[FoxHunt] ICOM 2AT and 02AT Output Power Mods

[Marvin Johnston]

ICOM 2AT and 02AT Output Power Mods

One of my goals for the US ARDF Championships is to have all the
transmitters output the same power at somewhere between 1 and 1.5 watts.
I managed to do that and thought it might be a good idea to share my
experience for any others that want to do the same thing ... or have a
better way to do it. The short story is that I much prefer the ICOM
02ATs modified to output 1.4 watts since the output doesn't vary with a
change in input voltage (9 - 14 VDC.) The 2ATs output will vary (after
modification) from about 1 watt to 1.5 watts with the same input voltage
variation. The ICOM BP-1 that I am using with the 2ATs outputs about
9.6VDC, and that was the voltage I used when adjusting the output
power.  All power measurements were made with a Bird 611 Termaline
Wattmeter. 

The mod for the 2AT is pretty simple. If you follow the high/low switch
circuit, one side goes to ground while the other side goes to a 2.2K
1/8w resistor. With the switch closed, the 2.2K resistor is in series
with a 100 ohm resistor and results in high power. Opening the switch
puts the unit into low power. I took the 2.2K resistor and replaced it
with a 1K trimmer pot, and then adjusted the output power to 1.4 watts.
The high/low power switch is left in the high position.

Note: A couple of the 2ATs had labels warning that problems could occur
when directly connected to a 12V battery (13.8 VDC.)

The mod for the 02AT is also simple. There are two pots to adjust the
high and low power outputs. Using high power, I could only adjust it
down to about 3.5 watts. Using low power, I could only bring it up to
about .5 watt. The high power pot (22K) is in series with an 8.2K
resistor. The 8.2K resistor is mounted vertically on the pc board. The
mod is simple; cut the resistor lead at the top and add in another 35K
resistor. The power output can then easily be adjusted to 1.4 watts with
the high/low power switch in the high position.

One other interesting thing was that at 1.4 watts output, the 2AT
required about 1A input current while the 02AT required about 1.5A.