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World of Ham Radio 1997
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WOHR97_AmSoft_(1997-02-01).iso
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mods
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kam
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kam_6.doc
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1997-02-01
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KAM.6
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