Base Changing the "look" of an antenna

[Greg T]

Yes! Glad you found that!

Can't fix a mistuned old antenna, but can help you in taking that antenna from non-50 ohm into what appears as a 50 ohm load - but those caps also act like "wave traps" for Common mode issues - a different story...

Well, I should have the amp here on Friday and the new SO-239s on Tuesday. I'm not going to even attempt anything until I get the filter repaired. I think the issue with the wandering mismatch stems from the bad connection.

http://bama.edebris.com/download/johnson/250-20/Johnson_250-20_LPF.pdf

http://amfone.net/Amforum/index.php?topic=19940.0

https://www.worldwidedx.com/threads...10-12-meters-on-a-sigma-4.127185/#post-328577

Thanx much, BJ. These sites will come in handy for sure. Gonna print that manual just to have on hand.

 

[Handy Andy]

I would take a moment and wipe and clean those trimmer caps - they are "air gap" types so it can be cleaned - but also - check the "MESH" the spaces between the plates you need to make sure they don't touch or get too close together so they won't arc when you put power thru them.

Alcohol, emission control clean or some type of De-oxit that @Slowmover uses can help..

Part of this is age and dirt and dust working against you - showing up in the "Impedance noise" the cap will exhibit when it's turned and the plates are dirty...

 

[Slowmover]

De-Oxit is the bees knees. Cleaned uncovered 3/8-24 mount threads had been months out in road dust a few days ago. Hello, bright shiny surfaces!

 

[Greg T]

I would take a moment and wipe and clean those trimmer caps - they are "air gap" types so it can be cleaned - but also - check the "MESH" the spaces between the plates you need to make sure they don't touch or get too close together so they won't arc when you put power thru them.

Alcohol, emission control clean or some type of De-oxit that @Slowmover uses can help..

Part of this is age and dirt and dust working against you - showing up in the "Impedance noise" the cap will exhibit when it's turned and the plates are dirty...

I usually use brake cleaner or MAF sensor cleaner for such tasks. Clean perfectly and leave no residue. But, this this is immaculate.

 

[Handy Andy]

Great pics - thank you...

This is why I mentioned the cleaner - it's not always dust or dirt (although the case is not airtight) but air and humidity which can carry in the dust and cause oxidation - you can't avoid it.

The only thing I wanted to point out was it's "marvelous" construction - pretty KEWL to look at...

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You just don't see this level of quality anymore - this is a multi-stage Pi filter network.

What you call chambers is the use of the "Cage" to make Feed-thru caps - that plastic on the left is the dielectrical separator to make that wall - a pass thru, to work as a feed thru cap as part of the filter...

Right now it looks as if the oxides are pretty much focused on the SO-239's - so yes, they will need some work.

That darkening on the SO-239 - if my memory serves, that is silver plated - it just may need to have the tarnish removed - but if you feel it needs to be replaced - then replace it.

It is my hope that whomever worked on this - when they refurbished it since it's so clean - you may have to tighten some of it's hardware but they did a good job in preserving it.

 

[Greg T]

Great pics - thank you...

This is why I mentioned the cleaner - it's not always dust or dirt (although the case is not airtight) but air and humidity which can carry in the dust and cause oxidation - you can't avoid it.

The only thing I wanted to point out was it's "marvelous" construction - pretty KEWL to look at...

View attachment 39221

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You just don't see this level of quality anymore - this is a multi-stage Pi filter network.

What you call chambers is the use of the "Cage" to make Feed-thru caps - that plastic on the left is the dielectrical separator to make that wall - a pass thru, to work as a feed thru cap as part of the filter...

Right now it looks as if the oxides are pretty much focused on the SO-239's - so yes, they will need some work.

That darkening on the SO-239 - if my memory serves, that is silver plated - it just may need to have the tarnish removed - but if you feel it needs to be replaced - then replace it.

It is my hope that whomever worked on this - when they refurbished it since it's so clean - you may have to tighten some of it's hardware but they did a good job in preserving it.

She's not been refurbed. I bought this new in 1966 after my neighbors were thinking the Russians were in their TVs!! I used it for a couple of years and then packed everything away as I had no time for radio any longer. It's sat for more than 3 decades in a box, as I was in and out of radio over the years. I pulled it back out a couple of years ago and set it on the shelf. This still has 1960s finger prints on it! But the one connector is broke. Look closely at the insulator in one of those pics.