[R-390] Another 390A Carrier meter question

[Bill Feldmann]

Hi All

On my R-390 non-A receiver I've been having a good time working on and 
testing it but was a little confused as to the proper setting of the IF gain 
pot which really controls the accuracy and span of the carrier meter.  The 
carrier meter pot only determines its zero setting.

I tried first setting the gain using my HP signal generator input to be 
100uv, normally a S9 signal on most older Collins receivers, on 75 meters. 
I adjusted the IF gain pot for a reading of 60db at 100uv input after 
zeroing the meter with no antenna input.  Then it was easy to give signal 
reports based on S units as 60db S9 and still had 40db above S9 on the meter 
for over S9 reports.  I then had run some 3rd order intermod and noise floor 
tests on my receiver.

Later a friend more familiar with R-390's told me the best method of setting 
the pot was using the line audio meter.  First set the pot for full IF gain 
with no antenna input and then setting the line audio meter for a db reading 
on the receiver's internal noise using the line audio control.  Also the 
selectivity should be at 16kc during this procedure.  Then to adjust the IF 
gain to drop the line audio meter reading  6db by adjusting the IF gain pot 
for the proper adjustment.

I tried this method and then used my signal generator to determine the 
carrier meter reading for 100uv antenna input.  It came out around 45db 
which makes some sense because 0db would be around a 1uv antenna input. 
After this adjustment of the IF gain pot I repeated my noise floor and 
intermod tests.  I noticed a very slight but hard to detect lowering of the 
noise floor by only 2db to 3db but the two tone 3rd intermod floor didn't 
change from the previous test.  So this method is slightly better for 
receiver noise floor performance.  But I went back and set the IF gain for 
40db with 100uv for easy S unit conversion since I'm a lazy guy.

But as Tim mentioned the R-390 to any old antenna is not a usable field 
strength measuring method since the antenna is not calibrated.  But the 
meter does seem to be accurate in db by my testing it when changing my 
signal generator input, so the antenna/receiver system should be good for 
comparisons.  I also found on my R-390 the sensitivity held fairly close 
after receiver alignment for the bands above 3mc.  Collins sure did a nice 
design job on the R-390.

Bill N6PY

----- Original Message ----- 
From: "Tim Shoppa" <shoppa_r390a at trailing-edge.com>
To: <r-390 at mailman.qth.net>
Sent: Sunday, November 06, 2005 3:39 AM
Subject: Re: [R-390] Another 390A Carrier meter question


>> It has plenty of utility even if not calibratable.
>
> Very true.  I have worked with radio equipment that was calibrated
> for measuring RF fields in terms of uV/meter, and it was an extremely
> convoluted and complex process that nobody would've ever paid for
> unless it was required by FCC certs :-).
>
> I think I exaggerated there a bit, I think that calibrated receivers
> are used for direction finding etc., but it's worth pointing out that
> the ANTENNA has to be calibrated too!  Not much point if just hooked
> to a random longwire...
>
> Tim.
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