Radio Shack TRS-80 Service Manual page 15

_ _ _ D_T_-- 1 - - - - - - - TRS-80
®
Service Manual
The oscillator is not working. Check for 7.328
MHz
at
pin 18 of U-12.
The Reset Capacitor CC-13) is installed backwards or is
shorted.
R-22 Missing or open. Some units may have 8751
microprocessors instead of
8~5l's.
875l's need the
pull-down resistor CR-22} to operate.
Keyboard Will Not Operate
Eliminate the Host, connections, protocol, etc.
Check keyboard Cable.
Make sure keyboard
is
being scanned by U-5. You must
press a key to see activity at this IC's output, OR add
pull up to +5 volts. This is an OPEN COLLECTOR circuit.
Check for two or more keys shorted or jammed closed.
Check for defective U-5 or U-6.
FSTRW* to pin 1,19 on U-6 not present.
EEPROM Will Not Retain Data or Data Stored in the EEPROM
Changes.
Power Fail Detect Circuitry not operating.
CR-1 or Q-1 shorted; Q-2 open.
Defective U-25. This part has a
l~,~~~
store cycle life.
Parallel Printer Port Will Not Operate Printer.
PSTRB* will not drive a heavy STROBE current from
printer. Add shunt resistance to ground on this pin. The
resistor added should match the resistance of the pull up
resistor in the printer.
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