[AMRadio] Low modulation


SBJohnston at aol.com SBJohnston at aol.com
Sat Feb 24 22:59:47 EST 2007


My Gates BC-1T still resists my troubleshooting efforts.  For the mod reactor 
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 kVon 
the low end of the modulation xfmr secondary - no difference, still very low mod 
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 the 
mod xfmr, then secondary, then each half of the secondary - in all cases I got 
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 rise 
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 feedback 
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 noticed 
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 way 
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 slots. 
 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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