[R-390] Another 390A Carrier meter question

[Roy Morgan]

At 07:28 PM 11/6/2005, DJED1 at aol.com wrote:
>
>I think your friend has an incorrect version of Chuck Rippel's procedure  for
>setting the IF gain.

Ed and all,

I did not get the impression that "the friend" claimed it was "Chuck's 
Method":  here is what the friend apparently suggested:

"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 ..."

Perhaps he really meant "set the RF GAIN full on with the antenna 
terminated by a resistor."

Just for the record, here is the procedure from Chuck's site, with the 
comments in parentheses added by me..

http://www.r390a.com/html/gain.html

"Procedure to set R390A IF Gain:

Once the receiver has been fully mechanically and electrically aligned, the 
final procedure to perform before "buttoning it up" is to set the IF gain 
control. Many otherwise very sensitive R390A's are thought not to be due to 
weak signals being covered by noise generated by excess IF deck gain.

  Allow the receiver to warm up for at least 1 hour then:

  1- Terminate the antenna input (with 50 0hms to ground with one terminal 
of the balanced input grounded, or with 125 ohms across the balanced input.)
  2- Set receiver for 15.2 mHz
  3- Set the "FUNCTION" control to MGC
  4- Select the 4kc filter with the "BANDWIDTH"
  5- Set "RF GAIN" control to 10 or maximum
  6- Peak the "ANTENNA TRIM" for maximum noise as indicated on the "LINE 
LEVEL" meter (Adjust the LINE GAIN control upwards to get a reading.)
  7- Set "Line Meter" switch to -10db scale
  8- Set "Line Gain" control to full CW or "10."
  9- Adjust IF gain control, R-519 to cause "Line Level" meter to indicate 
between -4 to -7 VU.
10- Re-zero the carrier meter control, R-523
11- Set controls above for normal operation and reconnect antenna.

Discussion:

This will yield the best compromise on all bands. I usually "poll" those 
bands which I normally spec out. Then, using an HP signal generator set for 
internal modulation of 800 hz @ 30%, "massage" the gain setting and even 
specific signal path tube selections for the best overall performance."

What this does is give you a modest amount of noise with the LINE GAIN and 
RF GAIN at full.  Any signal at or above that level you probably will hear. 
But no stage in the radio is working harder than it needs to.  The IF strip 
is contributing very little noise to the receiver output.  The majority of 
the noise you hear is from the first RF stage, where it should come 
from.  Adjusting the ANT TRIM for the noise peak assures that this is 
true.  If you get no peak with the ANT TRIM control, you have other things 
to fix first.

>  I believe what it does is to set the gain lower  in
>order to improve the sensitivity by reducing the IF noise contribution.

What it does is set the gain of the IF strip to be in balance with the gain 
of the other stages, and makes sure that the overall gain of the receiver, 
at full gain, is producing a modest amount of noise, mostly from the front end.

Roy

- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
Home: 301-330-8828 Cell 301-928-7794
Work: Voice: 301-975-3254,  Fax: 301-948-6213
roy.morgan at nist.gov --