Dan,
My MFJ-259B had a diode failure due to static and had to be returned
for repair. There is a simple test that you can do. It is based on
the W8JI alignment procedure but does not require that you open the
unit. Here is the procedu
1. Get the MFJ into test mode using the procedure described by W8JI (
http://www.w8ji.com/mfj-259b_calibration.htm )
2. Use the mode button to step to the R-S-Z display. Set frequency to
10 mHz. With nothing attached to the SO-239 you should see R around
230 to 250, S close to 0, and Z=255 or a bit less. S and Z need to
sum up to less than 255 no matter what resistance is across the input.
You will be off a little bit if alignment is required but you will be
way off if one of the diodes is shot.
3. Short the SO-239. R should stay the same but S=255 and Z=0. Note
S & Z sum up to 255. If your diodes are shot you will be way off as
mentioned above.
If you go to the trouble of replacing the diodes you might want to put
in a matched set of 50 ohm resistors in the bridge (R21,R22 & R24) in
order to get better alignment. Check the diodes for a match in
characteristics as well.
You have the wrong part number for the diodes. They are HSMS-2820 and
are in stock at avnet, newark and arrow (
http://www.arrownac.com) as
part hsms-2820-BLKG for 34 cents each - no minimum. You might be able
to talk Avantgo into sending you some as samples. You could always try
registering on their website for samples or you could call and speak to
one of their application engineers.
Hope this helps. Good luck - Roger
wrote:
My MFJ-259B has decided that a 100pF capacitor is a 600 ohm resistive
load at 2MHz, and really seems to be having problems with capacitative
reactances. Inductive reactances and pure resistances are a little
better, but something is distinctly wrong. Pure capacitances are
showing up with large resistive components across much of the frequency
range of the unit.
W8JI's calibration site suggests detector diode failure as a common
cause for sudden problems, but I wonder if anyone else has had this
happen? Is it likely the diodes?
Also, if I do need to replace diodes, where can I get some HSMS 2829
Schottky Detector diodes (I think these are the ones?) without having
to order 100 of them from Newark? I've looked through Digikey, Allied,
Newark, Mouser, Jameco... doesn't look good.
This thing is long out of warranty, it seems like MFJ service times are
in the month+ range at least, and i've got access to a temp controlled
soldering station and I'm not scared to swap surface mount stuff. Then
again, if I can't get the diodes or can't get them in less than a
month, I should probably just send it back to MFJ and pay their repair
fee; they've at least got the diodes.
Any comments or suggestions on what I should do? I'd rather not be
without it for an extended period.
Dan