Topband: MFJ-259B

[Dan Zimmerman N3OX]

>
> I have a MFJ-259B VSWR analyzer that the reactance is no longer X=0, but
> X=5 when a pure resistive 50 ohm load is attached. Makes spring time
> checking of low band antennas more difficult.


Before you start tweaking, try some other loads.  Try a capacitor of known
value.  Try a 200 ohm resistor and a 12.5 ohm resistor.

In fact, go ahead and try all the loads in W8JI's calibration procedure and
see if the test-mode settings are similar.  When something changes suddenly,
as W8JI says, you probably blew a diode, and there's no point in
re-calibrating with a blown diode.

I just repaired my '259B for a second time because of a blown diode/diodes,
but it was still indicating more-or-less 50 ohms on a dummy load.  When I
put a capacitor on it, though, it read mostly resistance!

It is possible, I suppose, for a trimpot to age or get jostled and for the
unit to go out of calibration, but  you'll save some frustration if you
try  other loads, resistive and reactive, and see if the results make any
sense at all.

If the readings don't make sense and indicate a blown diode it's not so hard
to do the repair.  The diodes are Avago HSMS-2820-BLKG, available from
Mouser, etc. for fifty cents or so.  There's a schematic here:
http://www.k0to.us/HAM/MFJ%20Diode%20measurements/11-17-sch_mfj259b-BW.pdf  and
I replaced D1 through D4.  They didn't all seem to be blown when I checked
them after.  This time I decided to try to select matched diodes but I
couldn't measure a difference at the 1mV level in the ten or twelve diodes I
checked so that's probably not necessary. They seem well matched.  Once the
diodes are replaced, then the calibration on W8JI's page will put you back
in working order.

It will be harder to do the calibration after diode repair if you calibrate
with a bad diode, so I'd do some extra diagnosis before you tweak anything.

73
Dan