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this was done properly, then start the troubleshooting
steps over again. Keep in mind that there may be more
than one trouble at a Lime, and that correcting one
trouble can sometimes bring previously undetectable
troubles to light.
This verification is especially important when the
original trouble had been intermittent, that is, was not
happening all the time.
Electronic troubleshooting of a video game essentially
consists of checking for the presence of various signals and
of examining their condition. A signal can be thought of as
acting like a "messenger" that carries instructions from one
unit or circuit to another. Many different types of signals
are produced in a video game, and for this reason several
unusual and perhaps unfamiliar types of test instruments
are used during troubleshooting. Each instrument has its
own set of advantages and disadvantages for examining a
given type of signal, and both the depth of the intended
troubleshooting capability and budget will determine what
instruments wil I be needed. Some instruments are basic and
essential, no matter what size of service facility, while other
optional instruments arc desirable because they make
troubleshooting easier and quicker.
(a) The Video Probe. This is a simple but invaluable
instrument having two leads--a test-clip lead and a test
probe lead. During troubleshooting of video signals the test
clip lead is left connected to the game side of the video
coupling capacitor. On the Steeplechase game this point is
the junction of resistors R 10 and R 11 on the PC B.
When the test-probe lead is then connected to any video
developmental signal, that signal wil I be coupled to the video
signal going to the TV monitor and a video probe picture
will appear on the TV screen. The shape and other charac
teristics of this picture will give information about the
nal being probed. The video probe is suited for trouble
shooting synchronization and image signals, but will not be
useful for extremely fast signals (such as the clock) or for
very slow analog or digital signals.
A video probe can be constructed in
these common electrical components: a length of 20 AWG
(American Wire Gauge) rubber-coated wire, a 4.7K-ohrn,
Vi-watt cdrbon resistor, and two lest leads. For the leads,
use a single Mouser lest clip (Mouser #l 31C 301 or 302)
attached to one end of the wire, <1nd a lest prod containing
the resistor on the other.
To assemble the video probe, proceed as follows:
Remove the cap from the test clip and strip 3/16 inch off
24
6.2 TEST EQUIPMENT
6.2.1 BASIC TEST EQUIPMENT:
fast verification of digital IC outputs. It is small, convenient
probe derives its power from the system under test; it has
two power leads, one for connection to ground and the
against a digital signal point, three colorcd lamps in the tip
will indicate the signal's condition or state, as follows:
A circuit shorted to ground will illuminate the white lamp
and an open circuit will illuminate the r-.:d lamp.
supply sources; a commercial model found satisfactory is
the Kurz-Kasch model LP 520.
size and shape to the logic probe, and it also derives its
power from the system under test. When the logic pulser's
tip is held against a digital signal point, the source and sink
point is driven to the opposite logic level. If the point that
the logic pulser is held against is low, pressing the switch on
the side of the pulser will introduce a high pulse. Conversely,
pulsing a high line will pull that line low momentarily.
tion of in-circuit ICs with a shaped digital pulse. For exam
ple, a certain feature of the game may not be working and
nal. Use the pulser lo imitate that signal and if the circuit
few minutes from
begins working, you have proved that the signal was in fact
missing and you can begin tracking it down. This technique
is very similar to jumping coils in electromechanical games
such as pinball machines.
another switch mounted on the logic pulser. When this
switch is �et in the "rep" mode, the instrument pulses the
d
each end of the wire. Solder one end to the post in the test
clip, thread the other end through the hole in the cap, and
snap on the cap. Next unscrew the plastic body of the test
prod from the point and trim both leads of the resistor
to a 3/16-inch length. Solder one resistor lead to the inside
of the point. Thread the other end of the wire through
the hole in the body and solder it to the other resistor
lead; screw the body back onto the point.
(b) The Logic Probe. This is a test instrument designed for
to carry, easy to read, and relatively inexpensive. The logic
other to +5 volts DC. When the logic probe's tip is held
• The red lamp lit indicates a high or logic level 1 (for
TTL components, this is +2.4 to +5 volts)
• The white lamp lit indicates a low or logic level 0 (for
TTL components, this is Oto +0.8 volt)
• The blue lamp lit indicates that the signal is changing
states
• No lamps lit indicate the grey region between 0 and 1
(for TTL this is between +0.8 and +2.4 volts)
The logic probe is readily avJilable from electronic
(c} The Logic Pulser. This lest instrument is similar in
capabilities of the pulser override any IC output and the
During troubleshooting the logic pulser allows stimula
you suspect that a circuit is not receiving the necessary sig
In addition to the regular "pulse" button, there is
digital signal point at a 5-Hz rate or 5 times per second.
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