PLUS 4 Troubleshooting Page 2
2) BAD VIDEO - Scrolling lines on screen - Random
blocks on screen - Blurred display
A) Check J1-J6 for shorts to ground or each other
O.K.
B) Check reset for correct operation
O.K.
C) Check U1 for proper operation
O.K. If not: 1) Check socket for good solder
2) Check for bad U1
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, U10 - signals at RP3
and RP4 should be simlar in frequency and
amplitude.
O.K. If not: 1) Suspect U9 or U10
G) Check ROM for chip select signal at pin 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 A0-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 B thru I above (Bad Video)