Star Tech Journal

S*TJ MARCH 1982 17
TROUBLESHOOTING WITH THE LOGIC COMPARATOR
by: BUGTRAP INSTRUMENTATION, SUNNYVALE, CA
For those of you interested in repairing your own games, and we assume that's most of you, test equipment is of
considerable importance.Obviously, a multimeter is a necessity, but you certainly can't troubleshoot complex digital
activity on your circuit boards with it. Oscilloscopes, signature analyzers, and logic comparators are usually the best aids
in troubleshooting the digital systems of your games. All three have advantages, but we will be discussing the merits of the
logic comparator in this article.
The logic comparator compares the output activity of a "known good" i.e. with that of the questionable i.e. on your
powered-up circuit board. The "known good" or reference i.e. ( straight from your stock of replacement i.c.s) is inserted in
the zero-insertion-force socket of the comparator and the comparator's test clip is clipped to the i.e. being tested. Flip the
switches of the comparator to" compare" for the output lines of the i.e. under test. Most testable i.c.s will have their switch
settings documented in the manufacturer's logic comparator reference manual. These settings are determined by
examining industry standard pin-out documentation for specific i.e. functions and packages. Although some basic
knowledge of i.e. function is necessary to determine undocumented switch settings, i.e. manufacturer documentation is
usually adequate to distinguish inputs from outputs on a particular device.
Circuitry in the comparator effectively "piggy-backs" the inputs of the two i.c.s and separates and compares the outputs
of the two. With the same inputs, the outputs of the two i.c.s should match perfectly. If not, one or more of sixteen LEDs on
the comparator will light, indicating miscompares on the corresponding output lines of the i.e. being tested. In some
instances, the output line appearing bad may be pulled down by a shorted input of the i.e. it is driving. If this is suspected,
simply cut and lift the pin of the i.e. you're testing. If it still miscompares, it's bad.
The logic comparator will accurately test such varied TIL and DTL i.c.s as multiplexers, shift registers, flip-flops,
adders, decoders, latches, and counters, as well as the simpler i.c.s which include AND, NANO, OR. and NOR gates,
inverters and b_uffers. It will not reliably test tristate or open collector devices, as they may share common bus lines on their
outputs. Their shared output activity would be read by the comparator as a miscompare.
With a logic comparator, unlike an oscilloscope, you don't need to look at and interpret the digital activity. The
comparator does this for you. Therefore, less experienced service personnel can troubleshoot much of a circuit board.
Those of you that have gone to the trouble and expense of replacing i.c.s that you suspected were bad, but really weren't,
will appreciate the comparator's verification of your suspect i.c.'s activity before you start the replacement process.
Although no individual test instrument can be 100% effective in all applications, the logic comparator has proven to be
a fast, effective tool for troubleshooting faults in the majority of TIL i.c.s used in modern electronics.
If you have any specific questions or would like additional information, we would be more than happy to respond.
Please send any inquiries to Bugtrap Instrumentation, 1173 Tasman Drive, Sunnyvale, CA 94086.
MIDWAY
CRT SUBSTITUTIONS
Subject: CRT Replacements
Game: All Garnes With 19" Color Monitors
Reviewing the difference in 19" in-line CRTs with internal degaussing shield versus exterior degaussing shield with CRT
manufacturers and with vendors (Electrohome and Wells-Gardner), it was found that the CRT with internal degaussing
shield can be used as a replacement in a monitor with an external shield with no need to modify the monitor in any manner.
This being the case, it is recommended that you purchase only 19" in-iine color CRTs with internal shields for replacement
purposes. This will simplify Midway's stocking requirement as well as reduce possible replacement errors.
Any of the below listed 19" in-line CRTs with internal degaussing shield can be used as a substitute for each other:
19VMNP22 (RCA)
19VMJP22 (Rauland)
510UJB22 (Hitachi)
Any of the above CRTs can also be used as a substitute for below listed CRTs, designed to be used with external
degaussing shield:
19VJTP22 (RCA)
19VMBP22 (Rauland)
19VMAP22 (Vendor not known)
MV19VLGP22 (Vendor not known)
MIDWAY
"BOSCONIAN" SERVICE VIDEO TAPE
A special video cassette recording in VHS format has been prepared and is being made available with the introduction of
"BOSCONIAN", Midway's newest 1 or 2 player, full color video game with vocalized instructions.
The cassette tape describes the many play action features, controls, and scoring table plus a new Self-Test display that
operators can access to with just the flick of a switch on the inside of the cash box door. This new electronic development by
Midway enables the operator to easily troubleshoot problems, and test control switches, as well as adjust credits, fighters,
rank - or play level - and bonus levels to suit his needs. Total running time of the tape is nine minutes and 2 seconds
(9:02) ..