[AMRadio] Low modulation

Bob Bruhns bbruhns at erols.com
Mon Feb 19 11:34:21 EST 2007

Hi Steve,

Try applying 122VAC to the secondary through a 1K, 20 watt power resistor.  The voltage to the windings should not drop much
from the no-resistor readings.  I am guessing that you should see at least 100V on the secondary and 67V on each half of the
primary, and these figures may even be closer to the no-resistor readings.  But if the transformer acts like a short, then there
is a problem.

   Bacon, WA3WDR

----- Original Message ----- 
From: <SBJohnston at aol.com>
To: <amradio at mailman.qth.net>
Sent: Monday, February 19, 2007 2:09 AM
Subject: [AMRadio] Low modulation

> Now an end to a long day of work on my Gates BC-1T transmitter.  After
> cleaning, painting, and reinstallation of the removed parts, the RF sections had
> shaped up nicely, but today I tried modulating the old girl.   No matter what I
> did I only got about 10 - 15% modulation as indicated on a scope and heard on a
> receiver.
> The modulator idle current had fallen right into line at the bias control's
> old marks, and applying some audio increased the current on each tube in
> proportion.  Turning up the audio into the range I would expect for normal program
> input made the modulator 833s glow in the usual amount.   I could even hear the
> groan of the power supply as the load increased.  I've had quite a few years
> of experience with AM rigs, and it all "felt right".  But very low modulation
> on the output...
> The plate and screen voltages on the audio driver stages look high thanks to
> the solid-state rectifiers, but the bias is right on the money.  The DC
> voltage on the grids and plates of the 833 modulators is as expected.  There's quite
> a bit of audio on the grids too, but I don't know what to expect there (any
> thoughts on what it should be, referenced to ground?).
> I started to suspect that the finals weren't getting modulated at all, as I
> think the driver modulation could be responsible for the 15% I am getting.
> Ohmmeter checks of the modulation transformer windings...
> Primary  (2 is CT)
> 1 - 2  28 ohms
> 2 - 3  38 ohms
> Secondary
> 4 - 5  64 ohms
> I then pulled out the transformer and applied 122 VAC line to the secondary,
> and that produced an even 82 volts on each half of the secondary - good.  That
> calculates to a voltage ratio of about 1.36:1, which is squared to get the
> impedance ratio 1.85:1.  I think that two 833s in push-pull would want about
> 9000 ohms load at the primary, and the secondary would be the PA's Ep/Ip or 5000
> ohms., and that's a ratio of 1.8:1  So the transformer "seems" OK.
> What am I missing here??  I'm too close to see it  -grin-
> Steve WD8DAS
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