# [RTTY] FSK Keying on an Icom 7800 Problem

Al Kozakiewicz akozak at hourglass.com
Sat Feb 1 12:36:14 EST 2014

```In order for any circuit to work that uses "parasitic" power, the host has to source a minimum of the switching transistor's  ICsat.  Use a VOM and measure the current when you ground the keying pin of the radio.  If it's not at least ICsat, adding gain (via a Darlington pair) or sourcing additional base current (with a lower base resistor) isn't going to drive the transistor into saturation.  The fact that it works on one radio and not another sounds an awful lot like you're right on the edge.

I would try Jim Hargrave's last suggestion - install a pull up resistor to an external power source. Here are the formulas you'll need to calculate the component values:

Pull up resistance=(Vcc-VCEsat)/ICsat  Vcc-power supply voltage + Use next lowest standard R value
Base resistor=(Vbb - 0.7)/(ICsat/Hfe)   Vbb-driver source voltage + Use next lowest standard R value

Vcc needs to be within the safe spec for the radio.  Keeping the supply equal to the measured open circuit voltage is the safe thing to do if you don't know the specification.

You also need to pick a transistor that has a VCEsat low enough to reliably signal a state change.  Typical silicon switch (e.g. 2N3904) is around 0.2 volts, which is sufficiently below the TTL zero level of <0.7V.

Al
AB2ZY

-----Original Message-----
From: RTTY [mailto:rtty-bounces at contesting.com] On Behalf Of Jim W7RY
Sent: Saturday, February 01, 2014 11:47 AM
To: rtty at contesting.com
Subject: Re: [RTTY] FSK Keying on an Icom 7800 Problem

I use darlington output opto isolators. They have always worked. Keeps things isolated too. The 4N25 works well with a 1.2K or 1.5K LED dropping resistor. LOTS of circuits on the interwebs for FSK and PTT circuits from com ports.

73
Jim W7RY

On 2/1/2014 8:09 AM, Jim Hargrave wrote:
> 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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