[AMRadio] Low modulation

Bill Smith hbco2 at sbcglobal.net
Sat Feb 24 23:17:07 EST 2007

I still suspect a phasing problem, perhaps a double error.  For example, the
feedback may not be crossed correctly at the input transformer, and to
compensate, the output transformer could be wired out of phase.  Do you have
a dual-trace oscilloscope and can you look at phasing throughout the
modulator stages?

Another thought is to temporarily disable one half of the push-pull
circuitry, perhaps by removing tubes.  If you get more output from one-half
the circuit, it would verify a phasing problem.

Wonder also if you have a large power resistor (200 w) of 2500-5000 ohms you
can temporarly substitute across the secondary of the output transformer.
Any modulation should heat it up fairly quickly and would proove the
function of the modulator circuit.


----- Original Message ----- 
From: SBJohnston at aol.com
To: amradio at mailman.qth.net
Sent: Saturday, February 24, 2007 7:59 PM
Subject: Re: [AMRadio] Low modulation

My Gates BC-1T still resists my troubleshooting efforts.  For the mod
I substituted a big power transformer's secondary which measured 45 H - no
difference.  I temporarily substituted a 1 uF 4000v cap for the 2 uF @ 3
the low end of the modulation xfmr secondary - no difference, still very low
yet the 833s are working reasonably hard.

I then used that big transformer to bench test the mod transformer as per
Don's suggestions.  I applied its output, 60 Hz 2700 VAC, to the primary of
mod xfmr, then secondary, then each half of the secondary - in all cases I
what I expected on the other end (and no tripped breaker or other bad sign).

Then I put it all together again and gave it one more try.  I hit the go
button and heard a low level, high-pitched squeal and saw a wisp of smoke
from the back side of the mod transformer.  I dropped the plates back off
immediately.  The squeak made me suspect that I'd accidentally swapped the
lines at the mod xfmr primary, and indeed I had.

This led me to inspect the transformer's backside a bit more closely.  There
is a terminal exiting each set of windings with a jumper between.  I'd
before that the terminals were at the extreme ends of "slots" in the outer
cover.  Now the terminals were no longer at the end, but about 1/3 of the
back across the slot.  The windings had shifted?  I bet there were not slots
there originally, but something made the windings shift before and cut the
 And the stress of my feedback error had shifted them back part of the way.

In spite of its reasonable behavior in my bench tests, I think I need a mod
transformer.  Any thoughts or other suggestions?

Steve WD8DAS

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