common component failure in lower power AM transmitters is the RF drive
potentiometer. This pot is usually a 4 watt wirewound type which controls
the screen voltage on the driver tube. Although this article details a replacement
for the DX-100, similar pots are found in the Viking I & II, Valiants,
Eico 720 and many others.
This pot recently crapped out in the DX-100 of Joe,
N2YR. After calling around and finding that no supplier could provide a
replacement in less than 30 days, without charging an arm or leg, we decided
to take another approach.
The replacement is very simple and requires only
three, low-cost parts. The key to the circuit is a garden variety horizontal
output driver transistor. These are available through any mail-order parts
house and also at Radio Shack. These transistors are
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designed to handle far more current than
will be needed for this application. Just make sure the one you select has
a voltage rating greater than the maximum screen voltage on the driver tube.
This is generally in the 300 volt range on lower powered AM rigs.
The circuit is shown in Figure
1. Remove the original drive pot and replace it with the 1 Meg pot.
This part is a standard one-half watt pot normally used in low power circuits
like mic gain. In the DX-100, the transistor can be mounted on a terminal
strip soldered on to the mounting flange for the original drive pot, as
shown in Figure 2. If you are working on another
rig, something similar can probably be done. Use your imagination. The nice
thing is that no extra holes need to be drilled to complete the modification.
That's it. No more than a half hour and you should
be able to have your DX-100 back on the air.
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