[Collins] AM Mech Filter for 75A-3, or LC net?

[Joel R. Hallas]

Guy,

Sounds like a very nice setup! The IF transformers in the 75A3 can do a
very nice job on AM without a mechanical filter at all. They are the same,
I believe, the 75A2, which I used for years, has no mechanical filters and
works great!

What I do on my 51J4, which is missing the 6 KHz AM filter, is just use a
coupling capacitor from input to output; .005 uF or so. On the J-4, as I
remember, it was also necessary to complete the path from grid to ground
provided by the coil in the filter. I just used a 100Kohm resistor or so.
It's possible you may need a resistor on the other end as well. I just
plugged the leads right into the filter socket and it works fine. It
doesn't provide quite the sharp skirts of the mechanical filter, but
better than most AM receivers of its day.

BTW, if anyone has a spare 6KHz filter for a reasonable cost, I'd be
interested.

73 & Good luck,
Joel, W1ZR



"Guy Olinger, K2AV" wrote:

> Looking for an AM mechanical filter for my 75A-3. It is being used
> with a Johnson Ranger & Courier, CW and AM. Not using it for SSB at
> all. At some point I will get something S-line for Collins SSB.
>
> The 75A-3/Ranger is a match for the first Ham rig I ever saw as an 11
> year-old.
>
> I got to operate it on ten meter phone with my school friend's dad, W8
> something, standing behind me. Had to move shortly after followed by
> three years of desperate family circumstances, but was hooked forever,
> and had a novice license at 14 due to the friendly ham shack of K4PNF
> (a real stand-alone shack with nuthin but radio in it).
>
> The 75A-3 currently has a F455B-08 and a F455B-3. The -08 is fine for
> CW. But the -3 is pretty narrow for AM, though just fine for SSB.
>
> Looking for a F455B-6 or a F455B-8 (think I have seen those mentioned)
> for outright purchase or to trade for the -3. I can only use two
> filters, as you know.
>
> Any corrections or tips about this appreciated.
>
> I realize some folks have used an LC filter in place of a mechanical
> filter for this, but I have no information on that, or on the relative
> merits.
>
> 73, Guy.
>
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