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Author Topic: Problem with Cushcraft A3S 20M trap?  (Read 4650 times)
VA3GUY
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Posts: 191




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« on: July 29, 2012, 08:51:53 AM »

I have an A3S up (with 40M add-on) that has been there for about a year.  SWR has been good on all 4 bands.  A few weeks ago during rain, my neighbor told me he saw 'arcing' coming from my antenna when I was transmitting.

Yesterday, I was able to finally get up on the roof and inspect the antenna.  I found that somehow, one of the 20M traps (TK) that is included in the 40M kit had rotated 180 degrees and the drain holes were facing up.  I also noticed that the end cap had been fried.  I removed the trap, open it, cleaned it up and inspected it.

There are 74 turns of what looks like #12 coated wire wrapped around a 3/4" insulator.  The wire is discolored around the end nearest the end cap.  I cleaned it up and did a continuity check using a multimeter.  It appears fine because the meter 'beeped' showing continuity.  I reassembled the trap, plugged the end with butyl tape, reassembled the antenna and put it back up on the roof (in a 5' roof tower).

Now, I get excellent SWR readings on 10M, 15M and 40M but 20M is >3.  I have tried a multitude of different adjustments to try and bring it down but to no avail.  I understand that the 10M and 15M traps don't really affect the 20M & 40M bands and that is reflected in the SWR readings during adjustments.

I decided to put the antenna back to the original settings suggested by Cushcraft's installation instructions.  The SWR readings I got were: 10M - 1.2 , 15M - 1.1 , 20M - >3 , 40M - 1.2.

It is obviously starting to look like the 20M trap is the culprit but I am at a loss as to what is going on.  It seems that there is a good electrical path through it.  I tried one last setting to see whether any of the tubing is affecting it.  I adjusted the furthest tube from the boom (inward) to see what bands it affected and how it affected them.  10M and 15M were unaffected, as suspected.  Adjusting that one small end tube changed the center frequency a little higher on 40M but SWR is still 1.1.  Unfortunately, there was no change in 20M.

Any idea as to what is happening?  Why?  And any suggestions on how to fix it?

Thanks much...Guy
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G4IJE
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Posts: 247




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« Reply #1 on: July 29, 2012, 10:45:25 AM »

Someone with knowledge of HF trapped beams will no doubt reply here soon. In the meantime I'll just make the point that the continuity test you used might indicate the inductance part of the trap is OK, but tells you nothing of the capacitance part. A short circuit of some kind would also beep your continuity test. Without looking up the resistance of #12 copper wire, I would guess you should see a few milli-ohms of resistance across 74 turns, perhaps just enough to show the difference between a normal trap and a dead short on a low ohms range multimeter.
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VA3GUY
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Posts: 191




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« Reply #2 on: July 29, 2012, 07:56:29 PM »

I decided to take the trap off and inspect it again.  It still looked fine and the resistance reading was less than 1 ohm.  I decided to put it back up WITHOUT the aluminum casing, just to see what would happen.  No change...but when I brought it back down, put it back in the aluminum shell and reinstalled it on the antenna, the SWR came down to around 1.2.  So I am figuring that maybe I did not have a good enough contact with the tubing and the problem was not the trap after all.  Not really sure.  Anyway, all is good now.  I made, and have added, a 6M parasitic antenna to the existing boom and it works great with an SWR of 1.6 through the band.  Not bad...5 bands on a tribander!
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NK7Z
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« Reply #3 on: July 30, 2012, 03:52:01 AM »

Speaking of A3S's...
I will be purchasing a used A3S in about a week...  I want to rebuild it from scratch, when I get it...  Has anyone done this before?  If so, can you offer any hints?
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Thanks,
Dave
For reviews and setups see: http://www.nk7z.net
AJ4CU
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Posts: 78




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« Reply #4 on: July 30, 2012, 04:46:00 AM »

Hi all,

i have owned the A3s with the 40M kit, while the performance of the A3 without the kit was ok and then with the 40m kit it was also ok, my experience was the design of the transition to mount the 40M kit, it was at best flimsy and gave me nothing but problems after a few months (wind etc...I had to take it down several times to re adjust and tighten), I got disgusted and sold it, I now use a Mosley TA-53 with 40M kit and the results are far better and the mounting of the kit is very substantial, I am so impresed with the performance I plan on getting the boom extender kit (adds length for better f/b and an extra element) and the 6M kit....
So my advice from experience is the A3 is ok by itself avoid the 40m add on or just go Mosley if you want multi band use...
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WX7G
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Posts: 6146




