[Johnson] Viking I durability?

[Richard Knoppow]

----- Original Message ----- 
From: <bcarling at cfl.rr.com>
To: <tbs50a at aol.com>; <johnson at mailman.qth.net>
Sent: Saturday, May 25, 2013 11:17 PM
Subject: Re: [Johnson] Viking I durability?


> it seems like that would be easy to fix with a phase 
> splitter circuit. throw away the transformer. I don't see 
> why a Viking one would be any less durable than any other 
> AM transmitter.
>
    From the data in the handbook the modulator is run Class 
AB2. This requires fixed bias and low resistance in the grid 
circuit since the grids draw current.  It is usual to drive 
Class AB2 or Class B amplifiers with transformers since they 
will meet the requirements above.  Also, it takes some power 
to drive a Class AB2 amplifier. Obtaining the necessary 
power from an RC coupled driver is difficult.  The circuit 
is a simple one and quite conventional.
    I wonder what the shortcomings of the transformers 
Johnson used were. Skimped cores can cause overheating which 
may in turn cause failure of the insulation or the 
insulation may have been skimped causing arc overs and 
shorting.
     It was common for years to design modulators, even in 
broadcast transmitters, on the basis of the rather low 
average modulation in unprocessed or lightly processed 
speech and music.  When fed with a processor that has heavy 
compression or worse yet clipping the whole modulator may be 
overloaded with consequent transformer failure.
     While the 807S in the modulator seem to be run on CCS 
ratings the transformers may still have been in adequate to 
cope with abuse from heavy processing or clipping.


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