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62736748382 · October 8th 06, 01:58 AM posted to rec.radio.amateur.antenna

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

Farnels has these diodes, they a off the shelf item.

Yeah those little detector diodes are impressive however they very
sensitive. I am surprised nobody has modified there analyzer with 1n23
1n119 top hat packaged diodes. The only advantage of the diodes used is
that the package is cheap! No surprises for MFJ. Macom has the biggest
selection of ceramic packaged detector diodes. They can withstand
static and overload pulses from electronic warfare emitters. Most
electronic warfare receivers which are homodyne or zero if use a
parallel pair of diodes to protect the detector diode. These diodes
typically operate -55 dbm Tss. i dont know if the MFJ uses a forward
bias of a few microamps to increase sensitivity. They also make zero
bias detector diodes which are just as sensitive as biased diodes. macom
have an excelllent range

I would think any detector diode would work, Macom have an exact
replacement for the HP unit with much better specs. Only problems is
getting parts from them. Since Tyco has just about bought up the whole
electronic industry everything is special order with lead times of 6 months.

Super High Frequency Parts company, if his still around may be able to
get them for you. He carries all the microwave parts for hams.

Greg

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