MATCHING TRANSFORMER

[Gerald C. Lemay]

About 20 years ago I used a public address amplifier to modulate an old CB
rig modified for 10 meters. In the process of experimenting with the
amplifier I hooked it up to a Measurements OP-182C. That thing is an audio
power meter with a variable impedance selector going from 2.5 Ohms to 20K
Ohms. I think the 70 volt line output was near 500 Ohms. If you want to get
an idea of what your transformer is showing for impedance, try the
following. What you can do is drive the 8 Ohm side of the matching
transformer with a known source and measure the secondary (or step up) side
of the transformer open circuited with a voltmeter or a scope. Then connect
a decade resistor or a pot across the secondary and adjust it until the
voltage read on the meter falls to half what it was open-circuited. Then
either look at the decade resistor setting or measure the pot and you should
be in the ball park.

-----Original Message-----
From: Heathkit Owners and Collectors List
[mailto:HEATH at LISTSERV.TEMPE.GOV]On Behalf Of Ed Richards
Sent: Saturday, November 11, 2000 5:08 AM
To: HEATH at LISTSERV.TEMPE.GOV
Subject: Re: MATCHING TRANSFORMER


Ralph;

Those are scarce as hen's teeth. I have been faced with the same problem
and solved it by using a Radio Shack line matching transformer #
32-1031, rated for 10 watts. They also have a 100 watt model. When I was
the radioman on Noah's Ark, the standard line impedance was 500 ohms. Now
it is 70 volt. I don't know the relationship between the two. I would
love to know. Perhaps someone can enlighten me. While they don't indicate
an impedance, the transformer has several high impedance primary taps and
several low impedance secondary taps labeled 4, 8 and 16 ohms. Since
there is a rule that maximum energy is transferred when the impedances
are matched, I do the following: temporarily connect the speaker to the C
and 4 ohm lugs and try all the primary taps for the loudest volume. Then
try all the secondary taps for the loudest volume. Repeat this again. You
will notice when you do this the tonal quality will change. If the volume
is similar, you can select the tap that gives the most pleasing tone. I
have a Hallicrafters SX-28 operating on one right now. I am using the
2.5W primary tap and the 4 ohm secondary tap. I hope this helps.

Ed Richards

On Sat, 11 Nov 2000 10:05:26 -0600 "Ralph L. Hopkins" <rh84859 at NAVIX.NET>
writes:
> ANY ONE ON THIS LIST HAVE A 4 OHM TO 500 OHM AUDIO
> TRANSFORMER.  I NEED ONE FOR A PHONE PATCH.
> TKS
> HOPPY W4YDN
>
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