I have an old MFJ-202B RF noise bridge I picked up used a while back.
I've been attempting to use it with my 10m transceiver, but I can't seem
to get any useful results. No matter what I connect to the UNKNOWN
jack, the noise in the receiver is lowest with the RESISTANCE knob
turned to the 0 ohms position. It never nulls completely, but as I turn
the knob clockwise the noise increases steadily from about S3 at 0
ohms to S7 at 200 ohms. I tried connecting a resistor to the output
jack, just in case the antennas I was trying to test were out of range,
and I still got the same result.

I can't figure out what I'm doing wrong, or what could be wrong with the
unit. It's a fairly simple circuit, and it does make white noise...

In article ,
says...
I have an old MFJ-202B RF noise bridge I picked up used a while back.
I've been attempting to use it with my 10m transceiver, but I can't seem
to get any useful results. No matter what I connect to the UNKNOWN
jack, the noise in the receiver is lowest with the RESISTANCE knob
turned to the 0 ohms position. It never nulls completely, but as I turn
the knob clockwise the noise increases steadily from about S3 at 0
ohms to S7 at 200 ohms. I tried connecting a resistor to the output
jack, just in case the antennas I was trying to test were out of range,
and I still got the same result.

I can't figure out what I'm doing wrong, or what could be wrong with the
unit. It's a fairly simple circuit, and it does make white noise...

One of the steps in setting up a (Heathkit) noise bridge
is to short the "unknown" jack with a short jumper inside
the unit. Maybe check if this was left in!

Meat Plow wrote:

On Sun, 01 Nov 2009 21:18:17 -0800, David Brodbeck
wrote:

I have an old MFJ-202B RF noise bridge I picked up used a while back.
I've been attempting to use it with my 10m transceiver, but I can't
seem
to get any useful results. No matter what I connect to the UNKNOWN
jack, the noise in the receiver is lowest with the RESISTANCE knob
turned to the 0 ohms position. It never nulls completely, but as I
turn
the knob clockwise the noise increases steadily from about S3 at 0
ohms to S7 at 200 ohms. I tried connecting a resistor to the output
jack, just in case the antennas I was trying to test were out of
range, and I still got the same result.

I can't figure out what I'm doing wrong, or what could be wrong with
the
unit. It's a fairly simple circuit, and it does make white noise...

Do those things actually work? I've never had hands on experience with
one because my rigs have built in DSP. I'm sure what you have was
designed to work on narrow SSB signals not voice as you might be
thinking (although you made no mention as to what mode you are
operating in).

The receiver is used as the detector for the noise level.

--
Best Regards:
Baron.

Meat Plow wrote:

On Sun, 01 Nov 2009 21:18:17 -0800, David Brodbeck
wrote:


I have an old MFJ-202B RF noise bridge I picked up used a while back.
I've been attempting to use it with my 10m transceiver, but I can't seem
to get any useful results. No matter what I connect to the UNKNOWN
jack, the noise in the receiver is lowest with the RESISTANCE knob
turned to the 0 ohms position. It never nulls completely, but as I turn
the knob clockwise the noise increases steadily from about S3 at 0
ohms to S7 at 200 ohms. I tried connecting a resistor to the output
jack, just in case the antennas I was trying to test were out of range,
and I still got the same result.

I can't figure out what I'm doing wrong, or what could be wrong with the
unit. It's a fairly simple circuit, and it does make white noise...


Do those things actually work? I've never had hands on experience with
one because my rigs have built in DSP. I'm sure what you have was
designed to work on narrow SSB signals not voice as you might be
thinking (although you made no mention as to what mode you are
operating in).
Yes they work, normally only if the noise is local..

In article ,
Jamie t wrote:

I have an old MFJ-202B RF noise bridge I picked up used a while back.
I've been attempting to use it with my 10m transceiver, but I can't seem
to get any useful results. No matter what I connect to the UNKNOWN
jack, the noise in the receiver is lowest with the RESISTANCE knob
turned to the 0 ohms position. It never nulls completely, but as I turn
the knob clockwise the noise increases steadily from about S3 at 0
ohms to S7 at 200 ohms. I tried connecting a resistor to the output
jack, just in case the antennas I was trying to test were out of range,
and I still got the same result.

I can't figure out what I'm doing wrong, or what could be wrong with the
unit. It's a fairly simple circuit, and it does make white noise...
Do those things actually work? I've never had hands on experience with
one because my rigs have built in DSP. I'm sure what you have was
designed to work on narrow SSB signals not voice as you might be
thinking (although you made no mention as to what mode you are
operating in).

Yes they work, normally only if the noise is local..

Y'all are confusing a "noise bridge" with a "noise canceller".

