[Milsurplus] [Boatanchors] Need Globe King 500C Modulator Question (off direct topic but related)

[Dennis DuVall]

Ran into an interesting situation a while ago with the MAK and MO1, a pair of early WW2 Navy transceivers, essentially Jr. versions of the TCS.  Single 807 modulated by a pair of 807s running class A in a Heising setup.  No room 
for the modulation choke on the transceiver chassis so it was mounted on the external dynamotor power supply chassis. Connections between the PS and the radio all ran through Amphenol 12 pin “octal” connectors which were 
in common use during that period.  The lead from the PS to the radio is thus carrying 500 volts from the dynamotor plus the audio voltages produced by the 807 modulator tubes.

The manual included dire warnings about "over modulating" the radio which would result in "destruction of the connector sockets."  Now, kinda hard to destructively “over" modulate with a Heising setup and while puzzling over this
I noticed a quarter-inch spark jumping from the plate lead on the PA tube over to an adjacent chassis bracket every time I un-keyed the radio I was working on and subsequent study of the schematic revealed that the 200ma load 
represented by the PA and modulator tubes was being abruptly disconnect from the 10 Henry modulation reactor but with no provision having been provided to deal with the resultant voltage spike as the flux in the reactor core 
collapsed.  The energy had to be dissipated somewhere and so an arc-over to the chassis had to take place somewhere in the wiring (or inside the reactor) and the close spaced pins in the connectors was apparently a common 
location.  A spark gap on the modulation choke might have sufficed here but I came up with a mod to keep the modulator tubes connected to the HV line and delayed the screen-grid turn off to allow the tubes plates to absorb or 
“snub” the voltage spike.  Worked FB.  Major design error here, though, and first time I’ve run into such in a piece of military equipment.

Dennis D.  W7QHO
Glendale, CA

****************


> On Nov 30, 2015, at 2:49 AM, Rob Atkinson <ranchorobbo at gmail.com> wrote:
> 
> 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?