Radio Shack TRS-80 Service Manual page 81

__ c_o_1_o_r_c_o_m ... p_u_t_e_r_2 _ _ _ _
TRS-BO
®
Service Manual
5.5 Cassette Problems
5.5.1 Motor Control Problems
Of all possible problems in the Color Computer 2, lack of
motor control is probably the easiest to isolate. Of
course, the obvious should not be overlooked, which includes
bad cable or bad cassette recorder. Actuation of the relay
can be tested by listening for the relay click while
depressing the Reset button. If the click is heard, but
continuity does not appear at pins 1 and 3 (the two outside
pins) of the cassette connector, J4, then either the relay
is bad or the PCB or connector is open. If the click is not
heard, then possible bad components are U2, Ul, and the
relay itself Copen coil), as well as PCB faults. The
following simple program will allow checking of these
components:
10 POKE 65313,60
20 POKE 65312,52
30 GOTO 10
After this program is keyed in and run, scope U2 pin 39 for
a squarewave of approximately 40 Hz. If it is not present,
then change U2. If it is present, then scope Ul pin 9 for
the same squarewave. If it is not there, then change Ul.
If it is there, then change the relay. If replacing the
suspect component does not correct the problem, then a PCB
fault is indicated.
5.5.2 Write Problems
Cassette write problems are also very simple to isolate and
repair. Again, don't forget to verify the obvious, such as
the cassette recorder and cable. If you are convinced that
a cassette write problem exists, key in and execute the
following program:
10 SOUND 200,255:GOTO 10
Now look for the waveforms as shown in Figure 5-17. If the
top waveform is there, but the bottom one isn't, look for it
directly at the chip (U3 pin 17). If it is at the chip, but
not at the connector, a PCB fault or connector fault is
indicated. If it is not present at the chip, insure that
pin 17 is not shorted to ground. If not, replace U3 and
re-run the program. If the waveform is still not present,
-----------1tad1elhaell----------
- 80 -