Play Meter
Issue: 1977 July - Vol 3 Num 13 - Page 44
The Valuable,
Versatile
Voltmeter
The volt-ohmmeter or V.a.M . is
the most important and versatile
piece of equipment in a serviceman's
tool kit. Because you cannot see the
current flow in an electronic circuit,
the meter becomes your "eyes" and
lets you look at circuits and
components to determine their
conditions. In addition to making
simple voltage and resistance checks
with it, you can use the V.a .M . to
test diodes and transistors as well.
Choosing Your Meter
Fortunately, it isn't necessary to
spend a whole lot of money on a
V.a.M . The more expensive meters
will have a greater degree of
accuracy and perhaps a larger
selection of ranges , but for most
applications a v. a . M . that costs
between $20 and $40 will perform
quite well.
The unit that I have found to be
especially well adapted to field
service is made by Lafayette Elec-
tronics and is affectionately known
as Model 99-50841. This meter is
inexpensive, has a wide selection of
ranges,a mirrored scale and a built-in
continuity tester.
This "buzzer" type tester is a
handy device for working on electro-
mechanical games . Because with it
you don't have to look at the meter to
read a continuity indication, you can
work between the mech panel and
litebox of a pin game without
running back and forth . An audible
continuity tester lets you check a
bank of fuses in seconds. It's a trick
feature that I've found to be very
useful at times.
The small "pocket-sized" v.a.
M .'s are okay in a pinch, but most of
them lack an adequate selection of
ranges, and they are notoriously
inaccurate . It's worth a little extra
investment to get an accurate piece
of test equipment.
Using Your V. O. M.
It is possible to check the
condition of diodes and transistors
by performing a static test with the
V. a. M . A static test is made with the
power off and the V. a . M . set to the
ohms or resistance scale .
To perform a static test on a diode,
set your V.a.M . to the X100 or X1 K
scale. Place the positive meter lead
on the cathode end of the diode, and
48
by Randy Fromm of Game Doctors
the negative meter lead on the
anode. Note the meter reading .
[See Figure 1Al Next, reverse the
diode (or meter leads and note the
meter again . [Figure 1 Bl
A good diode will show a meter
deflection (low resistance) one way,
and little or no deflection (high
resistance) when the leads are
reversed . This test is known as
checking the " front-to-back" ratio of
a semi conductor. If a diode gives
you a reading in both directions, then
it's shorted. Conversely, if you can
obtain no reading at all, the diode is
open . A leaky diode will show up as
having a poor front-to-back ratio . In
any of these cases, the diode is not
performing its intended function as a
one-way gate for electron flow, and
it should be replaced .
This same test can be applied to
transistors as well. Electronically
speaking , a transistor is the same as
two diodes " back-to-back ." [Figure
21 You test a transistor by measuring
between the base and collector, and
the base and omitter, and then
reversing the meter leads and
pertorming the same test. [figure 31
As with the diode, you will get a
low reading in one direction and a
high reading in the other. A typical
front-to-back ratio is 100 to 1 for
small transistors and slightly less for
power transistors. Those of you
who operate solid -state wall games
w ill find this method ideal for
checking the multitude of transistor
lamp drivers found in these games.
If you have a lamp that stays
illuminated all the time, or one that
doesn 't light at all , it' s probably just a
bad transistor. Even without a
schematic, it's easy to go down a
bank of lamp drivers and determine
which one is faulty . Explosion light
failu res on Sea Wolf can also be
traced to bad transistor lamp
drivers . These transistors are found
on the game board and can be tested
easily with the V .a .M .
Note: The negative lead of a
V. O. M. is often the more positive of
the two. This sounds stupid, but
many ohmmeters are designed that
way. If your readings are opposite
those shown in the illustrations, then
this is the case with your meter.
ffi
+
RX1K
FIG.IA
ffi
RX1K
FIG. IB
Versatile
Voltmeter
The volt-ohmmeter or V.a.M . is
the most important and versatile
piece of equipment in a serviceman's
tool kit. Because you cannot see the
current flow in an electronic circuit,
the meter becomes your "eyes" and
lets you look at circuits and
components to determine their
conditions. In addition to making
simple voltage and resistance checks
with it, you can use the V.a .M . to
test diodes and transistors as well.
Choosing Your Meter
Fortunately, it isn't necessary to
spend a whole lot of money on a
V.a.M . The more expensive meters
will have a greater degree of
accuracy and perhaps a larger
selection of ranges , but for most
applications a v. a . M . that costs
between $20 and $40 will perform
quite well.
The unit that I have found to be
especially well adapted to field
service is made by Lafayette Elec-
tronics and is affectionately known
as Model 99-50841. This meter is
inexpensive, has a wide selection of
ranges,a mirrored scale and a built-in
continuity tester.
This "buzzer" type tester is a
handy device for working on electro-
mechanical games . Because with it
you don't have to look at the meter to
read a continuity indication, you can
work between the mech panel and
litebox of a pin game without
running back and forth . An audible
continuity tester lets you check a
bank of fuses in seconds. It's a trick
feature that I've found to be very
useful at times.
The small "pocket-sized" v.a.
M .'s are okay in a pinch, but most of
them lack an adequate selection of
ranges, and they are notoriously
inaccurate . It's worth a little extra
investment to get an accurate piece
of test equipment.
Using Your V. O. M.
It is possible to check the
condition of diodes and transistors
by performing a static test with the
V. a. M . A static test is made with the
power off and the V. a . M . set to the
ohms or resistance scale .
To perform a static test on a diode,
set your V.a.M . to the X100 or X1 K
scale. Place the positive meter lead
on the cathode end of the diode, and
48
by Randy Fromm of Game Doctors
the negative meter lead on the
anode. Note the meter reading .
[See Figure 1Al Next, reverse the
diode (or meter leads and note the
meter again . [Figure 1 Bl
A good diode will show a meter
deflection (low resistance) one way,
and little or no deflection (high
resistance) when the leads are
reversed . This test is known as
checking the " front-to-back" ratio of
a semi conductor. If a diode gives
you a reading in both directions, then
it's shorted. Conversely, if you can
obtain no reading at all, the diode is
open . A leaky diode will show up as
having a poor front-to-back ratio . In
any of these cases, the diode is not
performing its intended function as a
one-way gate for electron flow, and
it should be replaced .
This same test can be applied to
transistors as well. Electronically
speaking , a transistor is the same as
two diodes " back-to-back ." [Figure
21 You test a transistor by measuring
between the base and collector, and
the base and omitter, and then
reversing the meter leads and
pertorming the same test. [figure 31
As with the diode, you will get a
low reading in one direction and a
high reading in the other. A typical
front-to-back ratio is 100 to 1 for
small transistors and slightly less for
power transistors. Those of you
who operate solid -state wall games
w ill find this method ideal for
checking the multitude of transistor
lamp drivers found in these games.
If you have a lamp that stays
illuminated all the time, or one that
doesn 't light at all , it' s probably just a
bad transistor. Even without a
schematic, it's easy to go down a
bank of lamp drivers and determine
which one is faulty . Explosion light
failu res on Sea Wolf can also be
traced to bad transistor lamp
drivers . These transistors are found
on the game board and can be tested
easily with the V .a .M .
Note: The negative lead of a
V. O. M. is often the more positive of
the two. This sounds stupid, but
many ohmmeters are designed that
way. If your readings are opposite
those shown in the illustrations, then
this is the case with your meter.
ffi
+
RX1K
FIG.IA
ffi
RX1K
FIG. IB
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