Prev: Sony Blu-Ray player drive replacement
Next: Washing M/C will not Spin
From: Jamie on 2 Nov 2009 22:20
Dave Platt wrote:

> In article <IrLHm.40$XH1.5(a)newsfe22.iad>,
> Jamie <jamie_ka1lpa_not_valid_after_ka1lpa_(a)charter.net> 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..


From: David Brodbeck on 3 Nov 2009 20:53
Jitt wrote:
> In article <TuGdnfQ5KqsH9nPXnZ2dnUVZ_oWdnZ2d(a)speakeasy.net>,
> gull(a)gull.us 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.
From: Dave Platt on 3 Nov 2009 21:38
In article <IOGdnTASGuAFQ23XnZ2dnUVZ_oqdnZ2d(a)speakeasy.net>,
David Brodbeck <gull(a)gull.us> 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 <dplatt(a)radagast.org> 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!
From: David Brodbeck on 14 Nov 2009 16:38
Dave Platt wrote:
> In article <IOGdnTASGuAFQ23XnZ2dnUVZ_oqdnZ2d(a)speakeasy.net>,
> David Brodbeck <gull(a)gull.us> 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.
From: Baron on 14 Nov 2009 18:31
David Brodbeck wrote:

> Dave Platt wrote:
>> In article <IOGdnTASGuAFQ23XnZ2dnUVZ_oqdnZ2d(a)speakeasy.net>,
>> David Brodbeck <gull(a)gull.us> 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.
First  |  Prev  |  Next  |  Last
Pages: 1 2 3
Prev: Sony Blu-Ray player drive replacement
Next: Washing M/C will not Spin