Play Meter

The diode is one of the simplest
devices of the solid-state world and
one of the most important.
It
changes AC (alternating current)
into DC (direct current) ; it allows
signals to go in one direction only; it
clamps voltages ; it suppresses.
noise; etc . The list is long, which
only attests to its usefulness.
Because of its many uses, ser-
vicemen often become confused
about how this device works, which
often leads to replacement of parts
that do not need replacing . Not
only valuable time and parts are
wasted , but the diode becomes
more of a mystery. The serviceman
is a little less sure about diodes and
their various uses.
One of the best teachers I ever
knew had a saying , '" Learn the
basics ." Once you know the basic
operation of a diode , its uses
become obvious and circuit analysis
becomes easier and less time is
needed to find faults . A diode is
simply a device which allows elec-
tricity to flow in one direction
only--an important point .
Because current flows in one
direction , the majority of diodes can
be checked satisfactorily with an
ordinary ohmmeter. When prepar-
ing to check a diode, first determine
which end is the cathode . The
cathode is almost always marked
one way or another .
When using an ohmmeter to test
a diode, make certain you are
absolutely sure of the polarity of the
ohmmeter leads . It is a good idea to
measure the ohmmeter leads with a
voltmeter to determine polarity .
Then mark the polarity of the
ohmmeter leads directly on the
leads or the ohmmeter case with a
felt marking pen . Th is is especially
helpful when using more than one
ohmmeter, since ohmmeter leads
can be either polarity .
Connect the negative ohmmeter
lead to Ithe cathode and the positive
lead to the anode . Thb hookup
forward biases the diode and the
ohmmeter reading should be low for
a good diode.
Connecting the
positive ohmmeter lead to the
cathode and the negative lead to the
anode, reverse biases the diode,
giving a high resistance reading .
It's important to check the resis-
tance in the forward direction on the
46
By PERRY MILLER
Rx1 scale of the ohmmeter . Other
scales will sometimes give erron -
eous readings because the ohm -
meter is not forward biasing the
diode enough to cause full conduc-
tion . Inversely, when checking the
reverse direction , use a high
enough resistance scale to deter-
mine the reverse resistance . The
ratio between reverse and forward
resistance should be at least 10 to 1.
Most diodes will have more than 100
to 1 back-to-front ratio. A typical
silicone diode will measure about 20
ohms forward resistance and over
100,000 ohms back resistance .
Diodes can be measured while still
in the circuit, but because of parallel
circuit resistance , the diode will read
differently then when the diode is
out of the circuit . Even so, the
diode will show a forward -to-back
resistance greater than the reverse
resistance . If tghere is any doubt in
your mind about the reading, lift one
end of the diode from the circuit and
measure the diode again .
" POWER SUPPL Y DIODES ARE
BURNING UP .. . and the two diodes·
near the connector burned up. On
one board they burned a hole
through the board . I replaced the
diodes with the ones sent from the
factory , but they lasted only a
week . "
This is a common problem with
the earlier Paddle Ball models . The
power supply diodes are under-
rated.
Replace the diodes with
silicon rectifiers rated at 50 PIV / 3
amps (Radio Shack #276 - 1141) .
You will have to drill out the holes
with a 1-16" drill because the diode
leads are a large diameter. Because
the drill will drill out the " feed -thru "
hole, make sure you solder the
diode leads on both sides of the
printed circuit board.
ATARl's TANK-- Power Supply
Diodes are Burning up. This is the
same problem that occurred on the
earlier Williams' Paddle Ball 2-Player
Tennis. The power supply diodes
should be replaced with silicon
rectifiers rated at 50 PIV / 3 amps
(Radio Shack #276-1141) . You will
have to drill out the holes with a
1-16" drill because the diode leads
are a larger diameter . Because the
drill will drill out the plated thru -hole
make sure you solder the diode
leads on both sides of the printed
circuit board .
CLEANING PLEXIGLASS. Most
manufacturers recommend a mild
detergent and a soft cloth for
cleaning plexiglass.
One of the
best " mild detergents" for the job is
Shaklee' s Basic-H . Basic-H will not
only clean the plexiglass, but also
clear up those minor scratches . Use
a quart container and mix YJ
Basic-H with % water and you ' re all
set to clean those dirty plexiglass
face plates .
INCREASE
INCANDESCENT
LAMP LIFE. Are you constantly
faced with annoying service calls to
replace burned -out bulbs?
Then
replace the bulbs with a higher
voltage rated bulb, i.e., #47 replace
with #1488 .
The bulb will be
dimmer, but you will increase the
life of the bulb by a factor of 25 .
Another solution to this problem
is to place a diode in series with the
lamp . Disconnect the lead from the
lamp socket and solder a 30 PIV / 1
ampere diode to the socket. Solder
the disconnected wire to the free
end of the diode . If the lamp will not
light, reverse the diode . Use tap or
shrinkable tubing to dress up the
solder connection between the wire
and the diode .
SHRINKABLE TUBING. Shrink-
able tubing is an insulated sleeving
which shrinks when heat is applied
and has replaced electrical tape in
just about every application . It is
used when exposed wire connec-
tions need to be insulated , such as
insulating solder connections on a
connector.
Before soldering the
w ire slide a piece of shrinkable
tubing over the wire , solder the wire
to the connector and then slide the
shrinkable tubing over the solder
connection . You can shrink the
tubing by using a lighted match .
Just place the match flame below
the shrinkable tubing and watch it
shrink .
A shrinkable tubing comes in
many different sizes and colors and
can be purchased at most electronic
stores.
Make sure the heat-shrinkable
tubing will shrink to half its original
size . Radio Shack has a package of
heat-shrinkable tubing with assorted
colors and sizes (#278-1627). Get a
package for your tool box . You will
find many uses for this material.