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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