[R-390] Distortion

[Roy Morgan]

At 10:14 PM 4/4/03 -0500, K2LNU wrote:
>Hello R-390A Fans,
>Can anyone tell me a probable cause for an R-390A going into distortion when
>the RF gain is turned up above 5?

1) Remove all tubes substituted for the originally specified ones in 
attempt to make the receiver "hotter". Replace with the correct 
tube(s).  Do not delude yourself in thinking that a higher gain tube is 
going to make the radio work better.

2) Make sure your IF gain adjust pot is not set too high.
See chuck Rippel's site for the instructions on how to set it.

<http://www.R390A.com/>
specifically:
<http://www.R390A.com/html/gain.html>
Which I quote here to make it REALLY easy for any one wanting to do this 
(parentheses are mine):

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 (put 120 ohms 
across the balanced terminals, or ground one and put 50 ohms from the other 
to ground)
                         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 (If you get no peak you have other 
problems, most likely alignment.)
                         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

                        (12- Please report to the list what success you had 
with this.)

(What this does is set the IF gain so that with the audio and rf gain full 
up, you get a modest but discernable noise from the front end, the first rf 
amplifier.  The overall gain of the radio is enough so you can hear any, 
repeat any signal that is just below, at or just above the noise of the 
radio itself.  And, the IF and AVC system can manage the gain of the 
various stages of the radio to keep distortion to a minimum.  Each stage of 
the radio is pulling it's fair share with out being run at too much 
gain.  No stage overloads more than necessary or at a signal level lower 
than is optimum.)

Roy




- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
Home: 301-330-8828 Work: Voice: 301-975-3254,  Fax: 301-948-6213
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