Commodore PLUS 4 Service Manual page 21

PLUS 4Troubleshooting
2) BAD VIDEO
blocks on screen -Blurred
A) Check
B) Check
C) Check Ul for proper operation
D) Check U2 for proper operation
E) Check
F) Check multiplexers
G) Check ROM for chip
H) Check that all ROM addresses
I) Check
3) NO POWER
A) Verify
4) BAD BASIC -Random
A) Check Basic ROM U23
B) Check
-Scrolling
JI-J6 for shorts to ground
O.K.
••
reset for correct
O.K.
••
O.K. If not: 1) Check
••
O.K. If not: 1) Check
++
RAM data lines for correct
O.K. If not: 1) Check
••
and RP4 should be similar
amplitude
O.K. If not: 1) Suspect
++
U23 and U24
O.K. If not: 1) Check
••
correct amplitude
O.K. If not: 1) Trace
++
U19, U23, U24 by replacement
good
voltage +5 and +9 volts
colors -Power-up
BB thru Iabove
lines on screen
display
operation
socket for good
2) Check for bad Ul
socket
2) Check for bad U2
for hot surface of RAM
2) Jump out RAM to verify
U9, UIO -signals
in frequency
U9or U10
select signal at p.in 22 of
for signal generation
at U20
problem address AO-A15
1) Check for shorts to ground
2) Check switch
3) Check power supply
characters on screen -Random
message
(Bad Video)
Page 22
-Random
or each other
solder
amplitude
at RP3
and
are present and
with known
is missing