[R-390] R-390a question (Tony Casorso)

[SHELLY199 at aol.com]

Hi Tony Casorso and everyone,

Hope this will give you an assist  with your AGC vs. MGC for your Lankford  
modified IF deck.  I  personally prefer the Lankford mod over the stock IF.  
The ssb is so easy  to tune it’s amazing.  The mod does alter the AGC 
characteristics.   With the mod, the AGC voltage for a specified rf input level is 
approx. doubled  that of an unmodified deck.  Also, the AGC delay no longer exists. 
  Here are some #’s I found in my notebook.  This data was taken comparing  
two decks.  One with the mod and one without.   Both decks had  been setup  to 
give -7 volts diode load voltage in MGC with 455 KHz into  P513 and 4 KHz 
bandwidth.  The signal level into the IF deck was varied and  the AGC monitored.
Results are below.  Note the rf and mixer stages are  not in the signal path.

Unmodified  deck 

Input  lel             AGC
145uv             -0.00    this level is threshold of delayed  AGC
150uv             -0.21
300uv             -1.09
600uv             -2.46
900uv           -  3.23
1mv             -3.42

Modified deck

Input  lel             AGC       Diode Load    Diode Load  in  MGC
150uv              -1.77          -3.32             -7.00   
300uv              -2.96          -3,81            -13.86
450uv              -3.70          -4.13            -20.83
600uv              -4.28          -4.29            -27.98
900uv              -5.11          -4.76          -  41.42
1mv            -5.33          -4.85            -45.70

In answer to your question about a reduction of 12 db from MGC to  AGC this 
is normal with this mod when a good antenna is connected.    Checking on two 
separate rcvrs, both with the mod and an antenna connected to  both rcvrs, 
switching from MGC to AGC decreased the line Lel meter by 11  DB.  FYI, In the AGC 
mode the AGC was -1.9 Vdc, the  diode load was  -1.8 Vdc and the carrier meter 
15 Db 

Going a step further I measured the  AGC and MGC at both rcvrs with no 
antenna connected.  That gave a reduction  of only 2 Db switching from MGC to AGC.

The measurements below were taken  with the antenna disconnected and with and 
without the calibrator on and the  rcvr tuned to 100KHz increment.  The 
results are below:

67  EAC                                               Imperial

Noise  only                                      Noise  only                  
AGC      Diode  Load                         AGC      Diode  Load             
       
-0.62        -0.19                                -0.55          -.47

Calibrator   on                                  Calibrator   on              
          
-6.3           -3.0                                   -6.5          -3.1   

If you get similar readings you have the Lankford mod in the IF deck.   Just 
one word of caution-watch the 1N4148 diodes.  Switching the AGC time  constant 
will deteriorate one of the diodes because of a fairly high voltage  
transient during the switch operation.  I once measured it because I got  tired of 
changing diodes but don’t remember the transient voltage value.   To evaluate the 
diodes you have to unsolder one end because of the parallel ckt  resistances. 
 I ended up with a 1N914 and a 1N5060.    If  you notice distortion on a very 
strong broadcast station check the  diodes.

Bye
Rich











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