|[AMRadio] Ranger Audio|
John E. Coleman (ARS WA5BXO)
wa5bxo2006 at pctechref.com
Thu Nov 16 20:07:01 EST 2006
Here is a quick test that might make some sense. Reduce the loading
to 50 percent of the current in the final so that the RF output is only
about 20 watts. And check the voice peaks then.
A bad modulation XFMR or bad modulators can cause some really weird
wave forms. And as others have said use a sine wave from a signal generator
instead of voice unless you can make a good OHHHHWEEE of near sine wave
quality. Really it is just a lot easier to trace with an audio generator.
Transformer saturation has a very distinctive look when low
frequency sine waves are passed through. One peak or the other of the sine
wave will flatten but the flattened part will have a slope. When a
pre-amplifier stage is over driven the wave flattens but does not generally
have a slope on the flattened part of the wave. When a transformer has
reached magnetic saturation there is a flattening as well but is will have a
distinctive slope some have labeled it the trapezoidal look not to be
confused with the pattern we use for modulation indicator.
Too much current in the final for the modulation XFMR is one way the
modulation XFMR can have saturation the other way is an unbalance in the
modulator tubes. If the two modulator have equal current then the magnetic
fields from each are canceled but if one is conducting good and the other is
not then not only will the audio be distorted due to the non linearity of
the tubes but the modulation XFMR will saturate at some point where it might
I have also seen improper wiring of the modulation XFMR cause this.
If the center tap of the primary is confused with one end of the primary in
wiring then you will have the scenario that you speak of but the DC currents
will be perfect. The center tap of the primary must go to B+ and have a
filter capacitor at that point. The ends of the primary must go to the
plates of the modulators If inverse feed back is in use from the secondary
then you can not switch the modulator plate wire.
The next thing to check is the saturation of the driver XFMR and to
make sure it has a filter capacitor at the B+ connection. Too small of a
cathode resistor on the driver tube will cause too much current in the
driver tube and they can saturate the driver XFMR real quick.
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Jack Schmidling
Sent: Thursday, November 16, 2006 5:54 PM
To: Discussion of AM Radio in the Amateur Service
Subject: Re: [AMRadio] Ranger Audio
John E. Coleman (ARS WA5BXO) wrote:
> A 150 watt bulb is going to be pretty dim with just 40 watts RF but does
> brighten up with modulation at all?
Sure. This would agree with my estimate of 50% modulation.
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