On Sat, 03 Sep 2011 09:25:31 -0700, Jeff Liebermann
wrote:

I have a 4" pigtail around the ground lug and a male banana plug on the
end that lives in the middle of the SO-239, unless I am making
measurements. I use the banana plug as a shorting stick to neutralize
any residual capacitive charge in the device/coaxial cable under test.
Center conductor to cable ground. If I am especially concerned I'll
groung the 259B to my extensive safety ground system. When dealing with
little diodes you always want to make sure none of the sparks go through
them. Ground, ground and ground.

Hmmm.... The problem might be the SO-239 connector. When you plug
something into that connector, it connects the center pin first, and
then the ground. If it were replaced by an N connector, the ground
would make contact before the center pink, thus offering some added
protection. I'll see if the connector can be replaced.

http://www.w8ji.com/mfj-259b_calibration.htm
Most Likely Failures

Other than manufacturing errors, the detector diodes clearly
stand out as the most common problem. They are the most easily
damaged devices in the analyzer. If you have a sudden problem,
it is most likely a defective detector diode. Diode damage almost
always comes from accidentally applying voltage on the antenna
port.

Why are the diodes so sensitive? In order for the detectors to be
accurate within a fraction of a percent (one bit), detector diodes
must have very low capacitance and a very low threshold voltage.
This means the diodes, through necessity, must be low-power
zero-bias Schottky microwave detector diodes. The same
characteristics that make them accurate and linear cause the diodes
to be especially sensitive to damage from small voltage spikes.
ALWAYS discharge large antennas before connecting them to the
analyzer! Never apply external voltages greater than 3 volts to the
antenna port!

and

Because the detector is broadband and because it is dc coupled
to the antenna, any external voltage across the input port
causes measurement errors. It is the accumulated voltage of
multiple sources that is most important, not the strength of
any individual signal. Because of that, large antennas should
be tested at times when propagated signals in the range of the
antenna's response are at minimum strength.

W8JI designed the MFJ-259b.



--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558