STAR*TECH JOURNAL/JULY 1983
7
tMNUFACTs 1114\
SEGA ELECTRONICS' TURBO TROUBLESHOOTING (PART 1)
By Mario Hudson, Sega Electronics, San Diego, CA
In this issue we will be discussing Turbo electronics trouble-
shooting hints. We will deal with troubleshooting by using some
of the more common problems seen with this game. For ease of
understanding, and to enhance your knowledge of the game, we
will also be describing some of the circuits involved. These are:
1. Power-Up Reset (CPU Addressing, Data, and Initial
Timing)
2. Object Display (Cars, Buildings, and Trees)
3. Steering Control
Throughout our discussion we will be making reference to the
schematic diagrams (by page number) in the Owner's Manual
(PIN 420-0681 ). The following test equipment will be necessary
to analyze circuit conditions within the game:
A. Multimeter
B. 30-50 MHz Oscilloscope, and a
C. Logic Probe would be helpful.
Although many troubleshooting methods may be familiar to
you, procedural logic is common among them, and might be
stated in this order: visual inspection, symptom recognition,
symptom isolation, function isolation, component isolation and
repair. Familiarity with the equipment will allow you to bypass
one or more of these steps, as any particular problem may be
obvious to you, or may have happened before. In general though,
these six steps form a good premise upon which to approach your •
"down" Turbo.
The first circuit to be discussed is the Power-Up Reset
Circuit This circuit appears on pages 136 and the upper portion
of 137. ltconsists primarily ofICs 91, 90, 64, 65, 66, 115, 116,
130, 120, 108, and the main program stored in the EPROMs
(ICs 76, 89, and 103). There are peripheral I Cs in this circuit, as
seen on page 136.
Power-Up Circuits are normally operative only when the
system is first turned on. The system must complete the power-
up reset sequence to insure that all of the test, control and
initialization operations necessary to bring the system up have
been performed. Failures in these circuits will not always
prevent the system from running. It may run for a short time and
then stop, or it may lock up in a meaningless program loop. This
failure is commonly known as a CPU halt, and is usually described
as "garbage" on the screen. A CPU halt is normally caused by
either IC 91 (the CPU block), the main program (EPROMs
1363-1365, and IC 90 [PA-06L]). The most expedient way to
isolate this problem may be to swap these components with
known good ones until the faulty item is found.
If component swapping is not possible, the following proce-
dure should locate the failure:
1. Locate the two LEDs on the CPU Board. The one closest
to the edge of the board is the Power-On LED and the other is the
CPU Halt LED.
The Power-On LED should be on continuously. The Halt
LED should flash on momentarily and then go off ( it is normally
oft).
2. Tum the game on, and watch the CPU Halt LED.
A. If the LED does not tum on at all, the problem is with
the CPU Block or its supporting ICs.
a Check pin 6 ofIC 91 for a clock signal.
b. Check IC 108 pin 8 for an active high signal. If both
signals are good, replace the CPU Block. If any of
these signals are incorrect or missing, trace the
problem through the defective circuit by referring to
the schematic on page 136 of the Owner's Manual.
B. If the LED comes on and stays on, the problem is with
the CPU Block IC 91.
C. If the LED flashes on and then goes off, the problem is
either in the main program ICs or the initial timing
circuit
a. Remove the CPU Block (IC 91) from the board.
b. Return the board to the game and power up.
c. Check pins 2-13, 15-19, 21, and23-25 fora signal of
approximately 1.5 to 2.0v as seen on your 'scope.
Trace any abnormal signals back to the preceeding
IC, verify its inputs and replace the defective IC.
d. Check 1Cs65, 66, 115, and 116 pins 11-15 for active
clock signals, and IC 64 pins 9 and 10 for the same. If
any of these signals appears incorrect (held high, low
or floating), replace the IC.
e. If all these appear correct, replace IC 90 (PA-06L).
f. Remaining ICs to consider are: ICs 78, 79, 106, 77,
124, and 104.
CPU Halt problems are relatively common in Turbo and can
generally be corrected by replacing the CPU Block (IC 91 ).
The next circuit we'll look at is the Object Display Circuit
"Object" refers to the buildings, cars, trees, etc. that move in the
display. This circuit is found on schematic pages 143, and 145
through 152. These EPROMs contain the stored information to
display all objects. Problems in this circuit usually show
themselves as white vertical bars, positioned where the normal
object should be. Often an object will appear much larger than
normal. For ease of troubleshooting, we will first describe circuit
operation.
To begin, Turbo is divided into eight object levels. Each level
controls a specific EPROM or pair ofEPROMs, thus controlling
a specific object display.
1. Object Level 0-EPROM 1246 or 1288 (IC 84) controls
the player car display.
2. Object Level 1- EPROM 1247 or 1289 (IC 86) controls
the two inner cars and ambulance (yellow and cyan).
3. Object Level 2- EPROM 1248 or 1290 (IC 88) controls
the two outer cars (green and magenta).
4. Object Level 3 through 7 - control of the EPROMs for the
buildings, trees, lamp post, etc.
A. These EPROMs are controlled as follows: all odd-
numbered EPROMs control the building displays.
a EPROM 1249 or 1291 (IC 90) controls the first
building.
b. EPROM 1251 or 1293 (IC 92) controls the second
building.
c. The remaining odd-numbered EPROMs (ICs 94,
32, and 34) control the third, fourth, and fifth
buildings respectively.
B. All even-numbered EPROMs control the trees, lamp
post, tunnel lamp and street lamp display.
a. EPROM 1250 or 1292 (IC 108) controls the first
tree, tunnel lamp, etc.
b. EPROM 1252 or 1294 (IC 110) controls the second
tree, tunnel lamp, etc.
c. The remaining even-numbered EPROMs (ICs 112,
4 7, and 49) control the third, fourth, and fifth trees,
tunnel lamps, etc., respectively.
These EPROMs are clocked and controlled by ICs 45, 50,
51 and 52 on page 143. This circuit develops the eight clocks
that are used to enable the EPROM circuits. CLK0 through
CLK7 are used to control Object Levels 0 through 7 respectively.
Moving now to some common problems you may encounter:
1. Objects appear too large.
Continued on next page.