the Values of R1 and R2
When I got my MT-3000 antenna tuner, I was unable
to adjust the metering function to get accurate readings for
reflected power. The problem, I eventually learned, was the bridge resistors R1 and R2.
The parts list / schematic calls for R1 to be 10 ohms, and R2
to be 47 ohms. Those were the values of the resistors in
my unit. I believe thatís an error in the manual.
The two resistors in a bridge circuit should be the same
When I made both resistors 47 ohms, the bridge
circuit was easily calibrated. If you are having problems
with your MT-3000A meters, check those resistors and make sure
they are both the same value, something close to 47 ohms.
has links to the
manual & schematic and the author comments on the strange
resistor values noted in the manual.
A Reasonably Effective
Run Coax from your Transceiver to the Input
Connect a 50 ohm dummy load to Coax 1
Set the Wattmeter button to the 200W scale
Select Coax 1 Direct &
Key the 100W TX
Adjust R4 (top potentiometer on the metering board) to read 100W
on the forward meter 200 watt scale.
Select the 2000 W Scale
Key the 100 W TX
Adjust R5 (middle potentiometer on the metering board) to show
100 W on the forward meter 2000 watt scale.
Now get a small screwdriver and adjust C2 to read close to zero
watts on the reflected meter (C2 is the bottom threaded screw-in setting
on the back panel. This looks like an inductor plunger, but
itís actually a trimmer capacitor).
NOW DO THE SAME THING IN REVERSE
Run the signal through the metering circuitry
Reverse the coaxes on the back. Put the transmitter output to
the Coax 1 jack, and connect the Dummy load to TRANSMITTER
Key the 100W TX
Adjust R6 (the bottom potentiometer on the metering board) to
100W on the Reflected meter
Now get a small screwdriver and Adjust C1 to read
close to zero watts reflected (top trimmer capacitor setting on
the back panel)
Finally, put the coax cables back where they
belong and readjust R4 and R5 again to read 100 W forward on the
200 and 2000 W. scales.
It would be best to pump 200 Watts, then
2000 watts to the dummy load to tweak the meters at full scale
itís not necessary to get close enough for basic meter