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

[Tony Casorso]

Hi Rich,

Fantastic! Thanks so much for this information. Looking at the circuit I can 
see that the Lankford mod would affect the delay but it looked like there 
would still be some offset. I wasn't sure though. Your data tells the story. 
This matches up with my receiver to a Tee.

I used 4148 diodes and promptly blew up the one across R547. I replaced it 
with a higher voltage type and it is OK so far. Yes it is a great mod for 
listening to SSB.

Thanks again.

Tony

----- Original Message ----- 
From: <SHELLY199 at aol.com>
To: <r-390 at mailman.qth.net>
Sent: Wednesday, February 13, 2008 2:06 PM
Subject: [R-390] R-390a question (Tony Casorso)



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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