Boxoffice-January.03.1953

I
:
a
,
a three-element tube, you should measure:
1. plate voltage; 2. plate current, 3. grid
voltage, grid current; 4. filament voltage.
When checking an indirect heater type,
screen grid, or pentode tube circuits, the
following additional measurements are
made: 5. cathode voltage; 6. screen-grid
voltage; 7. screen-grid current, and suppressor-grid
voltage. All these voltages and
currents are checked to see if they are same
as recommended by the manufacturer of
the amplifier.
THERE MAY BE A SHORT
When using voltage-current method for
analysis of theatre audio amplifier or preamplifier
defects, incorrect voltage or current
at the tube sockets is, of course, an
indication of a probable short in the circuits
or component; a high resistance
ground, defective or shorted condenser, resistor,
choke or transformer defect, causing
incorrect voltage or current reading.
Each stage should be carefully checked,
starting with the output stage, checking
until you find the stage giving the trouble.
This method of point-to-voint troubleshooting
points out the stage which is causing
the trouble. How to correct many amplifier
troubles you will find in back issues
of Modern Theatre.
Each month we try to present socket
connections of all the types of tubes used in
amplifiers and TV. In order to check voltages,
it is necessary that you know which
"pin" is the plate, grid, filament, etc. Diagrams
or schematics of amplifiers show the
bottom view of tube sockets, and that is the
way we are presenting these tube sockets,
showing plate cathode, grid. etc. Be sure
that all the tubes are in good condition before
starting either voltage or current measurements.
TURN OFF THE POWER
Those of you who have studied radio and
audio amplifiers will know it is wise to be
cautious when operating voltages are on
and you are checking these voltages. Voltages
used in many audio amplifiers in theatres
are high enough to be dangerous to
life. The best plan to follow, for safety's
sake, is to turn the power off at the supply
panel before changing tubes or arranging
amplifier chassis for checking operating
voltages. Of course, when ready to check
voltages at sockets, power pact, etc., it
will be necessary to have amplifier on, using
insulated test probes for testing voltages.
It is a good idea to stand on an
insulating board of some kind and touch
no grounded metal objects while checking
circuit voltages of any make of amplifier.
This advice also applies to checking photocell
and exciter lamp voltages.
Before checking voltages, be sure to let
your tubes warm up several minutes. Before
starting the show each day, the amplifier
should be turned on at least five or
ten minutes.
When checking capacitors ( condensers )
be sure to "short" these with a heavy insulated
screwdriver. This is a safety precaution
that should be kept in mind when
Continued on Page 42
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