TRS-80 PC-8 Troubleshooting Guide
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Faults on the TRS-80 PC-8 are covered below from Sharp's PC-1246/PC-1247 service manual. Sharp's manual has no symptom table; it gives a set of check items instead (standby and running current, memory size, the reset after a cell change and the auto power-off time), together with the CPU's pin functions and the circuit diagram.[1]
First checks
[edit | edit source]| Symptom | Probable cause | Action |
|---|---|---|
| Nothing on the display | Flat cells | Fit two fresh CR2032 cells, switch on and press RESET[1] |
| Any cell change | – | Switch on and press the RESET switch, as Sharp specifies after every cell change[1] |
| Switches off by itself | Auto power-off | Normal after about 11 minutes; Sharp's limits are 8 to 14 minutes[1] |
Sharp's manual has no fault table for the display or the keyboard. The parts involved are the LCD (LF7133E) and the rubber connector that joins it to the board (exploded-view item 8), and the single key rubber (item 14) over the key contacts on the board; TRS-80 PC-8 General Maintenance describes how they come apart.[1]
Sharp's check items
[edit | edit source]Current checks
[edit | edit source]Sharp measures the current drawn from a regulated supply connected to the GND and VGG terminals, with a microammeter in the VGG lead. The positive side of the supply goes to GND, because VGG is the negative supply line.[1]
| Check | Set-up | Limit |
|---|---|---|
| Off current | Slide switch at OFF; short the OFF pad to the B11 pad; supply VGG = −6.0 V | 7 µA maximum |
| On current | Slide switch at ON; ON pad shorted or open to GND | 115 µA maximum |
The manual prints the on-current test voltage as −0.6 V, against −6.0 V for the off-current test; the cells give 6 V.[1]
Memory check
[edit | edit source]Switch on, press MODE to enter PRO mode, type NEW0 and press ENTER, then type MEM and press ENTER. A working PC-8 (PC-1246) shows 1278. Sharp gives 3326 for the PC-1247, the 4 KB model Tandy did not sell.[1]
After a cell change
[edit | edit source]Switch on and press the RESET switch.[1]
Auto power-off
[edit | edit source]Leave the computer on without pressing a key. The display must not go out within 8 minutes, and should go out within 14 minutes.[1]
CPU pins worth measuring
[edit | edit source]| Pin | Signal | Function |
|---|---|---|
| 1–4 | V1–V4 | LCD driving power (inputs) |
| 6 | VDD | LCD drive power output; high during standby |
| 7 | GND | Power supply |
| 8 | VGG | Power supply, normally low |
| 9, 10 | X1, X2 | System clock oscillator output and input |
| 11, 12 | X3, X4 | Display clock oscillator output and input |
| 13–16 | K1–K4 | Key inputs, normally low |
| 21 | RESET | Reset input, pulled low; resets when high |
The system clock comes from the 1024 kHz crystal X1.[1] The RESET switch on the case drives pin 21; if the computer cannot be reset, check the reset terminal (part QCNTM1051CCZZ) and its contact.[1]
Interface
[edit | edit source]The PC-8 takes the TRS-80 PC-3's peripherals through its 11-pin connector.[2] The schematic names its pins MTOUT2, GND, VGG, BUSY, DOUT, XIN, XOUT, DIN, ACK, SEL2 and SEL1, matching the signals Kaiser lists for the PC-3's port.[1][2] If a printer or cassette interface does nothing, check the connector contacts before anything else.
Related pages
[edit | edit source]- TRS-80 PC-8
- TRS-80 PC-8 General Maintenance
- TRS-80 PC-8 Capacitor Replacement Guide
- TRS-80 PC-8 Documentation
References
[edit | edit source]- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 Sharp Corporation, Service Manual: PC-1246, PC-1247, code 00ZPC1247SM/E, December 1984. Hosted as File:Sharp PC-1246 PC-1247 Service Manual.pdf. Specifications p. 1; CPU signal description p. 2; PC-1246 circuit diagram pp. 3–4; parts and signal positions pp. 5–6; check items and cautions p. 11; parts list p. 12; exploded view p. 13.
- ↑ 2.0 2.1 Cameron Kaiser, "Tandy Radio Shack Pocket Computer PC-1, PC-3, PC-8", Floodgap Retrobits, Tandy! Pocket! Mania!. Retrieved 2026-10-21. The PC-8's 11-pin I/O port and compatibility with PC-3 peripherals; the 11-pin connector pinout given for the PC-3.