[R-390] A 6kc. mechanical filter for a R-390a?

[Al S.]

Hi Charles

Thanks for passing along all the information on the 6790 mechanical filters.

I purchased these from Gary Harmon? if I recall correctly. He refurbed
those receivers in the past and sold parts.

He did provide a table with the 7883-4 or 526 9957 040. 6kc filter, showing
the resonating caps for all six mechanical filters. 7883-1 to 7883-6.
The table indicates the 750 pf caps used for the 6kc filter.

Al
On Jun 16, 2017 11:09 AM, "Charles Steinmetz" <csteinmetz at yandex.com> wrote:

> Al wrote:
>
> I'll have to have a look at how this 6kc filter is terminated etc in the
>> Racal 6790 receiver. Its not one of the physically tiny torsional
>> filters. The only info that I have with the filter is that it needs a
>> 750 pf. cap for resonating on the input and output of the filter in the
>> circuit.
>>
>
> The 6790 terminates the filter inputs with 4.7k ohms and the filter
> outputs with 5k ohms (2x 10k in parallel).  The different filters use
> different resonating capacitors, from 82pF to 750pF (see table attached
> below).
>
> The manual does not provide sufficient information to say what the correct
> resonating capacitors are for the 6kHz filter (at least, not in the
> schematic and parts list -- I didn't read any of the text).  One of the
> filters does use 750pF + 750pF, but I can't say which one from the
> information at hand.  (Note that some of the filters use different value
> capacitors on the input and output.)
>
> Best regards,
>
> Charles
>
>
>
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