Electronics-World-1959-05

By WILLIAM C. CALDWELL
Rational troubleshooting based on clear -cut rules
can end the futility of hit- and -miss procedures.
GET OUT your meter, put on your
thinking cap, and let's see how we
can put those increasingly popular
transistor portables back into your customers'
pockets almost before they
miss them. It is possible to do this if a
few simple rules and transistor principles
are observed:
Rule 1. Always keep a voltmeter
handy and don't be afraid to use it. A
20,000- ohms -per-volt instrument is perfectly
satisfactory in most cases. In
fact, since it requires no periodic zero
adjustments, it can prevent errors and
save time when reading small voltages
and biases.
Rule 2. Use miniaturized tools for
miniaturized components -small screw-
transistors may also be divided into
three general categories. Low -power
types are generaly used in such stages
as r.f., local oscillator, converter, i.f.,
a.g.c. amplifiers, and small audio amplifiers
(under 50 milliwatts) like those
found in hearing aids. Medium -power
transistors may be used as audio
drivers 150 milliwatts or more) or in
audio -output stages of small portables.
High -power (or simply "power") types
are used in the audio -output stages of
car radios or larger portables that are
capable of one watt of output or more.
Troubleshooting Procedure
Now we are ready to take a typical
portable and apply some of the prin-
ciples involved. The first step always is
to check battery voltage under load
(radio on). This reading should be
within 10 per -cent of rated voltage if
standard zinc -carbon batteries are used
or within 5 per -cent of rated voltage if
mercury batteries are involved. Bad
batteries can cause any number of conditions:
weak, dead, or distorted sound,
or even instability and oscillation.
While at the batteries, it is a very
simple matter to find out how much
current the radio is drawing without
unsoldering any leads if the batteries
are mounted in clips. Simply insert a
piece of paper as an insulator between
one end of a battery and its clip. Switch
the meter to the appropriate milliam-
Save Time on Transistor Radios
drivers, pliers, and a small (20 to 40
watt) soldering iron. Nothing like the
right tool for the job.
Rule 3. Use a system; never just
"hunt or poke" around.
"But what sort of system shall I
use ?" you ask. Simply follow the rules
already mentioned, along with some
other common electronic principles
that will be pointed out here, and
you're in business. Before stating these
principles, let's review some working
facts about transistors:
1. There are two basic transistor
types commonly used in portable
radios, n -p -n and p -n -p. Of these two,
the n -p-n may be more closely compared
to a vacuum tube: one element
emits electrons that are influenced
(biased) by the base as they are attracted
toward a collector that has a
positive potential on it.
2. The p -n -p transistor differs from
the n -p-n primarily in the direction
that electrons flow through it. They
move from collector to emitter, but this
flow is still influenced by the potential
(bias) between base and emitter. With
this reversed internal operation, externally
applied voltages are also
turned around: the most positive potential
is applied to the emitter.
3. In terms of their use in the radio,
112
l
Fig. 1. Typical transistor stages.
Fig. 2. Open condition in base circuit.
Fig. 3. Open condition in collector.
Fig. 4. Open condition in the emitter.
pere range and measure across the
open battery connection, with no signal
and minimum volume on the receiver.
If more than 25 per -cent over the
normal drain is indicated you should be
looking for a short somewhere, not an
open condition. If several times the normal
value of drain is noted, you probably
have an "A" line short, such as in
an electrolytic capacitor.
Next, if any sign of instability is
present -oscillation, motorboating,
whistles, or howl on stations- substitute
good electrolytics across the ones
in the circuit. If not, isolate the trouble
to a stage or section of the radio.
Some manufacturers give a step -by-
step procedure for stage isolation in
their service literature. A good example
is the Delco Radio Division of
General Motors. This company describes
both a "click" test and a signal -
generator approach for auto -radio
portables. The "click" test (touching a
10,000 -ohm protection resistor from
certain transistor elements to ground
while listening for a slight noise as
this is done) is usually the fastest procedure
for a completely dead radio.
The signal -generator approach is often
best for a set that is merely weak. A
resistor installed in a test lead, with
clips on both ends, has proved very
ELECTRONICS WORLD