[Boatanchors] Boatanchors Digest, Vol 131, Issue 22

[Rob Atkinson]

hi Drew,

There have been two or three articles in ER about 75A-3 filters.
Usually they are LC filters.  It has become a kind of sport for me at
hamfests to lift the lids on any A-3s for sale and see how the filters
were made.

Mouser sells these small Murata 455 kc ceramic filters.  They are
categorized as "signal conditioners" and are in different bandwidths.
the ones to get are 2, 3 and 4 kc which you have to double for the AM
passband, i.e. 4,6 and 8 kc.  Murata gives the audio response for some
reason.  The skirts are not steep so 4 kc is not as restricted as it
seems.   they are in small black packages with 5 pins.  three in a row
are ground; the other two are signal.  there is no in and out, any way
works.   you have to construct a board with pins that seat into the
filter slot.   The mechanical filters can go any way but what you
build has to go in so the filter is in the signal path so study the
schematic and use a VOM to find which socket hole has B+.  That will
come from the plate of the driving stage and the socket hole across
from it will go to the grid of the next stage.  The other two small
holes are common and there are two large diameter holes for ground.
you need a blocking cap between the filter and the line in to it, and
loading resistors across the input to common and output to common.  I
apologize for being so sketchy but I can't remember the component
value details.  There was an ER article on AM with the 75A-3 that I
think explains how to build a LC filter.   Just be careful about which
pin has plate voltage on the filter sockets.

73

Rob
K5UJ

On Thu, Dec 18, 2014 at 1:00 AM, Drew P. via Boatanchors
<boatanchors at mailman.qth.net> wrote:
> Hi Rob,
>
> You wrote:
>
> "I have a homebrew ceramic filter in
> my 75A-3 that is kind of peaky so I retune a lot and worse, if I am
> using a sync. detector it has to re-lock all the time."
>
>
> Sounds interesting, where might I find information on building such a filter?
>
> Best Regards,
>
> Drew
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