The circuits affected by this relay are two in number. One set changes the sound output from
one projector to the other, and on the PG-1, 2 and 10 equipments, does this by short circuiting
the secondary of the projector which is not to be used, thus clearing the circuit on the
projector from which the sound is to be used. The other circuit transfers the pilot lamp
connections to indicate which projector is connected to the amplifier. The pilot lamps are
located behind red bulls-eyes, on the front of the amplifier. In the case of the PG-13, however,
the relay acts in a different manner, although the results are the same. The relay armature
changes the traveling contact over and selects either one of the output circuits from the
projectors, instead of shorting out the one not to be used. In this case the one not used merely
has an open circuit.
The difference in operation between relay and potentiometer faders is the fact that fading by
means of a relay gives an immediate change, as the relay operates instantaneously. Although
a slow change is possible with the potentiometer type of fader, it is not usually desirable, and
in fact is used very seldom. The fader arm is swung over rapidly, so that the effect is
practically the same as with the relay type which operates instantaneously.
Should "cross-talk" or failure of sound suddenly develop when fading, it is usually due to a
dirty relay contact. "Cross-talk" is characterized by the failure of either projector to "fade out"
when the fader switch is thrown, with the result that both projectors are in the circuit. Clean
the relay contacts by rubbing a card or paper up and down between the points. Do not use
emery or other abrasive.
118. Volume Controls.-The volume controls used on Photophone equipment are of both the
rheostat and potentiometer type of variable resistance, and are of the same construction as
potentiometer faders described in section 115, and illustrated in Figure 79. Potentiometer
connections are used on Types PG-1, 2, 10 and 13. These are used to vary the input to the
grid circuit of the tubes controlled. Rheostat connections are used on Types PG-3, 4, 6, 7 and
8, and are in the plate circuit of the tubes controlled. The actual location of the volume
controls on the apparatus is given in the table. On Types PG-6, 7 and 8, in which the volume
controls are located on the amplifier, two controls are used, one for each of the two voltage
amplifiers, and are located adjacent to each output fader.
Volume controls located on the input control panel (Types PG-3 and 4) are used without an
output fader, the method of changing from one voltage amplifier to another, in the case of
Type PG-3, being a switching arrangement consisting of a row of three tumbler switches. The
Type PG-4 has but one voltage amplifier.
Volume controls, when mounted on the amplifier rack itself, are conveniently located three or
four feet from the floor. In the case of the PG-13, the amplifier unit itself is mounted on the
wall and is much smaller than any of the amplifiers used with other types of equipment. This
means that the amplifier itself may be mounted in the position which would be occupied by the
input control panel on types of equipment using them.
Output faders are similar in construction to volume controls, and are placed across the output
of the voltage amplifier. They are used to give a smooth, noiseless change from one voltage
amplifier to the other.
Copyright 1930 RCA Photophone, Inc.