Weird RF noise bridge problem (or maybe operator error)

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).

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!

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

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Best Regards:
                     Baron.

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.

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Dave Platt                                    AE6EO
Friends of Jade Warrior home page:  http://www.radagast.org/jade-warrior
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Ok, excuse me . I must of jump the gun..

Yes, there is a difference..

Got it.

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.

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:

--
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!

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.

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:

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.