Boxoffice-April.07.1958

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loaded, either can or paper type of capacitor.
Capacitors will progressively deteriorate
with age and with the passage of time;
then replacement must be made. Sometimes
short circuits may occur with very
high quality capacitor, but are not likely
to do so as with a cheap brand. In modern
theatre amplifiers, only the very highest
quality capacitors and resistors should be
used for peak performance.
Projection rooms equipped with emergency
amplifier should use one amplifier
for a week, then the dual amplifier should
be used the same length of time. This gives
you a check on the operation and keeps
the condensers (electrolytic) in good condition.
Both amplifiers should have the
same power gain and quality of sound reproduction.
The power transformer and its filter networks
play a very important part in
your
amplifier. The voltage supply must be
exactly right for the B+ and B— supply
i"B" plus and "B" negative) for good operation
of your .sound system. Likewise,
the filament .supply voltage must be exactly
right for all the vacuum tubes. Transformers
should be very high quality in theatre
sound systems, because they are sometimes
in operation from six to 14 or more
hours each day. Generally, a power transformer
has taps for 125 V. 115 V and 105
volts. Connections .should be made according
to the AC supply voltage in order
that your transformer will operate efficiently
and supply the correct voltages. If
you have only 105-volt supply, then use
the 105 connections. Check your pKjwer
line voltage with a good voltmeter, or have
your local power supply furnish you with
this data.
In other words, connection to the power
supply terminals should be for average
line voltage taken during the operating
hours. Voltage—AC power supply line
will sometimes vary and it may be necessary,
in very rare cases, to install a voltage
regulator for good performance. This
is often necessary for drive-in theatres
where the power supply does fluctuate.
Today, modem dry-electrolytic capacitors
have replaced the so-called "wet"
electrolytic type, the exception being In
the filter section of the power supply, in
many ca.ses. The modern type capacitor
I
electrolytic employs a moist paste instead
of a .solution which sometimes leaked
out of the can. When making replacement,
polarity must be carelully observed.
You will find that dry electrolytic capacitor
leads are plainly marked like this:
Negative — and positive +. They have
various working voltages, the electrolytic
having the highest standard working voltage
from 450 to 600 volts. If a higher voltage
is applied, they will break down and
have to be replaced. Breakdowns are often
caused by connecting the wrong polarity
connections. Di-y condensers have flexible
leads and they are enclosed in a cardboard
box or tubular enclosure.
Defective Electrolytic Capacitors—
Continued on following page
You Can Build Your Own Testing Equipment for Checking Theatre Sound System
2-6B 2 Meg
:^D—r>
UPPER DECK
253-3 Filwr Flal Washer
100-Mt3 Battery Holder
2S3-7 Fiber Shoulder Washer
MIDDLE DECK
Exploded view ahowtng how
battery holder moonts on
meter studs.
LOWER DECK
Lock bra:-ket lo place wttb 252-10
speednut pressed over stud.
If you are interested in building your own multimeter for checking sound
equipment, t/ie instrument shown can very easily be assembled. You will note
the pictorial diagram shows the approximate location of each part. A large
pictorial diagram is always sent with each instrument, and all you need is a
soldering iron, pliers and a screwdriver. Before you build any type of test
equipment, first thoroughly familiarize yourself with the layout pictorial and
read the instructions completely through before starting. Many components
are supplied by the manufacturer with leads that are longer than the particular
application requires. Of course, all excess lengths should be removed with
side- cutters, after you have installed the part and know you have the correct
length. This type of tester is very handy and can be purchased in kit form.
Tube checker and other test equipment can also be built by purchasing kits,
but one should have some electronic background when assembling very complicated
circuits. But this tester, anyone can build. There are companies supplying
kits almost completely assembled, which simplifies assembling complicated
circuits.— Diagram, courtesy Heath Co.
BOXOFFICE :
: April 7, 1958 43