On Mon, 05 Sep 2011 11:47:21 -0400, Mike Coslo wrote:
On 9/5/2011 2:58 AM, Jeff Liebermann wrote:
What bugs me is that the diodes are blowing up despite this rather low
resistance to ground. Either hams are finding some rather high power
ESD sources with which to blow up their analyzers, or some other
failure mechanism is involved.
I've had more than one device damaged while operating around High power
equipment, with probable RF in the shack excursions. My MFJ analyzer, an
old Sony camera that had it's floppy drive head destroyed, and a couple
other things I cannot remember.
I used to have a Sony Mavica MVC-FD73 camera. Nice camera for 640x480
closeups. I gave it as a present to a friend, who bashed in the LCD
display. I repaired it, and she bashed it in again. I gave up.
Per one of my previous postings, none of the failures coincided with
any high power RF nearby. One was connected to an antenna when it
probably failed, but there were no transmitters active. I've tried
interrogating the owners and none claim that they did anything
potentially destructive. I've been assuming it was ESD, not RF that
is killing the diodes, but I'm not sure.
I just tested the diodes. 3 diodes were open, one was ok.
I haven't had any problem with the analyzer since installing my dummy
load on it.
Probably a good idea.
But that brings up something interesting. You're reading 50 Ohms on the
input connector? Isn't that going to make *all* readings somewhere near 1:1?
Owen answered that question. L12 (100 uH) is in series with the 50
bridge "load" and therefore allows measurements to be made without
padding the antenna with an additional 50 ohms.
I threw together a web page on the MFJ-269 repair:
http://802.11junk.com/jeffl/MFJ-269-repair/
Hopefully, this will help anyone following my succession of mistakes
and screwups. I'm not really done as I want to verify that it's
working correctly and hopefully not require recalibration.
Incidentally, no lock washers on any of the screws inside.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558