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

[Charles Steinmetz]

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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