Radio Shack TRS-80 Service Manual page 23

RS-232 INTERFACE
The RS-232 interface uses three simple level
converter circuits. Isolating the problem will be
an easy job of comparing the input to the out-
put.
Connect a DIN-type plug to the serial I/O jack to
short together pins 1, 2, and 4. Now, type in the
following test program:
5 POKE 3,1
10 FOR X=0 TO 10: NEXT X
15 POKE3,0
20 FOR X=0 TO 10: NEXT X
25 GOTO 5
Run the program and check pin 2 of U16; a
switching TTL waveform should be present. Pin
6 of U16 should have the same waveform
except that it will be switching from +Vcc to
-Vcc (the supply voltages of pins 4 and 7 of
U16). Also check pins 4 and 6 of U15. These
pins have the same waveform switching from 0
to +Vcc. The outputs of U15, pins 1 and 2
should show the original TTL signal.
CASSETTE INTERFACE
If you are having problems with the cassette
circuit, short together pins 4 and 5 of the cas-
sette DIN plug. Next run the program listed for
the RS-232 interface test. The output pin is
shared between RS-232 and cassette, so this
test program will produce a switching TTL
waveform on the output.
The output circuit is a simple attenuator, so the
likely failure here should be a short to ground.
The output on pin 5 of the DIN connector
should be switching for 0 to +1 volt.
The input circuit is a zero crossing detector. It
should convert the cassette signal back to the
original TTL waveform at pin 13 of U15.
POWER SUPPLY
The power supply circuit in the MC-10 has a
minimum component count so that a failure
should be readily identifiable.
The first place to check if the computer has no
+5-volt supply voltage is the output of the regu-
lator. With the power pack unplugged, measure
the resistance between the +5 output and
ground. If this shows zero resistance, the prob-
lem is a short to ground.
If the +5 output is not shorted, check the volt-
age with the power pack plugged in. You
should have 8 VAC at the power ON-OFF
switch. If this looks good, check the input to
the regulator. Here you should see a full wave
rectified voltage of 10 volts DC. If you do not,
one of the following components could be bad:
D1; CI [C1] or C5; U15, U16, or U17.
The only major remaining component is the +5-
volt regulator. If the regutator is receiving the
proper input voltage, and there is no +5 volts,
the regulator chip is bad.
Also, there is a simple circuit to supply an
unregulated negative voltage to the RS-232
interface. If you are missing the negative volt-
age but the +5 volts is OK, then one of the
diodes or the capacitors has failed.
MODULATOR
If clock does not appear at TP 2, there could be
a problem with the modulator.
Check the +5 volts (pin 11 of MC1372) and the
3.579 MHz clock (pin 1 of MC1372).
If you cannot see the 3.579 MHz clock, check
parts connected to pin 1 and pin 2 (R1, C2, C3,
X1) .
If the 3.579 MHz clock is OK but picture is dis-
torted, try to tune TV set by adjusting fine tun-
ing.
If picture is distorted even after fine turning
adjustment, replace modulator unit if you do
not have Spectrum Analyzer.
If you have a spectrum analyzer, you can adjust
inside of modulator.
Remove the modulator first.
Refer to disassembly instruction on page 10
and locate the four positions where modulator
is soldered to P.C. Board. Remove this solder
with solder wick or desoldering tool.
Bend the protruding edge of modulator until it
is parallel to the slot and remove modulator.
– 23 –