[Collins] Fw: Collins filters in Sline.

[David Knepper]

Everyone should read this email from Dr. Jerry from an inquiry from Evan.

Dave, W3ST
Publisher of the Collins Journal
Secretary to the Collins Radio Association
www.collinsra.com
Nets:  3805 Khz, Monday/Wednesdays 8 PM EDST
          14250 Khz Saturday, 12 Noon EDST

----- Original Message -----
From: "Dr. Gerald Johnson" <geraldj at ispwest.com>
To: <K9SQG at aol.com>
Cc: <cra at floodcity.net>
Sent: Thursday, October 07, 2004 12:25 AM
Subject: Re: Collins filters in Sline.


> The 32S-1 and 75S sometime before 3B used a round filter, Y case I think
> cylindrical, like a fat round pencil with solder terminals at each end. A
metal
> case. The filters were mounted under the chassis in both transmitter and
> receiver passing through a hole in a shield in the receiver for better way
out of
> passband rejection. In the receiver there were provisions for only two
filters
> and I don't recall that the CW filter was standard, I think it was
separate. The
> kit to install it was not hard to buy, because I remember buying the kit
for my
> dad's 75S2. It was the 500 Hz mechanical filter (round case but more like
3/4 or
> 7/8" diameter) and the required BFO crystal.
>
> By the time of the 75S3B, filter options were greater and the new FA
series of
> plastic cased filters were use in both transmitter and receiver. These
filters had
> three pins at each end arranged to fit a TO-5 transistor socket. Three
pins on a
> .200 " diameter pin circle, 90 degrees apart. That put the filters on top
the
> receiver chassis, but I see from my 32S3 manual that the transmitter
filter was
> still mounted under the chassis and the circuit wires soldered to the
pins.
>
> There were several filters available for the S-3B, including 1.2 KHz, 2.1
(stock),
> 3.1, and 6 (for AM). Then for CW, narrow filters of 200 and 300 Hz using
crystals
> and 500 Hz mechanical filters were available. With three filter positions
(one
> available for AM if desired in place of a couple IF transformers), a user
had
> several options. Those running wide shift (850 Hz shift) RTTY could use
the 1 KHz
> filter and the tunable BFO in the CW position while the CW op could use
the
> 200, 300, or 500 Hz filter.
>
> The plastic cased filter above the chassis and not a long ways from the
power
> transformer injected hum modulated noise into the IF. I chased that down
on
> my own and covered the filter with a little tent of mu-metal. Eventually
the
> factory made a mumetal box that fit over the optional mechanical filters
but
> won't fit over the crystal filters. My mu metal tent works with either.
There are
> also some FA filters in metal cases.
>
> All production used the same BFO frequencies planned to be 20 dB down the
> filter response curves so the filters should have had the same 2.1 KHz
response
> curves whether Y case or FA case.
>
> If the BFO is not at that 20 dB down point the radio can sound odd. These
LF
> crystals are known to sometimes drift with age and the cure has generally
> been to replace them. Replacement crystals are available. There are no
> provisions for adjusting either receiver or transmitter BFO crystal
frequencies.
> The audio passband should be 3 dB down at 300 and 2400 Hz. Those should be
> the audio corners, transmit or receive.
>
> When transmitter and receiver BFO crystals don't match frequency, its
certain
> that the pair won't transceive on the same frequency.
>
> The 32S3 and 32S3A are identical except for the second crystal board for
the
> HF mixing crystals. That crystal board was available as a kit for field
expansion
> of coverage.
>
> 73, Jerry, K0CQ, Technical Advisor to the CRA
>
> --
> Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
> Reproduction by permission only.
>
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