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« Reply #5 on: July 30, 2012, 06:31:32 AM »

You might just purchase a new trap.
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NK7Z
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« Reply #6 on: July 30, 2012, 09:40:53 AM »

Are parts still available for this antenna?
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Thanks,
Dave
For reviews and setups see: http://www.nk7z.net
KA5N
Member

Posts: 4380




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« Reply #7 on: July 30, 2012, 11:18:42 AM »

You said the wire on one end of the trap was discolored.  Your neighbor said he saw the
antenna arcing.   The trap was turned upside down. 
I suspect that the trap arced and there is a carbon path from the winding to the tubing.
Buy a new trap.   Clean the tubing before installin the new trap.
Enjoy.

Allen KA5N
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WB6BYU
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Posts: 13353




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« Reply #8 on: July 30, 2012, 12:51:55 PM »

Quote from: VA3GUY
...I decided to put it back up WITHOUT the aluminum casing, just to see what would happen.  No change...but when I brought it back down, put it back in the aluminum shell and reinstalled it on the antenna, the SWR came down to around 1.2...


The outer shell provides the capacitance that tunes the trap.  It isn't surprising
that it didn't work well without it.
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KE3WD
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Posts: 5689




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« Reply #9 on: July 30, 2012, 01:24:44 PM »

You said the wire on one end of the trap was discolored.  Your neighbor said he saw the
antenna arcing.   The trap was turned upside down. 
I suspect that the trap arced and there is a carbon path from the winding to the tubing.
Buy a new trap.   Clean the tubing before installin the new trap.
Enjoy.

Allen KA5N

There it is. 

The discolored lacquer on the coil wire is discolored for a reason, it overheated or arced to the point where the insulation is likely not able to function as an insulator when you put the power to it.  As you apply RF power, it is very likely that compromised area of the wire insulation is breaking down. 

Use of the DC Ohmmeter can only tell you if there is DC continuity, it cannot tell you anything about the coil's inductance, nor about the browned insulation effectiveness (or lack thereof). 

The proper replacement part is still available, locate and get it and install it. 


73
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VA3GUY
Member

Posts: 191




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« Reply #10 on: July 31, 2012, 08:26:56 PM »

As I stated earlier, all seems to be working well...so far.  But I do have a new question based on something KE3WD said.  He mentioned about the insulation.  Currently, there is no insulation inside that trap anymore but I am getting SWR around 1.4 at 14.010 up to 2.5 on 14.250.  Not too bad, really.  But is the insulation required?  What does it do?

Thank you...
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W9GB
Member

Posts: 2626




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« Reply #11 on: July 31, 2012, 10:32:47 PM »

Quote from: NK7Z
Are parts still available for this antenna?

YES, Cushcraft / MFJ
http://www.cushcraftamateur.com/Product.php?productid=A-3S

Quote from: KK7Z
I will be purchasing a used Cushcraft A3S in about a week...  I want to rebuild it from scratch, when I get it...  Has anyone done this before?  If so, can you offer any hints?

NoAlOx from electrical dept. at your local hardware store.
USE Sctchbrite Green pads, avoid steel wool, unless significant corrosion.
« Last Edit: July 31, 2012, 10:35:23 PM by W9GB » Logged
NK7Z
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« Reply #12 on: August 02, 2012, 06:30:02 AM »

Thank you sir!!!
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Thanks,
Dave
For reviews and setups see: http://www.nk7z.net
W9GB
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Posts: 2626




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« Reply #13 on: August 04, 2012, 11:18:48 AM »

Cushcraft A3S -- Refurbishment Photos
http://www.para.org.ph/membersarticles/A3S/

KG5U -- Narrative for restoration of Cushcraft A3S
http://users.hal-pc.org/~kg5u/a3refurb.txt

« Last Edit: August 04, 2012, 11:27:34 AM by W9GB » Logged
NK7Z
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« Reply #14 on: August 04, 2012, 12:21:26 PM »

THANK YOU!!!!!!!!!  I will be getting the A3S next week!!!
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Thanks,
Dave
For reviews and setups see: http://www.nk7z.net
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