[Collins] Finally

[Glen Zook]

The modification moves one of the two 455 KHz
transformers from the AM filter to the mixer.  This
was done in the later 75S-3B and 75S-3C receivers from
the factory.  All of the 75S-3, 75S-3A, and early
75S-3B and 75S-3C receivers were not built this way.

I have done the modification in my 75S-3A and one of
my 75S-1 receivers.  It definitely helps.

What I did was to compare the schematic of a later
75S-3B/C receiver with the ones from the 75S-1/2 and
75S-3/3A and rewire accordingly.

It does "broaden out" the AM response slightly if you
are using the internal AM filter (I use a wider
mechanical filter for AM in my 75S-3A).  But, that is
actually a "plus" if you use your 75S- series for AM.

My particular 75S-3A belonged to Collins' field
service technician who was the person who went around
to the various military bases "fixing" problems with
the Collins equipment that the military technicians
could not handle.  Every one of the changes that were
made to the first generation 75S-3B/3C receivers was
tried in this receiver.  Every one of them except one,
the addition of a zener to regulate the voltage on one
of the oscillators was removed since the changes
downgraded the performance of the receiver.  The zener
really doesn't do that much, but it definitely does
not "hurt" to regulate an oscillator's voltage.

The moving of the i.f. can was done in later
production so this was not "tried" when the receiver
belonged to the Collins technician.  I found this
difference when I was looking at the schematic of the
later receivers and decided to try it in my 75S-3A. 
It definitely helps.

Glen, K9STH


--- Fabio Liberatori <liber.fab at iol.it> wrote:

There is only one VERY useful mod, in my opinion :
using one of its internal transformers, it improve
receive performances, and it works GREAT on my unit !

Glen, K9STH

Web sites

http://home.comcast.net/~k9sth
http://home.comcast.net/~zcomco


		
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