[AMRadio] Modulation transformer ID and repair

[Donald Chester]

>
>The other end of the modulation transformer was tied directly to ground.
>
>What would be the difference?

The problem with tying the mod transformer directly to ground is that the 
secondary winding is permanently at ground potential, maintining the full 
HVDC across the insulation between windings, which is often nothing more 
than a few layers of paper that may be 50 or more years old.  If the bottom 
end of the mod transformer is tied to ground through the coupling capacitor, 
the voltage will  still appear across the insulation as a brief transient, 
but as the coupling cap charges, and the magnetic  field around the mod 
transformer core and mod reactor core reach stable values, the potential 
difference between mod transformer windings will quickly become the 
difference between modulator power supply voltage and final PA power supply 
voltage; if a common power supply is used, the voltage between windings will 
become zero.  This puts a lot less stress on the ancient insulation than 
permanently  maintaining a couple of kilovolts potential difference across 
the insulation between windings.  See John's drawing.  This is the circuit 
used by most BC transmitters.

Most BC rigs use a surprisingly small value for the coupling capacitor, 
often as low as 1 mfd and rarely more than 4 mfd.

Don K4KYV

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