POWER SUPPLY
TROUBLESHOOTING
CPU
CLOCK AND DIVIDER
Computer is dead. Disconnect the Power Supply
(PT1)
from
the
CPU
Board. Check
for
18.5VAC between pin 1 and pin 3
of
Connector J1A and 23.6V between pin 4 and pin 2
of
Con-
nector
J1A. If the voltages are missing, check the cable and
connections at Connector J1A and check for open
AC
power cord. If the correct voltages are present, reconnect
the Power Supply
to
the
CPU
Board and turn
On
the Com-
puter. Check for 5.0V at pin 3
of
Regulator
IC
(Z1),
11.9V at
pin 3
of
Regulator
IC
(Z2)
and - 5.0V at the anode
of
Zener
Diode CR2. If all the voltages are missing, check the Power
Switch
(S1).
If 5.0V is missing, check the voltages and components
associated
with
Regulator
Output
Transistor
(04),
Regulator Drive Transistor (03), Error Amp Transistor (05)
and pins 2 thru
7,
10,
11
and
13
of
IC
Z1.
If the 5.0V is incor-
rect, check the adjustment
of
the
5V
Adjust
Control
(R5),
refer to
"Miscellaneous
Adjustments".
If the 11.9V is missing, check the voltages and components
associated
with
Regulator
Output
Transistor (06) and pins 2
thru
7,
10,
11
and 13 of IC
Z2.
If the 11.9V is incorrect, check
the
adjustment
of
the
12V
Adjust
Control
(R10).
If the - 5.0V is missing, check Resistor
(R19),
Zener Diode
CR2
and check for possible short to ground.
MICROPROCESSOR (CPU) OPERATION
Check the 1.774MHz
clock
waveform at pin 6 of Micro-
processor
IC
(Z40).
If the waveform is missing
or
off
fre-
quency, refer to the "CPU Clock and Divider" section
of
this
Troubleshooting guide.
If the waveform is normal, check the operation
of
the reset
signal at pin
26
of
IC Z40. The
logic
reading at pin
26
should
be Low for approximately
.1
second when the Computer is
turned
On
and then change to and remain High. If the
logic
reading is incorrect, check Capacitor C42, Resistor
R47
and
check ICs
Z52
and Z53 by
substitution.
If the logic reading at pin
25
of
IC
Z40
is Low, the data lines
(pins 7 thru
10
and
12
thru
15
of
IC
Z40) and address lines
(pins 1 thru 5 and
30
thru
40
of
IC Z40)
will
be disabled.
When disabled the
logic
readings for the data lines are High
and the address lines are Open (No
lights
On). The
logic
readings at the
inputs
and
outputs
of
the address line buf-
fers, Tri-State Buffer ICs
(Z22,
Z38,
Z39 and
Z55),
will also
read Open (No
lights
On). If the logic reading at pin
25
of
IC
Z40 is Low, check Resistor
R58
and check for a possible
short to ground. The data and address lines from
IC
Z40 can
be checked by grounding pin
25
of IC Z40 and checking the
logic
readings for High on the data lines and
for
Open (No
lights
On) on the address lines.
If
IC
Z40
appears to start operating when the computer is
turned
On
and suddenly stops, pulses appear on data lines
then suddenly stop, check
ROM
AlB
IC
(Z1
B)
RAM ICs
(Z13
thru Z20) by
substitution.
Verify that the
CPU
Clock is operating properly by checking
for
a frequency
of
10.6445MHz at pin 6
of
IC
Z42. If the
clock
is
off
frequency, check 10.6445MHz Crystal
(Y1)
and
Capacitor C43. If the clock is not operating, check Crystal
Y1,
Capacitor C43, Resistors
R46
and
R52
and check IC Z42
by
substitution.
The 10.6445MHz
clock
is divided by six by the Divide By 61C
(Z56)
to
supply
the 1.774MHz clock, pin 8 of
IC
Z56, and fed
to
pin 6
of
Microprocessor
IC
(Z40).
Check for a frequency
of
1.774MHz at pin 8
of
IC
Z56.
If the frequency is
incorrect
or
the signal is missing, check the logic reading at pin 6
of
IC Z56 for a Low.
If
the
logic
reading at pin 6 is High, check
Resistor
R67
and check
IC
Z42
by
substitution.
If the
logic
reading at pin 6 is Low, check
IC
Z56
by
substitution.
Check
for pulses at pin 6
of
IC Z40. If pulses are missing, check
Resistor
R64
and check Tri-State
Buffer
IC
(Z72)
by substitu-
tion.
VIDEO
No video on the
Monitor
screen. Check the waveform at the
emitter
of
the Video
Output
Transistor (01). If the waveform
is normal, check the Video Connector (J2) for good connec-
tion
and check the video cable.
If the waveform is missing at the
emitter
of Transistor
01,
check for a video signal at pin 5
of
Driver
IC
(Z41).
If
the
signal is present at pin 5 of
IC
Z41,
check the voltages and
components associated with Transistor 01.
If
signal is
missing at pin 5
of
IC Z41, check for a video signal at pin 6
of
IC Z41. If the signal is present, check IC
Z41
by substitu-
tion. If the signal is missing from pin 6
of
IC
Z41, check the
waveform at pin 3 of IC Z30.
If the waveform is present at pin 3
of
IC
Z30
and the
logic
reading at pin 2
of
IC
Z30 is Low, check IC
Z30
by
substitu·
tion. If the
logic
reading at pin 2 of IC
Z30
is High, check the
Shift
Reg
(Register)
IC
(Z11)
by substitution. If the wave-
form is
missing
at pin 3
of
IC
Z30,
check the waveform at pin
15
of
Shift
Reg
(Register)
IC
(Z10)
and check
for
pulses at
pins
4,
5,10,11 and
12
of
IC
Z10. If the waveform and pulses
are present, check
IC
Z10 by
substitution.
Video display does not operate in the 32 character mode.
Type and run the following Basic program and check for
pulses at pins 9 and
14
of
Flip/Flop
IC
(Z59).
10 OUT 255,8: OUT 255,0: GOTO
10
If pulses are missing at pin 9
of
IC
Z59,
check
for
pulses at
pins 9 and 10
of
IC Z25. If pulses are missing from pin 9
of
IC
Z25, check Tri-State
Buffer
IC (Z22) and ICs Z23 and Z52
by
substitution.
If pulses are
missing
from pin
10
of
IC Z25,
check ICs Z36, Z52 and
Z54
by
substitution.
If pulses are
present at pin 9 and
10
of
IC
Z25, check IC Z25 by
substitu-
tion.
If pulses are present at pin 9
of
IC
Z59 and missing at pin
14
of
IC Z59, check
IC
Z59 by
substitution.
If pulses are pre-
sent at pin
14
of IC Z59, check Mux (Multiplexer)
IC
(Z43)
by
substitution.
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