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

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

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

At 10MHz, I get the following:

Open: R 255 S 003 Z 248

Shorted: R 232 S 255 Z 001

Decent 50 ohm termination: R 001 S 127/128 Z 121

At 3 MHz my 100pF +/- 10% capacitor reads164-j530 ohms on the Impedance
display ... it seems to get X right... Furthermore, setting the meter
to capacitance makes it read about 108pF... but there's this big
resistive component.

In the test menu, this cap reads R 255 S 023 Z 247 at 3 MHz

So maybe I just need an alignment... I suppose it wouldn't hurt to try
it if I can scare up some precision terminations.

I wouldn't consider these numbers "way off" ... this reading of
resistance at low frequencies had got me worried that something was way
wrong, but will an alignment fix this? I guess I can check through
the W8JI alignment procedure with the proper loads before I start
twiddling things.

Dan

Dan,

I repeated your test with the 50 ohm load at 10 mHz and got R 006 S
127 Z 126 which is pretty close.

With a 75 ohm termination reading is R50 S102 Z152
With a 200 ohm termination reading is R147 S50 Z204

With a 100 pf cap at 10 mHz I read R = 0 X=145 109 pf and R245
S083 Z242

With a 100 pf cap at 3 mHz I read R = 0 X=554 95 pf and R246
S023 Z255

Here are some things to check:

1. Try 75 and 200 resistors and see if you get readings close to mine.
If you do your diodes are probably OK but you need an alignment.

2. Use the 100 pf cap and starting from 3 Mhz start increasing the
frequency. Does it read R=0 some of the time - most of the time?? Try
another type/size of cap.

3. Attach about 20 feet of coax to the unit and put a 75 ohm resistor
at the other end. Your SWR should be about 1.4 to 1.5 depending on
coax type/length. Start at 2 mHz. and start increasing the frequency
to 30 mHz. The SWR should gradually get lower as you increase
frequency due to cable loss. However it will go up and down a bit (+/-
..1) at certain frequencies due to alignment and instrument error. R &
X should change in a smooth fashion as you change frequency (the cable
is an impedance transformer)


If you attempt alignment here are some tips.

1. Follow the procedure at W8JIs site - not MFJ. Tom was the designer
and his site has the latest info.

2. Use precision resistors placed deep into the connectors and short
leads for minimum inductance. I used metal film 1% and picked the best
ones with an accurate ohm meter.

3. Use a stable power supply to power the unit. If you try to use
batteries alone you will find the voltage dropping with time and you
will not get consistent results.

4. Be prepared to repeat the process several times in order to get the
best results.

5. It will never be perfect. The 259-b is an 8 bit implementation,
the bridge has inherent inbalance due to resistor/diode characteristics
and the log buffer does not match the diode characteristics perfectly.
This isn't a VNA but it is a useful low-cost instrument.

Hope you keep us updated. I am interested in your results.
Roger




wrote:
At 10MHz, I get the following:

Open: R 255 S 003 Z 248

Shorted: R 232 S 255 Z 001

Decent 50 ohm termination: R 001 S 127/128 Z 121

At 3 MHz my 100pF +/- 10% capacitor reads164-j530 ohms on the Impedance
display ... it seems to get X right... Furthermore, setting the meter
to capacitance makes it read about 108pF... but there's this big
resistive component.

In the test menu, this cap reads R 255 S 023 Z 247 at 3 MHz

So maybe I just need an alignment... I suppose it wouldn't hurt to try
it if I can scare up some precision terminations.

I wouldn't consider these numbers "way off" ... this reading of
resistance at low frequencies had got me worried that something was way
wrong, but will an alignment fix this? I guess I can check through
the W8JI alignment procedure with the proper loads before I start
twiddling things.

Dan

200 (197 on my DVM) ohm termination:

R 155 S 051 Z 197

75 (74.5 on my DVM) ohm termination

R 047 S102 Z 146

With the cap sweep, with two different caps (a 100pF doorknob and a
30pF air variable trimmer), R is almost never zero until I get up into
the 60MHz+ range and even then, rarely.

With the coax check, the SWR oscillation amplitude is about 0.2 or so;
1.1-1.3. R and X vary smoothly and more or less cyclically.. I guess
the oscillatory SWR variation with frequency is incorrect... I do get
more than you mentioned and more than one might expect... i've got
about 40 feet of RG-55U terminated in the 74.5 ohm termination that I
used above.

- - - - - -

From what you've said, it seems like alignment time. I guess the large
resistive component on capacitors at low frequency is a numerical
artifact of a slight misalignment... because the R S Z menu values are
really fairly close to yours. I'll build or find some precise SMT
terminations and go at it. Thanks much for your help and I'll post
after I do the alignment.

73,
Dan

wrote in message
ps.com...
.......................................
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.
.................................................. .....

MFJ sells replacement parts. You should be able to get some from them.

Tam/WB2TT

Hi All,

Found this forum and note when looking for some help on my MFJ-259B.

My MFJ 259B has developed a fault where the Impedeance meter Z reads 0 at all times and doesn't deflect at all. The SWR meter does read and dips when I tune through the resonant frequency of my aerial.

Using the instructions in this note for testing I get R240 S139 Z000 with Open circuit and R202 S039 Z000 with Short Circuit.

My thoughts are the Z000 matches the zero on the meter and so problem is probably the bridge and not the wiring or the meter itself?

Dismantle and check diodes?

Any help appreciated.

Paul

An