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From: David Brodbeck on 24 Nov 2009 23:35
Baron wrote:
> 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 ?
>

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.)
From: baron on 25 Nov 2009 05:10
David Brodbeck Inscribed thus:

> Baron wrote:
>> 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 ?
>>
>
> 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.
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