TRS-80 PC-5 Troubleshooting Guide
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There is no Tandy manual for the TRS-80 PC-5 online, so fault-finding relies on the PC-6 service manual, which Cameron Kaiser says applies generally to the PC-5, and on what is known of the differences between the two.[1] The full PC-6 fault table, test programs and pin functions are in the TRS-80 PC-6 Troubleshooting Guide; the table below shows what carries over to the PC-5 and what does not.
What carries over from the PC-6 manual
[edit | edit source]| PC-6 manual item | On a PC-5 |
|---|---|
| Fault table (p. 10): cells, battery contacts, heat seals, LCD, processors, key contacts | Same causes apply to the same kinds of part |
| Main and sub CPU split of the display and keyboard (pp. 5, 9) | Kaiser describes the same two-CPU split on both models |
| Processor part numbers HD61747B20 and B25 | Different: the PC-5 uses HD61747B10 and B11 |
| RAM HM6264ALFP-50, 8 KB | Different: the PC-5 has 4 KB |
| Memory check program (p. 11) | Written for the PC-6: it reserves 904 array elements with DEFM to fill the PC-6's memory, so reduce the count for the 4 KB PC-5 |
| Front RESET button | Not fitted on the PC-5 |
| Parts and schematic (pp. 20–26) | Use as a guide only; no PC-5 parts list or schematic is available |
Sources: the PC-6 service manual and Kaiser's description of both models.[2][1]
Faults by symptom
[edit | edit source]| Symptom | Probable cause | Action |
|---|---|---|
| No display at all | Flat cells; poor battery contact; peeled or broken heat seal; LCD; processor | Fit fresh CR2032 cells; clean the cells and springs and lift the springs; then check the heat seals[2] |
| Some characters or segments missing | Heat seal; LCD; processor | A gap of six characters in a row points to one of the two processors, each of which drives two blocks of six on the PC-6[2] |
| Calculations wrong or impossible | Processor | Run the operation check in the PC-6 manual (pp. 14–18) key by key[2] |
| One key dead | Key contact; broken track; peeled heat seal; processor | Clean the contact pads and key rubber[2] |
| Many letter keys dead or unreliable | The membrane panel in the lid or its connection | Inspect the membrane panel; Kaiser describes it as flexing in use[1] |
| Programs lost when the main cells are changed | Backup CR1220 flat | Replace the CR1220 while the main cells are still fitted[1][3] |
| Lid will not stay closed | Broken latch | See TRS-80 PC-5 General Maintenance#Hinges and latch[1] |
| No RESET button | Not a fault: the PC-5 never had one | –[1] |
Cassette and printer
[edit | edit source]The PC-5 drives the PC-4's 26-3651 cassette interface or 26-3652 printer through the 26-3671 link cable.[1] The PC-6 manual allows only one of the two to be connected at a time, and asks for the power to be off while connecting them.[2] For faults in the cassette interface itself, see TRS-80 PC-4 Troubleshooting Guide#Cassette interface (26-3651).
Related pages
[edit | edit source]- TRS-80 PC-5
- TRS-80 PC-5 General Maintenance
- TRS-80 PC-5 Capacitor Replacement Guide
- TRS-80 PC-6 Troubleshooting Guide
References
[edit | edit source]- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Cameron Kaiser, "Tandy Radio Shack Pocket Computer PC-4 and Tandy Pocket Scientific Computer PC-5, PC-6, PC-7", Floodgap Retrobits, Tandy! Pocket! Mania!. Retrieved 2026-10-21. PC-5 and PC-6 section: the PC-5's HD61747B10/B11 processors and 4 KB memory against the PC-6's HD61747B20/B25 and 8 KB; the two-CPU split of keyboard and display; the backup battery; the covered memory slot; broken hinges and latches; the membrane keyboard; the PC-6 service manual generally applying to the PC-5.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Tandy, Service Manual: Pocket Scientific Computer PC-6, catalogue number 26-3672. Hosted as File:Tandy PC-6 Pocket Scientific Computer Service Manual (26-3672).pdf. LSI descriptions p. 5; display p. 9; troubleshooting guide p. 10; test programs p. 11; operation check pp. 14–18; parts lists pp. 20–22; schematics pp. 24–26.
- ↑ Pocket Museum, "The Tandy PC-5 computer (26-3670)". Retrieved 2026-10-21. Power: two CR2032 and one CR1220 lithium batteries.