TRS-80 PC-4 Troubleshooting Guide

A working PC-4 shows READY P0 with the RUN and DEG indicators after power-on and ALL RESET, the display the service manual's first check expects (photograph Piotr433, CC0)

Faults on the TRS-80 PC-4 (Radio Shack 26-3650 and its later versions) and its 26-3651 cassette interface are set out below. Nearly all of it comes from Radio Shack's service manual, which lists the likely causes of each fault in order of probability and gives test programs for the RAM and CPU. The PC-4 has one CPU (HD61913), one RAM chip (HD61914), an LCD on a heat-seal connector, a rubber keyboard and two CR2032 cells. The service manual says nothing about leaking cells or dirty key contacts, so check those before suspecting the chips.

Preliminary checks

Before opening the case

Symptom	Probable cause	Action
Faint display even with the contrast control fully up	Cells exhausted	Replace both CR2032 cells
Blank or very slow display in a cold room	Low temperature	Let the computer warm up; it recovers at normal temperature
Keys ignored while a dash is shown	Computer busy	Wait; most keys are locked out while it is working
Nonsense on the display after fitting or removing the RAM module	ALL RESET not done	Switch on and press ALL RESET on the back
Switches off by itself	Auto power-off	Normal after about seven minutes without a key press

If cells have been in the computer for years, check the holder and springs for leakage before anything else; see TRS-80 PC-4 General Maintenance and Battery Explosion, Capacitor or Corrosion Damage.

Display faults

Radio Shack's first test is to switch on and press ALL RESET. A working PC-4 shows READY P0 with RUN and DEG lit; the CPU and RAM1 must both work for that. The causes are listed in the manual's order of likelihood.

Display faults (service manual 5-2 A)

Symptom after power-on and ALL RESET	Causes, most likely first
Indicators lit but the dot-matrix area stays blank	RAM1 (HD61914), then CPU (HD61913)
No display at all, or an unusual display	CPU
READY P0 shown but some dots missing	CPU, then LCD, then heat seal

The heat seal (part TH-07-1) is the heat-bonded flexible connector between the LCD and the board. The HD61913 drives the LCD directly; there is no separate display driver chip.

Keyboard faults

Key entry faults (service manual 5-2 B)

Symptom	Causes, most likely first
No entry at all	CPU, then the wiring between CPU and keyboard
Cursor moves but the number or character does not appear	RAM1, then CPU, then the wiring between CPU and RAM1
Only some characters can be entered	CPU, then the wiring between CPU and keyboard

The keyboard is two rubber key contact sheets (parts G420 A35389-1 and G420 A35390-1) over the board. Piotr Piątek's description of the Casio FX-700P, which uses the same CPU, explains why a single break can lose a group of keys: the keys are not wired as a plain row and column matrix, and each key connects a pair of the CPU's key lines K1 to K12.

Memory and CPU tests

RAM1 (built-in)

1. Remove the RAM module if one is fitted.
2. Press ALL RESET.
3. Enter DEFM 62 and press EXE.
4. Type in this program:

10 FOR I = 1 TO 87
20 A(I) = I
30 PRINT A(I)
40 NEXT I
50 PRINT "RAM OK"
60 END

1. Check that the number of steps remaining is 0.
2. Run it. The display should count from 1 to 87, one number per press of EXE, and then show RAM OK.

If it cannot be done, RAM1 may be faulty.

RAM2 (26-3653 module)

1. Fit the module and press ALL RESET.
2. Press MODE 1 and check that 1,568 steps remain. If 544 remain, the module is not working or the connection between the CPU and the module has failed.
3. Enter DEFM 190 and check that the display shows ***VAR:216.
4. Type in and run:

10 FOR I = 1 TO 215
20 A(I) = I
30 PRINT A(I)
40 NEXT I
50 PRINT "MODULE OK"

The display should count from 1 to 215 and then show MODULE OK. If not, the module may be at fault.

CPU or RAM1

If the unit passes the checks above and a fault remains, the service manual separates the two chips like this.

1. Calculate 1+2×(3+4×(5+6×(7+8))). The answer is 767.
2. In DEG mode, calculate TAN(COS(SIN(LOG 100))). The answer is 0.0174550616.
3. Press ALL RESET and run this program, which should finish by displaying END:

10 FOR I=1 TO 2:FOR J=1 TO 2:FOR K=1 TO 2:FOR L=1 TO 2
20 NEXT L:NEXT K:NEXT J:NEXT I
30 GOSUB 40:PRINT "END":END
40 GOSUB 50:RETURN
50 GOSUB 60:RETURN
60 GOSUB 70:RETURN
70 GOSUB 80:RETURN
80 GOSUB 90:RETURN
90 GOSUB 100:RETURN
100 GOSUB 110:RETURN
110 RETURN

If the calculations fail, suspect the CPU. If the calculations work but the program does not, suspect RAM1. The program uses the full depth of four FOR loops and eight subroutine levels, the limits given in the specifications.

The manual also gives a longer test program in program areas P0 to P9, kept on tape and loaded into the unit under test with the RAM module fitted; the listing and procedure are on pages 11 and 12.

Error codes

The PC-4 reports errors as a code number with the program area and line, for example ERR2 P0-10. Code 8 is not used.

PC-4 error codes (service manual p. 15)

Code	Meaning	Causes
1	Memory or stack overflow	Not enough steps for the program; memory cannot be expanded; a calculation too complex for the stack
2	Syntax error	Mistake in the program; left and right sides of an assignment in different formats
3	Mathematical error	Result 10100 or greater; argument outside a function's range; indefinite or impossible result
4	Undefined line number	GOTO or GOSUB to a line that does not exist
5	Argument error	Argument outside the range a command or function accepts
6	Variable error	Use of a memory not yet expanded with DEFM; one memory used as a number and a string at once
7	Nesting error	RETURN outside a subroutine; NEXT outside its FOR loop or with the wrong variable; subroutines nested more than 8 deep or FOR loops more than 4
9	Option error	PRT mode or SAVE and similar commands used with no printer or cassette interface connected

If ERR9 appears with the cassette interface fitted, the computer is not detecting it; work through the "No function at all" checks below.

Supply and signal reference

The PC-4 runs on a negative supply. GND on the schematic is the positive side of the two cells, which feed VDD1 through a 1S1588 diode and a 100 Ω resistor.

HD61913 supply and clock pins (service manual pp. 5 and 22)

Pin	Signal	What to expect
73	GND	0 V reference (positive side of the cells)
76	VDD1	−5.5 V input
77	VDD2	−5.5 V while on; 0 V when switched off or after auto power-off
20–24	V5–V1	LCD drive voltages, from the resistor chain on VDD2
25, 26	OSO, OSI	Clock oscillator; a 56 kΩ resistor between these pins sets the frequency
78, 79	φ1, φ2	Clock outputs to the RAM; the CPU clock is 400 kHz with a 200 kHz machine cycle
74, 75	CE1, CE2	Chip enables: CE1 selects RAM1, CE2 the module's RAM2

VDD2 comes from the HD61913 itself. If VDD1 is present at pin 76 and VDD2 stays at 0 V with the power switch on, the CPU is not running.

Cassette interface (26-3651)

The cassette interface uses an HD43110 that encodes data at 300 bits per second as 1,170 Hz and 2,340 Hz tones (Kansas City standard), from a 32,768 Hz crystal. The manual lists its two AA cells as the power source for the remote control, and the HD43110 runs from −3 V.

Cassette interface faults (service manual p. 21)

Symptom	Check	Action
SAVE does not work	Jack cable open circuit	Replace the cable
	HD43110 signals	If the signals are correct, replace the HD43110
	Recorder head	Clean it
LOAD does not work	Recorder output level	Turn the volume to maximum
	Jack cable	Replace if open circuit
	Recorder	Test it with a music tape
	Clock	Replace the crystal
	Signal C3 at the HD43110	If C3 is faulty, check the shaping circuit; if C3 is correct, replace the HD43110
Remote control does not work	Relay click on SAVE or LOAD	If there is no click, check signal C1; no C1 means a faulty HD43110, a good C1 means a fault in the relay driver (Q1, Q2)
	Jack cable	Replace if open circuit
No function at all	PC-4 seated in the interface	Connect it properly
	Connector contacts	Replace the connector if contact is poor
	Clock	Replace the crystal

The cassette interface's owner's manual warns that a wrong connection will not damage the computer or the recorder, but programs may fail to save or load and data may be erased.

Related pages

• TRS-80 PC-4
• TRS-80 PC-4 General Maintenance
• TRS-80 PC-4 Capacitor Replacement Guide
• TRS-80 PC-4 Documentation