[Boatanchors] Need Globe King 500C Modulator Question

[Donald Chester]

 > I ran into the same fear of blowing the mod. transformer when I first
 > put my 3-400 rig on the air. This is a rig homebrewed by an unknown
 > ham in S.C. in the early 1960s. There was so much audio I was
 > constantly risking clipping the carrier and had to watch my modulation
 > envelope on an oscilloscope like a hawk.> 
 > Rob
 > K5UJ

A common practice for protecting the modulation transformer is to place spark gaps across the windings.  The gaps should be wired across the primary, using two gaps, one from each end of the winding to mid-tap, not just one gap across the entire winding.  No gap should be placed across the secondary.

The reason is that when a spark discharge abruptly shorts the secondary winding, the transformer carries the full peak current of the surge. The corresponding surge of magnetic flux may induce a high voltage kick back to the primary, kind of like the induction coil on a chain saw engine, just for a fraction of a millisecond, but still enough to destroy the transformer insulation.

RCA broadcast modulation transformers typically have a pair of spark gaps across the primary, but none across the secondary.  The only RCA transformers I  ever saw with a gap across the secondary are those WWII surplus 5500-ohm 1:1 ratio ones that must have been released by the thousands after the war.  Those have three sets of gaps, one each from midtap to each end of the primary winding and one across the secondary.  I have never heard of anyone blowing up an RCA broadcast mod transformer, but I have blown a couple of the 1:1 ratio ones myself, and picked up a couple more hamfest finds that were already shorted internally when I tried them.  Perhaps with one of those transformers it would be wise to set the spacing of the secondary gap considerably wider than that of the primary gaps.

BTW, those RCA 1:1 transformers are not broadcast quality.  They are more on par with typical amateur radio quality mod iron, such as the UTC "S" series.  The reason is that they weren't designed for voice at all, but for single- tone modulation, manufactured for MCW (tone modulated CW) service in Liberty Ship transmitters during the war.

Don k4kyv