[RTTY] FSK Keying on an Icom 7800 Problem

[Dennis]

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