REVISED JANYARY 13, 2004

The two hand drawn schematics relate to my Kachina 505DSP radio, but apply to 
many radios.

1) mic_mod_schematic.jpg

I use an electret "Computer" headphone / (electret) microphone for my regular 
QSO mic. 

I have used a different arrangement in the past, but recently changed to the
one shown.  There are two features to this modification.

1. It uses the (cw) KEY jack.  While this eliminates the straight key jack
this is not a problem for me, as I do not use a straight Key.  If you want
to still use one occasionally you should note that the "DAH" paddle contact
is the same as the KEY in.

2 K1ND pointed out to me that there is a regulated 5 volt source on the
PC board in the control head.   According to the schematic this is
used only by the microphone preamplifier,  what better source for power
for the electret microphone!

I cut the foil for the key jack, to separated it from the dah,
but leave L803 and C809 to maintain rf protection on the input.

I made the new connections by soldering the component leads to the edges of
the surface mount L and C.  In a similar fashion I soldered a wire to pin
1 of U801 the 5 volt regulator.

I found that a value of 4.7k provided adequate bias for a standard computer
microphone.  If you decrease this value it will provide more voltage
to the microhone, but also lower the audio frequency resistance to ground.

The 3.3u cap is picked to be at least 10 times the value of the series input 
capacitor C803 (.1 uf) inside the Kachina. I used the 3.3 because it was
on hand, a 1.0 should be adequate.  While the electret Microphone
requires DC+ on the audio line.  The Kachina has microscopic 5k pot
on the audio line that could be damaged by DC+.  This capacitor blocks
DC from that POT.  One of the Techs at Kachina told me I needed to block
the DC this way. 

NOTE
I previously was doing something very similar using a resistive voltage
divider on the 12 volt supply.  I did not realize that the lack of
regulation on that line was causing problems in my radio, until after
I switched to using the 5 volt regulated supply, and some annoying
problems went away.


2) sound_card_interface.jpg

I have used this circuit on My Kachina for about a year.  It works for any sound 
card function, including: voice recorder, SSTV, and PSK31. 

The Kachina has a balanced input on the accessory plug that requires a fairly high 
level of drive. I experimented with several types of connections and found that 
using a 8 ohm to 1,000 ohm miniature audio transformer and the Speaker output 
on the computer provided the best results.  If you prefer a heftier
transformer look for for a "70 volt" audio transformer at Radio Shack
and hook the speaker side to the computer, and experiment to find the correct
tap value for the radio side.

[The mute circuit was found to be unnecesary and is no longer in use at
KA9CAR but it worked fine. (1/2004) ]

Notice the "mute" circuit.  I included this to allow easy cut off of PC audio from 
the radio.  Since I don't have a linear amplifier, but I did have a relay in the junk 
box I used the relay, driven by the Radios Amp relay driver output, to allow me to
control the mute function from the radio menu.  On the Kachina TX menu, when 
the amplifier is on PC audio passes to the Radio.  When the amplifier is off, the
balanced inputs are tied together for silencing.  The reverse bias diode protects the
radio from the magneto effect of opening the relay coil circuit.  I also included an
LED to show when the relay coil was energized.

PTT is a simple transistor and resistor INSIDE the db9 plug.  The resistor is to
limit the base to emitter current.  I did not include a reverse bias  diode, as the 
Kachina does not expose this input to a relay coil.  If your radio uses a relay,
 or if you are unsure, include a reverse bias diode like the one I show on the
control relay.

For audio to the PC I found a simple patch cable was all that was required.
WIRE ONLY the tip and sleeve of the stereo plug.  The Kachina has a very
high output level, and it works fine with my sound cards.

On the schematic, RS means Radio Shack part number.

John
KA9CAR