[Lowfer] rx transmission line maintenance, no pin holes here.....

[Michael Sapp]

Pat N4LTA wrote:

>On Sat, Jan 26, 2013 at 10:03 AM, pbunn <pbunn at matrixei.com> wrote:
>> It makes corrosion worse. About a month ago I came in and found the power supply led off on my DC coupler. I figured the power supply had died but >when I unplugged the coax it was OK, so then I knew I was looking at a short at the probe or in the coax. Looking at the way the probe circuit works, I >didn't see a way a failed transistor could result in a short that low in resistance.
>>
>> I emailed Jack Smith and asked hid opinion and he said 90 % likely that I had a coax problem and the DC bias would corrode a breach in the shield. >After a couple hours with the multimeter, I found a pinhole in a buried section of the Belden quad shield. Just as Jack had indicated, the DC current had >caused the aluminum shield to turn to powder for s couple of feet. I ended up replacing the entire 300 foot run with new cable.
>>
>> Pat
>> N4LTA

    Pat: That is certainly a plausible scenario in your particular situation. 

    However, I went the the rx antenna transformer box, disconnected the RG6 and placed a plastic bag over the open connector end, then went to the other end of the RG6 in the shack and peformed a megger test at 1KV. After about 2 minutes of applied 1KV, the DC leakage resistance was measured at 2.5 GIGA OHMs and still climbing slowly, with no glitches or other signs of arcing.  At 30pf per foot and +175 feet of coax, that coax is a > 5250pf capacitor, so it "charges" slowly after initially reaching 1.75 Giga-Ohms or so in the first few seconds of applied 1KV DC.  In my book a capacitor with +2.5 Giga-Ohms of leakage resistance qualifies as good.  

    As a related example, most of the .1uf caps (mostly RF bypass and coupling) that were originally in my HD-1420 VLF converter measured 10 to 50 mega-ohms, for DC leakage, so they were all replaced with poly-film caps that had +2 giga-ohm mewasured DC leakage resistance. The performance improvement was immediately noticeable with the new  .1uf caps in the converter.   It's no iron-clad  guarantee of a good cap at RF, but it is a good start.   

    Since I do a procedure where I also repeatedly tap the DC power clip lead to the feedline center pin several times with a larger PL-259 adapter, it's posible the 11:1 transformers are upping the impulse volts and bleeding the volts off through the antenna ground and termination, perhaps cutting though some wire to connector oxide in the process.....I only apply a combination of limited impulse and constant current DC for 30 seconds to a minute once a week or so, so there is no long term electrolysis going on.....

        Like I said previously, the only reason I tried the coax zap procedure was previously owning a TS-430 and knowing that a few ma of 12 VDC on the rx relay cured the lossy relay contact resistance issue on the receive side realy contacts. That mod is well documented, it should be at dk.mods...

  Regards,  Mike wa3tts