The MFJ-202 is one of the former. It's a type of impedance measuring
device. It generates broadband RF noise (via a zener diode),
amplifies it (a three-transistor circuit) and then feeds it to a
bridge circuit. An unknown impedance (typically an antenna) is
connected to one side of the bridge, an adjustable impedance is
connected to the other side of the bridge, and your radio receiver is
connected to the center of the bridge. You listen to the hissing
noise from the receiver, tweak the R and C controls, and when the
noise level drops (ideally, is completely nulled out) you can read the
unknown impedance's R and X values from the adjustment knobs.

It's a substitute for (e.g.) an MFJ-259 impedance meter (much less
expensive).

It's also usable as an "antenna tuner-tuner". Connect it between your
rig and your antenna tuner / transmatch, set the knobs to "50 ohms R,
zero ohms X", and adjust your transmatch to null out the hissing.
Just MAKE SURE that you disconnect / bypass the noise bridge before
transmitting, or you'll burn it up for sure!

It's not an active noise-cancellation / nulling device for over the
air signals, like the MFJ-1025 and -1026 are. Those do an entirely
different job, in a rather different way.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Dave Platt wrote:

In article ,
Jamie t wrote:


I have an old MFJ-202B RF noise bridge I picked up used a while back.
I've been attempting to use it with my 10m transceiver, but I can't seem
to get any useful results. No matter what I connect to the UNKNOWN
jack, the noise in the receiver is lowest with the RESISTANCE knob
turned to the 0 ohms position. It never nulls completely, but as I turn
the knob clockwise the noise increases steadily from about S3 at 0
ohms to S7 at 200 ohms. I tried connecting a resistor to the output
jack, just in case the antennas I was trying to test were out of range,
and I still got the same result.


I can't figure out what I'm doing wrong, or what could be wrong with the
unit. It's a fairly simple circuit, and it does make white noise...

Do those things actually work? I've never had hands on experience with
one because my rigs have built in DSP. I'm sure what you have was
designed to work on narrow SSB signals not voice as you might be
thinking (although you made no mention as to what mode you are
operating in).


Yes they work, normally only if the noise is local..


Y'all are confusing a "noise bridge" with a "noise canceller".

The MFJ-202 is one of the former. It's a type of impedance measuring
device. It generates broadband RF noise (via a zener diode),
amplifies it (a three-transistor circuit) and then feeds it to a
bridge circuit. An unknown impedance (typically an antenna) is
connected to one side of the bridge, an adjustable impedance is
connected to the other side of the bridge, and your radio receiver is
connected to the center of the bridge. You listen to the hissing
noise from the receiver, tweak the R and C controls, and when the
noise level drops (ideally, is completely nulled out) you can read the
unknown impedance's R and X values from the adjustment knobs.

It's a substitute for (e.g.) an MFJ-259 impedance meter (much less
expensive).

It's also usable as an "antenna tuner-tuner". Connect it between your
rig and your antenna tuner / transmatch, set the knobs to "50 ohms R,
zero ohms X", and adjust your transmatch to null out the hissing.
Just MAKE SURE that you disconnect / bypass the noise bridge before
transmitting, or you'll burn it up for sure!

It's not an active noise-cancellation / nulling device for over the
air signals, like the MFJ-1025 and -1026 are. Those do an entirely
different job, in a rather different way.

Ok, excuse me .
I must of jump the gun..

Yes, there is a difference..

Jitt wrote:
In article ,
says...
I have an old MFJ-202B RF noise bridge I picked up used a while back.
I've been attempting to use it with my 10m transceiver, but I can't seem
to get any useful results. No matter what I connect to the UNKNOWN
jack, the noise in the receiver is lowest with the RESISTANCE knob
turned to the 0 ohms position. It never nulls completely, but as I turn
the knob clockwise the noise increases steadily from about S3 at 0
ohms to S7 at 200 ohms. I tried connecting a resistor to the output
jack, just in case the antennas I was trying to test were out of range,
and I still got the same result.

I can't figure out what I'm doing wrong, or what could be wrong with the
unit. It's a fairly simple circuit, and it does make white noise...

One of the steps in setting up a (Heathkit) noise bridge
is to short the "unknown" jack with a short jumper inside
the unit. Maybe check if this was left in!

Good thought! No such luck, though. Looking at the schematic, if there
were such a jumper it also would null at 200 ohms when the "range
expander" switch is on, and it doesn't.

I think when I get time I'm going to check the transformer windings for
continuity...maybe someone transmitted into it and blew a winding. It
looks from the schematic like this little toroidal transformer would be
"first in line" to get zapped.

In article ,
David Brodbeck wrote:

Good thought! No such luck, though. Looking at the schematic, if there
were such a jumper it also would null at 200 ohms when the "range
expander" switch is on, and it doesn't.

I think when I get time I'm going to check the transformer windings for
continuity...maybe someone transmitted into it and blew a winding. It
looks from the schematic like this little toroidal transformer would be
"first in line" to get zapped.

