[Collins] Reducing the output power

[Gerald]

On Fri, 2005-04-22 at 14:58 -0500, Jim Isbell, W5JAI wrote:
> I would like to use a ICOM AH-3 antenna tuner with my KWM-2 in mobile
> service.  I can build a control circuit to make the tuned do it's
> thing but I need to have the output of the transmitter to be less than
> 10 watts while its tuning.  Yes I could back off on the gain, but I am
> concerned that I might just "forget" someday.  So I want to activate a
> relay in the control box that can close..OR..open a contact to a line
> going back to the KWM-2 to a circuit that will drop the output to 5
> watts, no matter what the gain control is set to.  That way it will be
> automatic, just punch the button and wait till its done tuning.
> 
> Anyone have a suggestion as to the best way to do that in the KWM-2
> with a single line (that could be taken to ground, or to ground thru a
> resistor or to +12 or to +12 through a resistor, some of the options)?

Two ways come to mind. You could put a DC bias (negative going) on the
ALC line to reduce the power. Its a technique often used with rice boxes
and VHF and up transverters. The power may vary with band but so long as
its enough to let the tuner tune and not so much as to burn the tuner's
relay contacts, it should be fine. Probably something in the range of -8
to -12 volts should do, but I've not tried it.

Or you can drop the PA screen voltage. Its brought to a pair of phono
jacks on the back as Kurt suggests. They are jumpered together inside so
you have to cut that jumper. Then with a relay and a pot you can set the
output power to most anything. With the jumper open the power will be
virtually zero. In fact the 62S-1 doesn't take power from the RF output
connector, it takes it from a phono jack coupled with a small capacitor
the to PA grid tuned circuit. And the power is probably only milliwatts.

I'm sure one could change the tune power level by changing the PA screen
divider in the radio but then hitting output on full CW or LOCK KEY will
still blast the tuner. Getting to the ALC input or the PA screens should
be most reliable and neither calls for serious radio modifications.

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer