[AMRadio] Low modulation

Jim candela jcandela at prodigy.net
Mon Feb 19 13:47:39 EST 2007


   In cases like this I like to pull out my 1000X HV AC compensated probe.
There is a suitable probe on Ebay right now similar to mine:
Ebay, Item number: 320084300029. Mine is good for 15 kv whereas this one is
good for 7 kv.

A probe like this will allow you to use your scope and safely measure DC and
AC up to 7 KV peak. Wouldn't it be nice to probe your RF modulated B+ to
troubleshoot your problem?

Some years back a fellow at an Austin Hamfest had a big box full of these
for sale brand new for $5.00 each!! These came from the trash heap at
National Instruments where there was a lawsuit because some idiot used the
probe to measure a differential signal where both points were at high
voltage. This made the (-) side of his meter HOT, and he got shocked, hurt,
and then the lawyers took over. After I bought one, I reminded him that
those were stolen from National Instruments, and he could go to jail. His
eyes got big and his mouth dropped as I moved on to another table.. I should
have bought more.


-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net]On Behalf Of SBJohnston at aol.com
Sent: Monday, February 19, 2007 1:09 AM
To: amradio at mailman.qth.net
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
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
input made the modulator 833s glow in the usual amount.   I could even hear
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
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.
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
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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