[Milsurplus] LM-18 and AC PS Questions.. More

[J Forster]

brunneraa1p at comcast.net wrote:

> From: J Forster <jfor at quik.com>
> > That still does not answer the question why are the capacitors switchable, when,
> > if properly set up, there is no need to change them. It it a design error?
>
> The transmitter power supply load is mostly resistive, but must have some reactance which enters the equation.  The capacitors are set by observing filament voltage change key-up and key-down, selecting capacitance for best regulation.

It's a radar. It's turned on or off.  There is no monitoring of the filaments, just an AC line voltmeter in the early control boxes.

> Load reactance might change a bit with different transmitter loading, necessitating different capacitance settings.

The transmitter loading is fixed at installation with a slotted line. The transmitter does not change frequency.

> That's my best guess.  The capacitance value could also be too critical to be satisfied with a generic factory value.  It could also change if there is other apparatus connected to the 120 volt 800 cycle line.
>
> Richard Brunner, AA1P

Again, the only rational purpose I see to having switchable capacitors is to adjust the AC voltage input to the set.

-John