Commodore Plus 4 - Page 22

PLUS 4Troubleshooting
Page 33
5) NO COLOR or BAD COLOR
A) Check Ul
frequency
O.K. If
..
pin 14 for 14.31818 MHz with
counter
no
t:
1)
Check solder joints of CTI
and adjust for correct frequency
2) Check crystal, Ql and Q2
3) Check clock circuit for
opens or shorts
for Color Out signal.
1) Swap Ul w/known good
B) Check
O.K.
••
C) Check
Ul pin 13
If not:
O.K.
••
D) Check FB4 and
is present.
modulator Ml pin
55
for Color In signal
and pin 6for Color Out signal
If not: 1) Check Ml operation
CN7 pin 6to see if color signal
1) Check for shorts
6) NO SOUND or BAD SOUND
A) Check Ul pin 33 for SND signal
O.K. If not: 1) Check socket for open circuit
•• 2) Swap Ul w/known good
B) Check audio circuit for short to ground or loss of
signal.
O.K. If not: 1)
••
C) Check modulator Ml
1)
Check Q3 -Be sure emitter and base
are not shorted to 55
v.
pin 2for SND signal
Adjust I.F. can (top right of
modulator) for clean, loud volume
2) Ml pin 2to ground should read
approximately 480 ohms
3) Check Ml for component failure
7) SERIAL FAILURES
A) Check FB23-26 for shorts to shield or each other
B) Check U7, U2 and CN2