[RTTY] FSK Keying on an Icom 7800 Problem

[Vladimir Sidarau]

Dennis,

1. Make sure, the transistor is not fully closed but appears fully open,
instead, at start of your PC. In most of cases like this the transistor
opens up widely and keys down your FSK line permanently but never returns
back to "zero" stage. Such a phenomenon is clearly seen if you make the
interface for CW. Then you have the permanent KEY DOWN status. As for the
FSK keying, you may not notice the HIGH status but rather see lack of change
of statuses.

2. A simple single transistor interface will do perfectly, as long as you
establish a proper control voltage level at the base. You may start with 1k
resistor in series and then change it to 1.5k, then 2k  and so on. But
regular low current diodes are even better. Just have in mind, 1 diode
reduces the input voltage by approx. 0.5 V. Adding more diodes in series
would allow you to establish a precise transistor's opening level which
should be not too high and too low. 
As an example, one of my similar interfaces like this consists of a simple
transistor with 1.8k resistor and 3 low current diodes in series with the
base.
        
Hope it helps.

73,

Vlad VE3IAE

--




> 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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