Re: {Collins} 75A-4 Filter Pin Outs



Richard,

According to the 3rd edition of the 75A-4 manual revised 15 March 1957, the connections are pins 1 and 2 for the input, and 6 and 7 for the output. All the unused pins of the 9-pin miniature socket are grounded. Make sure there is no DC voltage on the pins the filter uses. A 1000 pF capacitor is used in the 75A-4 to decouple the 2nd mixer plate voltage. This was a change from the original design to prevent filter damage. I don’t know whether the Collins mechanical filters are isotropic or not, but if they are, that would mean that if the input and output connections are reversed the filter would still work. Perhaps someone more knowledgable than I am could comment?

73,
Bob AD3K

> On Oct 31, 2017, at 2:56 PM, Richard Dillman via Collins <collins@xxxxxxxxxxxxx> wrote:
> 
> 
> We have a receiver that we think has been modified with a socket intended to accept a 75A-4 mechanical filter (the filter is missing).  Can someone supply the pin-out connections for the 'A-4 filters?  With that information we can compare the wiring of the socket with the pins on the filter to see if our theory holds water.
> 
> Thanks,
> 
> RD
> 
> =================================
> Richard Dillman
> Maritime Radio Historical Society
> http://www.radiomarine.org
> =================================
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