Commodore Plus 4 - Page 21

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