Yup. Not an uncommon way for these devices to go to Tumbolia. The
pot might also have gotten cooked.

http://www.qsl.net/wq1rp/noisebrg.htm has a nice article on construction
of a homebrew bridge, including instructions for winding the
transformer. If yours is smoked, it ought to be relatively easy to
replace.

I built a fancier derivative of the above-mentioned circuit a couple
of years ago - one with some amount of protection built in, and a
periodic chopping oscillator which turns the output from a simple hiss
into a chuff-chuff-chuff sound (slightly easier to null out under
noisy-band conditions). The schematic and a writeup are available
in the following newsletter:

http://www.fars.k6ya.org/relay/Relay0710.pdf


--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Dave Platt wrote:
In article ,
David Brodbeck wrote:

Good thought! No such luck, though. Looking at the schematic, if there
were such a jumper it also would null at 200 ohms when the "range
expander" switch is on, and it doesn't.

I think when I get time I'm going to check the transformer windings for
continuity...maybe someone transmitted into it and blew a winding. It
looks from the schematic like this little toroidal transformer would be
"first in line" to get zapped.

Yup. Not an uncommon way for these devices to go to Tumbolia. The
pot might also have gotten cooked.

Well, I'm stumped. The pot tests fine. The windings on the toroidal
transformer have continuity, and none are shorted to each other. It
seems like it ought to work, but I continue to not get a useful null for
resistance -- it just gets quieter the closer to zero I turn the knob.
The inductance/reactance knob works, though -- I can get sharp nulls
with it.

David Brodbeck wrote:

Dave Platt wrote:
In article ,
David Brodbeck wrote:

Good thought! No such luck, though. Looking at the schematic, if
there were such a jumper it also would null at 200 ohms when the
"range expander" switch is on, and it doesn't.

I think when I get time I'm going to check the transformer windings
for
continuity...maybe someone transmitted into it and blew a winding.
It looks from the schematic like this little toroidal transformer
would be "first in line" to get zapped.

Yup. Not an uncommon way for these devices to go to Tumbolia. The
pot might also have gotten cooked.

Well, I'm stumped. The pot tests fine. The windings on the toroidal
transformer have continuity, and none are shorted to each other. It
seems like it ought to work, but I continue to not get a useful null
for resistance -- it just gets quieter the closer to zero I turn the
knob. The inductance/reactance knob works, though -- I can get sharp
nulls with it.

What are you trying to test it with ?

--
Best Regards:
Baron.

Baron wrote:
David Brodbeck wrote:

Dave Platt wrote:
In article ,
David Brodbeck wrote:

Good thought! No such luck, though. Looking at the schematic, if
there were such a jumper it also would null at 200 ohms when the
"range expander" switch is on, and it doesn't.

I think when I get time I'm going to check the transformer windings
for
continuity...maybe someone transmitted into it and blew a winding.
It looks from the schematic like this little toroidal transformer
would be "first in line" to get zapped.
Yup. Not an uncommon way for these devices to go to Tumbolia. The
pot might also have gotten cooked.
Well, I'm stumped. The pot tests fine. The windings on the toroidal
transformer have continuity, and none are shorted to each other. It
seems like it ought to work, but I continue to not get a useful null
for resistance -- it just gets quieter the closer to zero I turn the
knob. The inductance/reactance knob works, though -- I can get sharp
nulls with it.

What are you trying to test it with ?


I tried a few different fixed resistors connected to the "unknown"
connection. I also tried leaving the "unknown" connection open and
engaging the range expander feature, which should give a null at 200
ohms. (The range expander just parallels a 200 ohm resistor with the
unknown.)

David Brodbeck Inscribed thus:

Baron wrote:
David Brodbeck wrote:

Dave Platt wrote:
In article ,
David Brodbeck wrote:

Good thought! No such luck, though. Looking at the schematic, if
there were such a jumper it also would null at 200 ohms when the
"range expander" switch is on, and it doesn't.

I think when I get time I'm going to check the transformer
windings for
continuity...maybe someone transmitted into it and blew a winding.
It looks from the schematic like this little toroidal transformer
would be "first in line" to get zapped.
Yup. Not an uncommon way for these devices to go to Tumbolia. The
pot might also have gotten cooked.
Well, I'm stumped. The pot tests fine. The windings on the
toroidal
transformer have continuity, and none are shorted to each other. It
seems like it ought to work, but I continue to not get a useful null
for resistance -- it just gets quieter the closer to zero I turn the
knob. The inductance/reactance knob works, though -- I can get sharp
nulls with it.

What are you trying to test it with ?


I tried a few different fixed resistors connected to the "unknown"
connection. I also tried leaving the "unknown" connection open and
engaging the range expander feature, which should give a null at 200
ohms. (The range expander just parallels a 200 ohm resistor with the
unknown.)

Fixed resistors are not very frequency sensitive. Try with a tuned
circuit, say an inductance in series with a capacitor and see how you
get on. Ideally the tuned circuit should be resonant some where within
the tuning range of the receiver.

--
Best Regards:
Baron.