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Du Mont 13
While no fast rule can be laid down, once the section
failing has been decided on it will generally be found that
a systematic check correctly interpreted will locate the
fault. A voltage check of the suspected circuit along with
the checking of the tubes employed will probably be the
next step. Then, if the voltages are correct and cathode-
ray oscillograph is available it can be used to trace the
source of the trouble.
At this point several factors affect our procedure and it
will be necessary for us to divide the service field into
two classes which we will call the Field and the
Laboratory. The factors in question are as follows: First,
considerable special equipment will be needed. Second,
not all of it is readily available. Third, due to the amount
of investment required the division between Field and
Laboratory must be decided by the service organization
contemplating television work.
Field Service
Most servicemen and: dealers will come under this
classification at present. Until improved methods and
inexpensive equipment can be developed we advise this
group to confine their work to the actions covered by this
manual and not attempt adjustments of the critical circuits
which require special equipment. It is quite probable that
the majority of service problems will fall within this range
in spite of this limitation, as the correct adjustment of the
regular control knobs along with the replacement of tubes
and parts will provide the answer to nearly all troubles. It
is recommended that the adjustment of the trimmer
condensers in all circuits be left to the laboratory group
which should have the necessary equipment for a
complete job.
Equipment (Field)
Regular service tools.
Regular service oscillator.
Ohmeter.
Voltmeter.
Oscillograph
Ultra High Frequency Oscillator.
Diode equipment for oscillograph or a vacuum tube
voltmeter
Discussion
In addition to the regular service tools the regular service
oscillator will be found helpful in checking the audio I.F.
if it covers three megacycles. Incidentally the audio
receiver is so like the average high fidelity broadcast
receiver little trouble should be experienced in servicing
this section.
An Ohmeter is convenient for checking the
size of various resistors and it should have a range that
includes the high resistance values (see the component
parts list).
A good voltmeter is also of value and it too should have a
high range. The Weston Model 722 can be used, thus
combining both of the above instruments. This meter is
now equipped with safety prods (good insulation is a
necessity where high voltages are checked). Sensitivity of
20,000 ohms per volt is provided along with a range of
5,000 volts which adds to the uses of the-instrument. A
unit called the Televerter is available to present owner's of
the Model 772 which will provide the high voltage range
and safe test prods.
Regarding the oscillograph, several models are available
and no particular one will be stressed for this section. The
matter of price is usually paramount with the field group
and it is well to bear in mind that the more extensive the
range the more uses to witch the oscillograph can be
applied.
Another useful piece of apparatus is the Ultra High
Frequency Oscillator. It should have the following
features in order to justify its purchase or construction.
Calibration and reliability are just as important as its
covering the entire band of television frequencies and
fundamental frequencies (not harmonics) should be used.
Provision for external modulation will be convenient,
especially if it is capable of handling television
frequencies. Internal 400 cycle modulation is essential.
Battery (self contained) operation will aid portability and
is an advantage. The Weston Model
787 will be found to possess these characteristics. The
value of this equipment can .be judged by the following
uses. Being portable it can be set near the antenna. and
used to check the antenna and feeders for actual
operation. Using internal 400 cycle modulation, the
receiver can be checked on both the video and audio
channels. The video modulation can be roughly checked
using the internal 400 cycle source, but due to the fact that
the modulation is sine in character the black bars
produced will taper off gradually each side of the center.
The use of- a square wave signal applied externally will
be necessary if even color, sharp cut bars are desired. This
checks not only the modulation circuit but the sweep
linearity.
Regarding diode equipment for use with an oscillograph
or its substitute, the vacuum tube voltmeter, it is advisable
to be sure that they will operate at the high television
frequencies before purchasing. If usable, either of these
units, will prove a valuable aid in locating the point where
a signal is lost or diminished.