Heath HM-102 Wattmeter

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From: Mike Strong (mstrong@raiders.micro.ti.com)
Date: Thu Jul 22 1993 - 14:15:26 EDT


I have a Heathkit HM-102 Wattmeter that I am trying to convert to use with QRP
transmitters. If you are not familiar with this meter, it is a 200W/2000W
wattmeter and SWR Bridge. The sensor circuit consists of a large (12ga) wire
connecting the input and output SO-239s. This wire passes through a toroid coil
with 17 windings on it that forms a transformer, providing a RF signal to the
circuitry. Clear as mud? Anyway, in my infinite wisdom, I figgered that a way
to improve the power sensitivity of this meter would be to add more turns to
the toroid. I removed the coil, and thought (key word: thought) that I
understood how the coil was wound. I thought that the coil was just a center-
tapped winding on the toroid, providing balanced power input to both sides
(forward / reverse) of the SWR circuit. I removed the old windings from the
toroid, and rewound the coil with more turns. The wattmeter now works fine,
with relatively accurate readings at the 1 watt level. However, the SWR
bridge is now worthless. I get full scale deflection on forward and reverse.
I suspect that I have wound the coil wrong. Does anyone have one of these
venerable old meters who would like to check out the coil and tell me for sure
how it is wound? Or, do you have any other ideas? I have checked the obvious
things: the dummy load is OK on another SWR bridge, the switch,etc are wired
OK...

I would appreciate any help. I am feeling kind of stupid right now. But,
nothing ventured, nothing gained. I feel like I am just this || close to having
a really nice QRP Wattmeter/SWR Bridge.

72 de Mike Strong KT5H

mstrong@micro.ti.com

TI - Houston (713) 274-3317


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