KAM.5

The  AM/FM  push-button  switch  on  a  KAM  panel  (S2)  changes  not  only 
compression  rate  of the input amplifier system (U1-14,  Q5  and  Q6),  but 
signal  input  sensitivity.  Reduction  of the first is  desired  for  a  CW 
reception to avoid "swamping" of the circuitry (over-compressing);  however, 
the second effect could create a problem in interfacing with a transceiver.

I  use ICOM-751 transceiver and feed the KAM from high impedance output  (10 
kohm)  before  AF gain control and power amplifier (pin 4 of  24-pin  plug). 
Since the impedance of KAM audio input is only 600 ohms,  the signal is much 
attenuated.  To  develop  the required 20 mVpp on the input pin  for  non-CW 
reception  (the switch in FM position),  the transceiver has to output  0.35 
Vpp (20*10600/600).  But, if the switch is in AM position, the de-sensitized 
input (about 100 mVpp) requires at lest 1.8 Vpp.  This value, which is close 
to distortion limit of the transceiver,  I could obtain with strong  signals 
and with disconnected AGC (in OFF) only. 

I have slightly modified the KAM circuitry by disconnecting the AM/FM switch 
from  ground and connecting this point to the R1-R13  junction.  The  switch 
changes  rate  of  compression only.  The sensitivity of  AF  input  remains 
constant at 20 mVpp. 

Now, I could:

-    copy  very weak CW signals which hardly move the S-meter,

-    use large dynamic range of the transceiver automatic gain control and

-    enjoy easiness and speed of tuning to a CW station.


This  is how I understand the circuit.  Sorry for the long text.  You  could 
skip it.

The original circuit:

If signal level at U1-14 exceeds 0.6 V in peak,  the transistor (Q5)  starts 
feeding  the FET transistor (Q6).  The FET by-passed by R27 and with  series 
R13 acts as a variable attenuation pad (voltage divider). Timing of this AGC 
(or  compression)  is determined by R19-C10 (2.2 ms attack) and R21-C1 (1  s 
release), and this is independent of AM/FM switch position. 

Since  an  op-amp  (U1-14) has amplification of 100,  peak  voltage  at  its 
positive  input pin is not much higher than 6 mV (12 mVpp).  Diodes CR4  and 
CR5  are  inoperative in this low range of voltages.  They protect  the  KAM 
circuitry against large voltage,  which could be accidentally applied to the 
input. 

Only  the  FET (Q6) fulfils a function of the gain control.  When the  AM/FM 
switch is off, the attenuating pad (R13-Q6) works in a range 1 to 1000 (0 dB 
to 60 dB according to the specification). When the switch is on, the initial 
attenuatiom is 7.7 and the controlling feedback is much reduced.



Modified circuit:

When  R27  by-passes R13 (instead of Q6 as in  original  cct),  the  initial 
attenuation remains the same (=1),  but attenuation ability in reduced.  The 
circuitry  treats  the  signal with more "respect".  The compressor  is  not 
"over-doing" its function.

Actual modification:

1.   Cut  the PCB track which grounds the AM/FM switch as close as  possible 
     to the switch,

2.   Connect this cut-off switch pin to junction R1-R13 with thin  wire-wrap 
     wire. 

The modification works well not only for me; Art, VE3RX modified his KAM and 
is satisfied with its results. 

I will be glad hearing your opinion:

     VE3MRM @ VE3NUU (BBS)     on 145.03 MHz or 
     VE3MRM-1        (my PBBS) on 145.59 MHz both in Toronto.

Enjoy and 73