[Boatanchors] General Radio 1931-A Modulation Monitor

[Richard Knoppow]

----- Original Message ----- 
From: <bcarling at cfl.rr.com>
To: <boatanchors at mailman.qth.net>; "rbethman" 
<rbethman at comcast.net>
Sent: Tuesday, May 06, 2014 10:24 AM
Subject: Re: [Boatanchors] General Radio 1931-A Modulation 
Monitor


> It seems like either USA or UK amateirs were at one time 
> required to have an "Absorption
> Wavemeter" at their shacks to take alleged frequency 
> readings.
>
> I never had one either place!
>
> Bry AF4K / G3XLQ

   I will have to do some research but I suspect the wave 
meter was a very early requirement. Commercial stations also 
had decrement meters to measure the purity or damping of arc 
and spark transmitters. I think stations were required to 
have some means to insure operation within the limits of the 
ham bands but I think the use of commercially made and 
calibrated crystals may have been sufficient. Old QST 
magazines have a fair number of articals on how to measure 
modulation percentage including some early application of 
CRTs.  Its possible to measure modulation percentage by 
comparing a true avarage meter to a true peak reading meter. 
At least for sine wave modulation the indication of the 
avarage meter will not vary with modulation while the peak 
reader will indicate either upward or downward modulation 
depending on the polarity of the input signal. The average 
meter is set to read the carrier.  I am pretty sure the GR 
1931A works on this principle.  There are ads in pre-WW-2 
ham magazines for similar modulation monitors for ham rigs, 
I think Triplet made one.  I don't know what the regulations 
required but I suspect one can find out from old ARRL 
handbooks. In any case I will look.
    Crystal calibrators seem to have been rare before about 
1945, perhaps because the crystals were too expensive. The 
regulation of broadcast station frequency was tightened up 
beginning about the late 1920s and the Radio Service 
Bulletin of the Federal Radio Commission printed monthly 
lists of stations meeting tight specifications. There was 
one list for stations with 100hz or better control and 
another for stations with 50hz or better control. There were 
also lists showing what sort of control the stations had. 
Some were beginning to use crystal control and others also 
had crystal monitors.  Western Electric advertised a crystal 
frequency monitor beginning around the early 1930s.
     Conversations I had years ago with some of the local 
broadcast pioneers (like Ben McGlashan) told of the amount 
of wandering around of early stations.  A particular PITA 
was Sister Aimee Semple MacPherson's station KFSG (Four 
Square Gospel) which tended to fire up anywhere they felt 
like including right on top of other stations.  KFSG was 
eventually put in the hands of a manager who was independent 
of the owners.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com