On 07 Sep 2011 13:27:08 GMT, dave wrote:
If you do not short the center conductor (on the DUT) to ground
immediately before connecting to the analyzer you risk discharging a
large "capacitor" directly into the instrument.
Good point except that none of the 3 analyzers that failed were
(allegedly) attached to equipment. They were attached to antennas.
Antennas normally do not have BFC's (big fat capacitors) attached, but
do build up static charges. With the humidity currently at 60% or
more, I don't think that's likely. Also, any antenna design, that
would fry an analyzer, might also fry a radio front end. It's
possible, but unlikely.
If I am at an antenna
farm I am grounding all my test gear prior to putting it into service.
There is a Ground lug on the 259B right next to the SO239. They
provide a Type N adaptor with a new meter, too, BTW.
I carry a neon lamp line tester in my tool box. It's useful for
checking AC power, but also for detecting high voltages on antenna
terminals. It's amazing what I find at broadcast sites.
The plot thickens. I checked the accuracy of the MFJ-269 and is was
off on the real part of anything it was measuring about +10%. A good
50 ohm load would read about 56 ohms. It didn't take long to find the
culprit. Instead of 51.1 ohms (1%), R88 showed 54 ohms, while R85 and
R86 read 52 ohms (or there about within the limits of my cheapo
ohms-guesser). It therefore seems possible that the owner had
transmitted into the instrument (even though he denies it). That also
might explain how 3 out of 4 diodes were found fused open. Most
diodes I've seen end up shorted when the junction is punctured. I'll
be replacing all the 51.1 ohm resistors.
Meanwhile, I added a grounded load cap, returned the instrument to its
owner, extorted a free lunch, applied the requisite instrument safety
lecture, and warned him of the accuracy problem. The dip in the VSWR
curve is all he needs at this time, which is unaffected by bridge
imbalance.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558