, Ohmite is the manufacturer now. I had to order three. I'll keep one as a spare.

Does anyone else want one?

They have the holders too.

Thanks!

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