[AMRadio] Low modulation

SBJohnston at aol.com SBJohnston at aol.com
Mon Feb 19 02:09:27 EST 2007

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 

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

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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