[RTTY] FSK Keying on an Icom 7800 Problem

[Jim Hargrave]

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 at 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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