TRS-80 PC-1 Troubleshooting Guide

A TRS-80 Pocket Computer with the 26-3505 printer and cassette interface.

Fault-finding for the TRS-80 Pocket Computer (catalogue number 26-3501, later called the PC-1) and its 26-3503 cassette interface. The computer is a Sharp PC-1211 and the interface a Sharp CE-121, so the Sharp service manual cited on the Sharp PC-1211 page covers the same circuit. The tests below come from Radio Shack's own service manual. Routine care is on TRS-80 PC-1 General Maintenance.

Symptoms

PC-1 symptoms and where to look

Symptom	Likely cause	What to do
Nothing on the display after ON	Flat cells, or corrosion on the cell contacts	Fit four fresh cells (MR44 in Radio Shack's specification) and clean the contacts. See Battery Explosion, Capacitor or Corrosion Damage.
Display faint, or dark all over	LCD drive voltage (VDISP) out of adjustment	Set VDISP with the 250 kΩ trimmer: −3.74 V at 20 °C, viewing the display from 30° off vertical.
Dark patches growing round the display traces, sometimes covering the whole display	The LCD itself; Kaiser attributes it to poor sealing of the LCD assembly, letting the liquid crystal dry out	No adjustment cures it. The display has to be replaced, for example from a donor PC-1 or PC-1211.
Fresh cells go flat in storage	Excess current with the computer off	Measure the current: under 12 µA after OFF at 4.72 V (see below).
Tape keeps running before a save, does not start on CSAVE, or does not stop	The relay control from the computer	Check the computer first, then the relay drive (see the cassette test).
No sound from the recorder while saving	Recording circuit of the interface	See the cassette test.
Saved tapes will not verify	Recording or playback circuit of the interface, or the recorder	Try a tape recorded on another machine to tell the two apart (see the cassette test).

Current drawn

Radio Shack gives the current at a supply of 4.72 V:

Current consumption

Condition	Current
After pressing ON	Under 850 µA
After pressing OFF	Under 12 µA

Feed the computer from a bench supply set to 4.72 V through a meter in place of the cells. Opening the case is described on TRS-80 PC-1 General Maintenance.

Check program

Radio Shack's check program is a sequence of key entries with the display that should follow each one. It exercises arithmetic, program entry and running, a labelled program run from the keyboard, and the buzzer (BEEP). ALL RESET at step 2 clears memory, so save anything wanted first. Keys are shown as on the keyboard; SHFT " is the double quote.

Check program (service manual p. 22)

Step	Keys	Display
1	ON	>
2	ALL RESET	>
3	5/9 ENTER	5.5555 and further fives (the right of the line is cut off in the scan)
4	MODE	>
5	1 P . SHFT " O K SHFT " ENTER	1:PRINT "OK"
6	2 SHFT " Z SHFT " P .	2"Z"P.
7	SHFT " S SHFT " ENTER	2:"Z"PRINT "S"
8	3 B E E P 2 ENTER	3:BEEP 2
9	MODE	>
10	G R A D ENTER	>
11	SHFT SPC 0 1 4 7 ENTER	:0147
12	MODE	>
13	SHFT Z	S
14	MODE SHFT SPC	0147
15	CL R A D . ENTER	>
16	R . ENTER	OK
17	ENTER	S
18	ENTER	>
19	OFF

A step that gives the wrong display, or a key that does nothing, narrows the fault to the keyboard, the display or the two CPUs. The manual's pin tables for CPU I, CPU II and the display chip give the signals to look for.

Cassette interface test

Radio Shack's procedure saves a program to tape and verifies it, with the interface and recorder connected as shown in each step:

1. With the computer off, connect it to the interface.
2. Press ON. With the EAR and MIC plugs connected, wind the tape to where the recording is to go.
3. Connect the REMOTE plug and press PLAY and RECORD on the recorder. The tape should not run.
4. Type CS. SHFT " A (CSAVE "A").
5. Press ENTER. The tape should start and the recording sound should be heard.
6. When > appears, the sound should stop and the tape should stop.
7. Disconnect the REMOTE plug and rewind the tape to the start of the recording.
8. Reconnect REMOTE and press PLAY.
9. Type CLO.? SHFT " A (CLOAD? "A", which compares the tape with memory).
10. Press ENTER. The display should clear, the tape should start and the recorded sound should be heard.
11. When > appears, the sound and the tape should stop.
12. Disconnect the interface.
13. Press OFF.

Radio Shack's notes on the result:

1. If the tape keeps running at step 3, does not run at step 5, or does not stop at step 6, check the computer.
2. If no recording sound is heard at step 5, check the recording circuit of the interface.
3. If no sound is heard at step 10, play another recorded tape. If that is silent too, check the playback circuit of the interface. If it can be heard, check the recording circuit.

Relay control

The interface starts and stops the recorder through a relay on its REMOTE socket. To check the control signals, the manual uses this program, which writes data to tape in a loop (the line wraps in the scan):

10:PRINT #"D";A(201):PAUSE 10:GOTO 10

1. Type RUN. The tape should not move yet.
2. Press ENTER. The recording sound should be heard.
3. The sound should stop and "10." should show for about a second, then the cycle repeats.

While it runs, pins 5 and 12 of the TC4528BP change level between recording and display. Pin 6 (relay terminal 5) should give a pulse at the start of each recording, which closes the relay and starts the tape, and pin 10 (relay terminal 3) a pulse at the end, which opens it. Each pulse should last at least 1 ms.

Related pages

• TRS-80 Pocket Computer (PC-1 main page)
• TRS-80 PC-1 General Maintenance
• TRS-80 PC-1 Capacitor Replacement Guide
• TRS-80 PC-1 Documentation
• Sharp PC-1211