[R-390] Testing Collins Mechanical filters:

[Jon Schlegel]

Hi Frank,

Another way to check the mechanical filters is to actually measure 
the RF signal going in and out to determine insertion loss.  You will 
need a Hi-Z AC voltmeter like one of the HP410 model VTVMs or a scope 
with with a 10x probe to make this measurement.  I think you should 
probably see an insertion loss of 20 dB or perhaps better.  Assuming 
the input and output impedances of the filter are the same, this 
means you should see the ratio of filter input voltage to output 
voltage of no greater than about 10:1.

This is a dicey measurement because of the impedances involved.  If 
the probe is not high enough impedance, it's loading effect on the 
circuits could make the measurements invalid.  Also, the trimmer caps 
may need re-peaked due the extra shunt C that probe presents to the circuit.

For reference, I've measured filter impedances in the rather wide 
range of 10k to 50k.  The standard Collins measurement setup requires 
a 100k source and load on the filter.  In-circuit, the filter sees 
the plate resistance of V501 which as I recall is listed at around a 
1 Megohm and the output sees only the grid input impedance of 
V502.  It's clear Collins didn't want any more loading on these 
filters than absolutely necessary.

Regards,
Jon Schlegel WA3MVM


At 04:47 PM 4/24/2009 -0400, Tim  Shoppa wrote:
>KIORQ writes:
> > But the audio is not very loud.
> > Anyone have any ideas?
>
>Ohmmeter tests:
>
>Across input and output coils of each filter, it should
>be 50 to 100 ohms. (Web says 50 to 60 ohms, my
>memory says closer to 100 ohms...). This is the
>DC resistance of the transducer coil.
>
>Between the signal pins and can should be
>very very high resistance, bigger than megohms.
>If it's leaky at the megohm level AGC action will
>reduce the gain.
>
>If there is leakage but only between one end and
>ground, it may be possible to isolate the filter can
>from ground.
>
>Standard reference on rebuilding filters: 
>http://www.delphelectronics.co.uk/filter390a/
>
>I kinda wish that I would get laid off of my job
>so that I could have time to fix my bad mechanical filters
>(and all the other old radios laying about the shack.)
>As it is others are getting laid off and I have to work extra
>hard to pick up the slack.
>
>Tim N3QE
>
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