O Wise Ones,
I'm in need of advice on recalibrating an SWR meter. It's a Daiwa
cross-needle one about 15 years old. The forward scale is calibrated
0 to 20 W and the reverse scale is 0 to 4 W. Inside is a screened
box with holes allowing adjustment of two trimming capacitors.
Orange and green wires go from this box to a PCB with preset resistors
to adjust the forward and reverse readings. Connection is made via
two SO239 sockets on the back, marked Ant and Tx.
Here's what I've done; did I get it right?
I connected a 50 Ohm dummy load/Wattmeter to the Ant socket and my
TS530S to the Tx socket. With 20W out I adjusted the forward sensitivity
pot to give 20W on the forward scale, and one of the trimcaps (the one
which had a effect) to give a zero reading on the reflected scale.
Then I turned the power down to 4W, reversed the connections to the Tx
and Ant sockets, adjusted the reflected sensitivity to give 4W on the
reflected scale and the other trimcap to give a zero reading on the
forward scale. I assumed that the trimcaps are used to set the input and
output impedances to 50 Ohms.
The reason why I want to recalibrate it is that, years ago, I fixed it
when it wasn't broke. The meter was also available in a switchable 20W/
200W version. I took the case off mine to find the PCB had holes for a
switch and some extra components, so I added these. How I chose the
values, I don't know. Anyway, it didn't work properly after that. Now
I want to use it again and right the wrongs of years ago, or perhaps
even make it switchable from 20W to 2W - that would be the honourable
thing to do. Is my reasoning correct or have I messed it up further?
An inquiring mind needs to know.
72 de John G4EDX
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