[Collins] Suggestion of best 75A receiver for AM reception

[Dr. Gerald N. Johnson]

In the 51J-4 both first IF tube plate current and AGC bias to the next 
IF stage go through the filter coils as originaly wired. Some have taken 
the IF or mixer tube plate voltage and plate current out of the 
mechanical filter for better filter life. There it would be best to use 
an RF choke on the input and s 47K to 270K resistor on the output side. 
Or an IF transformer in place of the filter. The 75S receivers used a 
10mh plate choke and 100K grid resistor keeping DC off the mechanical 
filter. The 75A-3 is wired like the original 51J-4 with DC on the 
filter, so choke and resistor would be needed or a two winding IF 
transformer to replace the mechanical filter. The standard AM circuit 
for the 75S-3B used a couple IF transformers. There is a mod about to 
wire those transformer to be in the circuit for all modes improving the 
IF rejection more than just the mechanical filter.

73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association

On 7/20/2015 12:59 PM, Joel Hallas wrote:
> FWIW, before I found a 6 kHz mechanical filter for my 51J4, I just used a
> coupling capacitor to bypass it (perhaps with a resistor on one or both
> sides as needed, I forget) and plugged that into the socket. The AM
> broadcast reception was quite nice. I eventually found a 6 kHz filter from a
> different 500 kHz IF radio and adapted it -- much better on 75 AM, but not
> as good for broadcast.
>
> So, if you have an A3, A4 or J4, all is not lost, use a 0.01 uF capacitor in
> place of the 6 kHz mechanical filter and you will get the hi-fi(ish)
> reception you seek, if not the QRM reduction.
>
> Regards, Joel Hallas, W1ZR
> Westport, CT
>
>
> -----Original Message-----
> From: Collins [mailto:collins-bounces at mailman.qth.net] On Behalf Of
> billriches
> Sent: Monday, July 20, 2015 12:31 PM
> To: geraldj at netins.net; collins at mailman.qth.net
> Subject: Re: [Collins] Suggestion of best 75A receiver for AM reception
>
> I am fortunate to have an R-725 - the Servo modified R-390A and a 390A in
> the same rack and do notice the difference in noisy reception.  725 is
> quiet!
>
> (The R-725 was a modified R-390a with an R-390 IF deck. The mech filters
> rang and sang and were unusable for diversity direction finding work.  I
> believe only about 1000 of the r-725s were built.)
>
> 73,
>
> Bill, WA2DVU
> Cape May
>
> -----Original Message-----
> From: Collins [mailto:collins-bounces at mailman.qth.net] On Behalf Of Dr.
> Gerald N. Johnson
> Sent: Monday, July 20, 2015 11:02 AM
> To: collins at mailman.qth.net
> Subject: Re: [Collins] Suggestion of best 75A receiver for AM reception
>
> I suspect the mechanical filters of that era, like those in the S-line rang
> like a bell when hit by lightning static and power line noise. I know the
> standard filter in my new 75S-3B did that in 1964.
>
>
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