[RTTY] FSK Keying on an Icom 7800 Problem

[Al Kozakiewicz]

Bipolar transistors are current operated devices, not voltage,  You want enough current through the base-emitter junction to drive the transistor into saturation.  At 45.45 baud, the higher recovery time from driving a transistor too far into saturation is really immaterial.  The current needed is calculated by ICsat/Hfe (gain). Ohms law does the rest.

Al
AB2ZY


-----Original Message-----
From: RTTY [mailto:rtty-bounces at contesting.com] On Behalf Of Vladimir Sidarau
Sent: Saturday, February 01, 2014 5:48 PM
To: rtty at contesting.com
Subject: Re: [RTTY] FSK Keying on an Icom 7800 Problem

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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