Dennis,
I have had good success with a 2N5306 darlington transistor. I use a 3.3k
base resistor to insure full saturation.
Check the voltage on the FSK line on both radios. You could have a
problem with
the one that does not work,
I assume you have tried them on the same computer/adapter. If not,
switch them
around and see if it helps isolate the problem.
Another option is to install a pull-up resistor on the radio side to
insure good
transition between Mark/Space This will allso assist in a slight
increase in
collector current that will reduce the internal resistance.
Jim w5ifp@gvtc.com
On 2/1/2014 7:43 AM, Dennis wrote:
I have two Icom 7800s that I want to use on FSK RTTY. I have tried
several different
keying interfaces on each, i.e. simple transistor, FET transistor,
several different
Opto-isolator chips; on one of the 7800s, they all work, and on the
other 7800,
none of them work. The only thing that all the interfaces have in
common is that
they are all "passive"; none of them require any external power.
For the 7800 that won't key, grounding the FSK lead does get the tone to
change, so they actually circuitry of the 7800 seems to be okay; the
problem
seems to be that none of the keying interfaces pulls the 7800 FSK lead
close
enough to ground to get the FSK keying to work.
Can someone point me to a circuit that brings the Icom FSK lead close
enough to
ground to key the circuit? A no-power circuit is preferred, but I can
also
do a powered circuit if that is the only way to do it. I also haven't
investigated
using a reed relay or similar, as I'm not sure it is fast enough to
keep up with
the keying signal.
Dennis W1UE
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