75S-1 CW Filter Suggestions

[Glen Zook]

The optional CW filter for the 75S-1 and 75S-2 was a
mechanical 500 Hz.  There was an optional BFO crystal
which was used with this filter.

For some reason, Art Collins would not allow a 75S-1
or 75S-2 to be shipped from the factory with a 500 Hz
filter installed.  You had to purchase the receiver
and then either buy the CW filter kit and install it
yourself or else you could return the receiver to the
factory to have it installed.  But, Art would not
allow any of those receivers to be shipped with the
filter originally installed.

I have a 75S-1 that actually cost a Dallas Collins
Radio employee his job.  He got the radio through the
company employee purchase program and since he "knew"
people at the Cedar Rapids plant he got them to
install the CW filter and optional BFO at the plant
before it was shipped to him in Dallas (this was
before the Richardson plant).  Art found out about the
filter being installed when the unit was built and
called the employee "on the carpet".  The employee
decided that all the "hastle" was not worth it so he
quite his job.

After a short period of time the former employee
traded off the receiver to another amateur radio
operator in one of the Dallas suburbs.  That person
already had an S-Line and soon stored the receiver in
his garage (basically brand new).  The receiver sat in
the garage until about 4 years ago when it was
"discovered" and traded to another amateur radio
operator for some closed circuit television equipment.
 The "new" owner then traded the receiver to me, along
with some other equipment and some "cash", for a
commercial 450 MHz FM repeater that I had retired from
commercial service.

I was checking out the receiver on the test bench when
I discovered the CW filter.  The filter had obviously
been installed at the same time as the receiver was
built because not only was the wiring "factory", but
the glyptol used on the connections had been applied
in exactly the same color and the technique was
identical to all other inspection marks in the
receiver.  It was then that I "found out" the history
of the receiver.  It had not been turned on for over
45 years (since about 1958) when I got the receiver in
late 2003.  The only thing that was wrong with the
receiver when I obtained it was that the insulation on
the power cord had cracked due to it being stored in
an un-airconditioned garage with the hot summers of
the Dallas area.

Now Art Collins changed his position and EVERY 75S-3
and 75S-3A receiver was shipped with the 200 Hz CW
filter installed.  Then the 200 Hz filter was made a
high-priced option in the 75S-3B and 75S-3C receiver. 
Any 75S-3 or 75S-3A receiver that does not have the CW
filter has had the filter removed, probably to sell at
a pretty good price.

Because the 75S-1 and 75S-2 do not have the variable
BFO (as well as the crystal controlled BFO) as in the
75S-3- series, you pretty much do need an optional BFO
crystal.  Collins used a 455.8 KHz crystal to produce
an 800 Hz CW tone.  The 500 Hz mechanical filter is
centered on 455 KHz.

Go to

http://www.collinsradio.org/archives/manuals/75S-1cwfilter.pdf

for the PDF of the instruction sheets on installing
the CW filter and optional BFO crystal in the 75S-1
and 75S-2.

Glen, K9STH


--- Steve <stever2k at MCHSI.COM> wrote:

Does anyone know what CW filters will work in a
Collins 75S-1?  Will any Mechanical Filter designed
for 455 KHZ work  as long as it can be mounted? Is the
center freq of the filter exactly 455 Khz?  I picked
up a very nice 75S-1 and it has no CW filter in and I
miss not having one and thought I might try and find
something suitable that will work..
 
Does the filter have to be a Mechanical filter in
order to have the right input and output impedances or
could I possibly use a crystal filter with the right
matching network?









Glen, K9STH

Website:  http://k9sth.com


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