Titre: pacmod
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From: EB5IGU@EB5IGU.EAV.ESP.EU
To  : MODS@EU

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KPC2 and radio modification: the RCV led

I put on the air a packet station consisting of a Kantronics KPC2 and a
modified Motorola HT220 radio with a 6W power amp.  I found that the
reciever squelch is a little slow to unsquelch, so I set the TNC to
detect via its software (SWDETENA command) signals from the unsquelched
radio.  This works fine, except that the RCV led on the TNC is
constantly flickering.  This is annoying, so I made this mod:  I
identified the squelch switch transister in the radio.  This transister
shorts to ground a bias circuit in the audio amp circuit, thus making
the radio quiet.  (this bias circuit also has the audio signal riding
on it, but the bias change does the real squelching)  I cut a trace on
the pcboard, isolating this switch transister, then ran a wire from
this transister collector to a new buffer circuit in the TNC.  This
buffer is just a pair of transisters connected in cascade to act as a
non-inverting buffer, the collector of the last transister is connected
to the non-ground side of the RCV led and the led side of the led's
resister.   When the radio's squelch switch transister is turned on by
the lack of a recieved signal, the TNC's RCV led is kept off by the
buffer shorting the led to ground.
                                          _________
                                          |   |+5v|
           antenna--|                     r   |   r   resister--led driver ic
    |--------------------------|          |   r   |----|
    | radio    Af--af amp--------->aud in |  _|--K   RCV LED
    |            L_x_________________-----+-K_____|____|___gnd 
    | squelch det---K  sq sw   |               TNC 
    |                |         |<-----mic in-----------
    ________________gnd________|<-----ppt--------------

    -K is a transister   r is a resister   x is the cut trace

Of course, the same thing could be done by rewriting the TNC's microprocessor
code so the LED would light only on recieving a packet signal.  Not being a
software type, I found this hardware mod easier.
Now, the TNC recieves complete packets, and the RCV led really indicates a
packet comming in, and not just reciever hiss noise.  It probably took longer
to type this (vi has got to be the worst word processor program ever written
by man!) than it took to design this mod, and only a little less time to
actually build the mod!  Yea, it's trivial, but it got rid of an annoyance.

73 de WA2ISE