DIYbanter - View Single Post - Weird RF noise bridge problem (or maybe operator error)

David Brodbeck

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.