[NLRS] (now) re: keying multiple radios

[tosca005]

On 23 Jan 2007, Glen Overby wrote:

> I use a 4-port TV/Audio switch from Target ($15-$20).  Audio goes
> on L and R and the key on video.  I've been tempted to connect one
> headphone channel to the IF rig, since thats mainly where I use it,
> and switch the other.

Thanks for the tip, I will definitely look into it. But my problem was not
with the switching, it was with getting the keyer to key the radios
correctly. I didn't want to invest a lot of effort into building a
switching circuit when the thing being switched wouldn't work in the first
place when plugged in directly! Yes, I could have switched the paddles
themselves between radios with a simple 2-pole 3-position rotary switch,
but I ultimately want to switch more than just the key, so I'd be building
something twice. So I just fabricated a simple cable adapter to the paddles
that would allow me to plug in the "near end" of the radio keying cable
from either radio as needed, instead of running around the back of the desk
and moving cables at the backs of the radios.

With the IC-251a connected to the keyer, the radio gave the illusion of
sending CW as instructed by the keyer, but in reality it was only playing
the CW tones out the monitor speaker and not really transmitting it.  I
didn't even know that this was happening until I tried to work W0GHZ a few
contests ago and he could not hear any CW from me, even though it was an
easy SSB shot on the same band and radio. When keyed by either shorting the
ends of the cable together, or connecting them to a straight key, the radio
works as it should on CW.

With the FT-847 and the IC-706MkII, either radio goes into continuous
transmit mode as soon as it is plugged into the keyer, as if the output of
the keyer was permanently shorted.

The problem is somewhere in the output keying circuit of the keyer. The
funny thing is that it gives every impression of working like it should
when tested on the bench. When not keyed, the output shows about 20 megohms
of resistance, which should be sufficiently high to act as an open circuit,
and when keying, it shows less than 1000 ohms of resistance, which should
be sufficiently low to act as a closed circuit. If I turn the keyer's
internal speaker volume all the way so I can't hear it, set my DMM into
continuity-beep mode, and trigger the keyer, I hear perfect CW come out of
the DMM as the keying circuit closes and opens.  Neither of the modern
radios (the ones that key continuously) will key until the resistance
between the input contacts falls below 3,000 to 5,000 ohms (tested with a
resistor across the end of the cable furthest from the radio, i.e. the end
that would be going into the keyer.  I spoke with the tech from MFJ and he
was basically stumped also, but is sending me a set of replacement
transisitors to see if that will fix the problem.

I also have a different model of MFJ keyer that uses much of the same
circuitry (as per the schematics provided with both models), and it
functions perfectly with the IC-706MkIIg that is the IF radio mounted on my
10 GHz system. Since I had no desire to tear apart the 10 GHz system for
testing, I don't know for sure that it would work equally well with the
older IC-706MkII or the FT-847, but I would be very surprised if it didn't.

73 de W0JT