[Elecraft] Re: OH Dummy Load Mod

[Don Brown]

Hi

OHR does not have the instructions for the mod to their dummy load. I posted
the instructions several months ago but here they are again:


There was a question about modifying the OHR 100 watt dummy load so you
could use the OHR WM-2 wattmeter to indicate up to 100 watts for testing and
calibration of the KPA100. I think I have found a simpler and better way to
do this. By converting the load into a pi network attenuator with 20 db
reduction the WM-2 will read 100 watts when the load is dissipating 100
watts. This can be done with just one resistor and a BNC chassis mount
connector. You will need a 62 ohm 2 - 3 watt resistor MOS or carbon comp
type (you can use a 51 ohm 2 watt and a 10 ohm 1/4 watt in series) and a
chassis mount BNC connector. Mount the chassis mount connector next to the
SO239 connector on the back of the dummy load. Remove the circuit board and
make a cut in the circuit trace between the last resistor and the ground
screw at the back right of the board. Then make a cut in the trace between
the 4th and 5th resistor from the back right. This will isolate 4 - 1K
resistors in parallel from ground forming a 250 ohm resistor with one side
connected to the dummy load that is now 62.5 ohms. connect the 62 ohm
resistor across the BNC connector and run a short wire from the isolated
side of the 250 ohm resistor (the outside end of the last resistor in the
string on the right. This completes the mod. You now have a pi network with
a 62 ohm power resistor to ground and a 250 ohm resistor in series with
another 62 ohm to ground on the output. The input and output is 50 ohms and
a 20 db reduction in power. If you are using the load without the WM-2 then
you will need to terminate the output BNC with a 50 ohm 2 watt resistor or
the load will be a couple of ohms high If you want to make the load as
accurate as possible then add a 2500 ohm 1 or 2 watt MOS resistor across one
of the load resistors but this should not be necessary in most cases.

To use the load connect the WM-2 to the BNC connector and place a 2 watt or
higher 50 ohm load on the WM-2 antenna connector. set the WM-2 to the 1 watt
full scale range and it will read from 0 to 100 watts. Use the 10 watt scale
and multiply by 10. The accuracy is quite good through 15 meters but falls
off a little above 12 meters probably due to stray capacitance or the WM-2
response may fall off a little at the high end.


Don Brown
KD5NDB



----- Original Message -----
From: "Meier, Peter H." <peter.meier@unisys.com>
To: <DolfinDon@msn.com>
Sent: Friday, May 16, 2003 6:13 AM
Subject: OH Dummy Load Mod


> Hi Don,
>
> Does the Oak Hills Dummy Load instructions detail the modification for
using
> the WM-2 at 100 watts?
>
> If not can you send me the details on this mod? I assume this mod just
> shunts a lower voltage reading to the WM-2?
>
>
> Pete Meier  WK8S
>
>