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[ct-user] LPT Keying Cable? Problem

To: <ct-user@contesting.com>
Subject: [ct-user] LPT Keying Cable? Problem
From: Dallas Carter" <ludal@dmv.com (Dallas Carter)
Date: Fri, 4 Aug 2000 09:48:08 -0400

> I missed it.
> What is the solution ?
> What was positive - what was negative ??
> After all of this, please share the details.
> Robert E. Naumann
> N5NJ / V26O

I have seen several requests for a repeat of the suggestions that I provided
with regard to Franks keying problem.  Hope this reprint of the message to
him sheds some light on the solution.  W3PP...

Frank, I think I have read most of the responses to your query, and your
responses to them.  I take it you are still in a quandry as to what the
problem is.  I am not the worlds best theorist, and usually just apply
emperical solutions, so forgive me if I am not theoretically pure in my
It would seem to me that first you  must determine whether the problem lies
in the s/w, computer, interface or the radio.  Here is the process that I
would adopt, then I will provide, what I believe is background, explaining
some relativily common problems.

    Unplug the keying interface from the radio.  With an ohm meter on the x1
ohms scale, connect one lead to the keyed element (ring or tip) and the
other to the sleeve of the phone plug.  Send a message and note if the meter
moves to the rythm of morse.  You may have to reverse the leads.
            If the meter is moving, you know that the s/w and computer are
ok, and that the interface is doing something.  You have obviously checked
to see that shorting these two points together while connected to the radio,
keys the radio.  If so, then it is the interface that, while working, is not
sufficient to key your radio.  I will explain this later.

    If the meter is not moving, put the meter on a low voltage, ie 10vdc
scale, and put the black lead on the emitter lead of the transistor in the
interface, and the red lead on the computer side of the base resistor.  Send
another message and check to see if the meter reads the keying voltage.  If
it is, then you have again confirmed that the s/w and computer are ok.  If
not, then you need to check out that computer & s/w.

    In both cases above, if the meter is responding, the problem lies
between the computer and the radio, or, the interface.  As I recall, the
interface is an NPN transistor with a base resistor of 1K or so that is
connected to an LPT pin (the keying source).  The emitter is grounded, and
the collector is connected to the radio key line.  The base resistor limits
base current so that you dont burn up the transistor, and as the voltage on
the base goes positive, the transistor turns on, shorting the collecter to
ground, keying the rig.  Collector voltage (VCC) is provided by the rig, and
is normally plus 12 vdc.  Here is the point...   If the Beta (gain) of the
transistor is low, and the base resistor is too high, there will not be
enough base current to turn on the transistor.  This may also be a function
of the radio limiting the collector current and/or voltage.  If this is what
you are seeing, lower the value of the base resistor and see if that
corrects the problem, else try a different transistor.

    I have eight interfaces that I use here.  Some of them will key my
FT-1000MP's but not all of my TS-940's.  One in particular would not key a
guest ops 940, and for the contest I just swapped it out with the cable from
one of my 940's.  I ultimately got it to work on his rig by changing the
base resistor.  His 940 keying circuit aparently takes more keying current
than my other rigs.  One more thing.  If you dont see the voltage at the LPT
pin, first suspect that the transistor base is not grounded.  It may be that
the computers lpt port is not wired like most.  Should be easy to check that
and find a ground source.  This is by no means a complete trouble shooting
effort that I have outlined for you, but I hope it gives you some ideas.

    very 73,
Dallas  W3PP

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