Radio Shack TRS-80 Service Manual page 83

__ c_o_1o_r_c_o_m ... p ... u_t_e_r_2 _ _ _ _
TRS-BO
®
Service Manual
flowchart. If it is there, but there is no negative
portion to the RS-232 output waveshape, then Ul is most
likely at fault.
5.6.2 Input Problems
RS-232 input problems can be checked in a static condition.
Ground pin 1 of J3 and insure that Ul pin 5 goes high. Then
connect pin 1 of J3 to +5 volts and insure that Ul pin 5
goes low. Repeat the test with J3 pin 2 and Ul pin 4. If
any of the conditions are not met, replace Ul. If problems
still exist, then change PIA U2 or check for PCB faults.
5.7 Sound Problems
Almost any sound problem in the Color Computer 2 can be
traced to U3. Other possibilities include PIA U2 and the
modulator. As a quick check, key in the following program
and look for the waveform shown in Figure 5-17 at the
modulator, pin F:
10 SOUND 200,255:GOTO 10
If the waveform is present but no sound is heard, replace
the modulator (did you check to see if the volume control on
the T.V. was turned up?). If the waveform is not present,
then change U3 or U2 as necessary. If the problem still
exists, check for PCB faults.
5.8 Joystick Problems
As with sound problems, joystick problems are almost always
associated with U3. Additional possibilities include U2 for
joystick pots, and U7, CR7, and CR8 for fire button
problems. Of course, PCB and connector faults could also be
a problem. To test the joysticks, connect two KNOWN GOOD
joysticks and key in and execute the following program:
5 CLS
10 A=JOYSTK(O)
20 B=JOYSTK(l)
30 C=JOYSTKC2)
40 D=JOYSTK(3)
50 E=(PEEKC65280)AND1)
60 F=(PEEK(65280)AND2)/2
70 PRINT
@
O,A,B,C,D,F,E
80 GOTO 10
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