[Boatanchors] Need Globe King 500C Modulator Question

[Rob Atkinson]

I was always under the impression Globe Kings could barely get to 90%
up and down and I figured that was by design to protect the mod iron.
Maybe the stock couplates had something to do with that.

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.  My solution was to haul out
a broadcast peak limiter I had and use it to limit my negative audio
peaks.

You can find old used broadcast limiters for sale; I think there's a
CRL PMC300 on eBay now.

You really need to watch the modulation envelope with an oscilloscope.
One trick I learned from Don K4KYV is to look at your mic audio (if
you  are using a crystal mic like a D104) by running the microphone
right into the vertical input on the scope.  There will be enough
there to drive the high Z scope input.

Speak into the mic and observe the audio wave form.  A male voice will
have some asymmetry; maybe a lot.  In my case, I saw huge negative
going audio peaks, 3 times as high as the positive ones.  That told me
I had to flip the phase somewhere in the audio line to the rig or I'd
constantly clip the carrier.

Take a look at raw mic audio on a scope and see which way any
asymmetry is going.  If it is down, find a point where the audio is
balanced either outside or inside the rig and reverse or flip the
lines.  You may have to cross the plate caps on the 811As.  Check the
modulation envelope on the oscilloscope.  first line of defense is to
get high asymmetric peaks going positive.

73


Rob
K5UJ

On Sun, Nov 29, 2015 at 9:23 PM, Dennis DuVall <duvallddennis at gmail.com> wrote:
> Hmmm…  Sounds like Globe/WRL was having trouble with blown mod transformers.   A lot of  iron here and care must be taken when initiating and especially when interrupting current
> flow through the transformer windings, otherwise destructive voltage transients (spikes) can occur.  Took a look at the 500C schematic and not obvious the designers included ANY
> snubbing or other protective provisions on or around the modulation transformer. The somewhat huskier Military T-368 has spark gaps spaced  at 0.0625 inch on both the mod xformer and
> the HV filter choke.  Might be a place to start.
>
> Takes me back  60 years to when I was in college.  A ham friend of mine in the same class was a well off “townie” and he was the very proud owner of the first version of the
> Globe King, the one with a pair of V-70-Ds in the PA.  His sad story was that the modulator blew up the first time he ever tried to operate the thing on phone and he
> would only run the radio on  CW after that.  Anyone else have history to relate?