[Collins] 75S3 Questions

[Glen Zook]

The 1N4007 diodes are fine.  You might find some of
the original types but if you don't find them
everything will be OK with the 1N4007 units.

As for the "S" meter:  You need to see if "S" meter
reads S-9 when a 50 microvolt signal is applied to the
input.  Now this is with a signal generator with a 50
ohm matched output.  If the output of the signal
generator is not 50 ohms then you need to use the
"scheme" used by Collins which employs resitors that
match the signal generator output to 50 ohms.  The
voltage applied to the resistor network is 100
microvolts which will result in an actual 50
microvolts being applied to the receiver.

It will take a signal level of 500 microvolts for a
"20 over" reading, 5,000 microvolts for a "40 over"
reading, and 50,000 microvolts for a "60 over"
reading.  Now on the "S" meter in the 75S-3 S-9 is the
"40" reading on the dB scale, "20 over" is the "60"
reading on the dB scale, "40 over" is the "80" reading
on the dB scale, and "60 over" is the "100" reading on
the dB scale.

Now the "S" meter in the 75S-3 will not read anywhere
near as high as many "modern" receivers. 
Unfortunately, the manufacutrers of most receivers
these days started "inflating" the "S" meter readings
in an attempt to say that their receiver was "more
sensitive" than the next manufacturer's receiver. 
Then the other manufacturers followed suit until the
actual signal strength required for an S-9 reading is
like between 5 and 10 microvolts on many receivers
these days.  The "dB over" readings are even more
insane.  I have seen a number of "modern" units that
read "40 over" with under 100 microvolts and "60 over"
with well under 200 microvolts.

As I pointed out above, when the "S" meter is
calibrated for S-9 = 50 microvolts it takes 500
microvolts for a "20 over" reading, 5,000 microvolts
for a "40 over" reading, and a whopping 50,000
microvolts for a "60 over" reading.  Frankly, you are
not going to get a "60 over" reading unless you live
next door to someone running full legal power and your
yagi is pointed right at their yagi.

Unfortunately many amateur radio operators these days
are used to getting the very inflated "S" meter
readings and they are "insulted" if you don't give
then at least a "20 over".  All of my "S" meters are
calibrated for S-9 = 50 microvolts and they are pretty
accurate in the "dB over" category and I seldom see
signals that are much over S-9 unless the station is
local.  A few signals may reach "20 over" but the
majority are in the S-7 to S-9 range and not a few are
well under S-7.

It is the signal to noise ratio of the signal that
determines how well the other station can be copied
and not the "S" meter reading.  If the noise level is
reading S-9 and the incomming signal is also reading
S-9 then it is going to be very difficult to
understand the signal.  Frankly, especially on the
higher bands (i.e. 10 meters) sometimes the other
station doesn't even move my "S" meter but they are
very easy to copy because of the low noise level.

Because of the "insult" factor I do not like to give
"S" meter readings.  If "pushed" I will usually say
that the other station is S-9 if I don't have any
problems copying them.  In fact, years ago when many
receivers did not have an "S" meter that is how we
gave "S" reports.

Now I do have a better than average antenna system and
I can copy stations a lot better than a good number of
other stations in this area (see
http://k9sth.com/uploads/newantenna.JPG for some
photos of my antennas).  But, since my "S" meters are
calibrated for the "olde tyme" standard of S-9 = 50
microvolts and are accurate in the "dB over" category,
I don't get the very high "S" meter readings that are
so common these days.

The audio output tube does run VERY hot in the S-Line
receivers.  Definitely do not even think of touching
it unless the receiver has been "off" for a while.

The illumination of the "S" meter in all 3 of my
S-Line receivers is sufficient to see in a dark room
but not that bright.

The power transformer gets warm, but not that bad. 
Since you had a blown diode I suspect that the
electrolytic capacitors are leaky.  I definitely
recommend replacing all of them in the receiver.  The
cost will be well under $20 and to keep from burning
out the transformer they need to be replaced very
soon.

Glen, K9STH


--- Bill Stewart <bstewart at ipass.net> wrote:

> I recently received a 75S3 (s/n 12xxx) which was
> blowing fuses. I found one of the B+ diodes shorted
> and replaced both with 1N4007 diodes. This rcvr used
> 1N1084 S/T B+ diodes which fit into a open type fuse
> holder. The schematic which I have, apparently not
> for this rcvr, calls for 1N1490 diodes. Also noted
> is that this rcvr uses 6U8A's instead of the 6EA8's
> as noted on the schematic I have. Now it's working
> fine, but I have a few questions.
> 
> Question 1: Is there anything special about the B+
> diodes. The 1N4007's are working ok and the current
> rating is well
>                   above rcvr requirements. Can the
> older 1N1084 diodes still be found should I want to
> return it to orig. config.
> Question 2: I am hearing plenty of signals, but the
> S-meter is very stingy...with only a few sigs going
> over S-9. Is this 
>                   normal S-meter action or could the
> RF need some tweaking. 
> Question 3: Another S-meter question- Should there
> be enough light from the meter lamp to read the
> meter in the dark. 
>                  Mine has very little light coming
> around the meter face and is about useless as far as
> reading it in the dark.
>                  Anything special about the meter
> lamp make up. 
> Question 4: The 6BF5 audio tube runs real
> hot...makes spit sizzle. Is this normal? I'm using a
> Radio Shack 'scanner'  
>                   speaker and am guessing its 8
> ohms.
> Question 5: Last one for now...the power transformer
> runs pretty warm...it gets uncomfortable after about
> 15-20
>                   seconds...is this a normal temp.? 
> 
> I haven't received a manual yet so I'm operating in
> the dark a bit. Would this rcvr be toward the end of
> the plain S3 series being it has a high ser. no. and
> diode and tube changes. Thanks for any help...









Glen, K9STH

Website:  http://k9sth.com


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