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TRS-80 PC-3 Troubleshooting Guide

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Radio Shack TRS-80 PC-3 Pocket Computer (Joe Haupt)
A TRS-80 PC-3, 26-3590 (Joe Haupt, CC BY-SA 2.0).

Fault-finding for the TRS-80 PC-3 (catalogue numbers 26-3590 and 26-3589) and its 26-3591 printer/cassette interface. Radio Shack's owner's manual covers operating faults and the reset procedures;[1] no Radio Shack service manual for the PC-3 has been found. For faults inside the computer (supply, current drawn, display bias, keyboard, memory board), use Sharp PC-1251 Troubleshooting Guide, which is based on Sharp's service manual for the PC-1250 and PC-1251. Routine care is on TRS-80 PC-3 General Maintenance.

Resetting

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The PC-3 has two levels of reset, both using the RESET button on the back:[2]

  1. Hold down any key and press RESET. This resets the computer and keeps programs, variables and reserve memory.
  2. Only if that does not bring any response, press RESET without holding a key. This clears the program, data and all reserve contents.

Press the button with something like a ball-point pen; Radio Shack warns against points that break easily, such as mechanical pencil leads or needles.[2]

Symptoms

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PC-3 operating faults (owner's manual, Chapter 10)
Symptom Try, in order
Nothing on the display after switching on Check the slide switch is at RUN, PRO or RSV; press BRK in case automatic power-off has acted; replace the cells
Display present but keys do nothing CL; CA (SHIFT and CL); off and on again; hold any key and press RESET; RESET alone (clears memory)
No response from any key Hold any key and press RESET; if that fails, RESET alone (clears memory)
A calculation shows in BASIC line format (colon after the first number) The computer is in PRO mode; switch to RUN for calculations
A program displays something and stops Press ENTER

The table is from the owner's manual.[3] A display that stays dim even with the contrast control fully anticlockwise means the cells are exhausted.[4] After fitting new cells, if the display is blank or shows anything other than the prompt, take the cells out and refit them.[4]

Static electricity or external noise can hang the computer with all keys dead; Radio Shack's remedy is to hold any key and press RESET.[5]

Printer/cassette interface

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Radio Shack lists three causes for the 26-3591 stopping and the computer hanging: low batteries in the interface, strong electrical noise, or an LPRINT or LLIST command given while the interface is connected but switched off. ERROR 8 may appear on the computer. CL usually clears it; sometimes ALL RESET is needed. Restore power to the interface before using it again.[6]

  • ERROR 8 is the I/O device error: a fault in transfer between the computer and the printer or tape. Check the interface's low-battery indicator and all the connections.[7]
  • If the low-battery indicator is on, the interface will not work until it has been recharged, which takes about 15 hours with the supplied adapter.[8]
  • Connecting the computer to the interface with the computer switched on can hang it; clear it with ALL RESET.[8]

PC-1 programs

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The PC-3 loads PC-1 tapes, but a PC-1 line such as IF N=LPRINT A (meaning "if N equals L, display A") is read by the PC-3 as an LPRINT command and stops with ERROR 1. Rewrite it as IF N=L THEN PRINT A.[9]

Inside the computer

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For faults the reset procedures do not cure, the Sharp pages have the circuit-level checks: Sharp PC-1251 Troubleshooting Guide covers the supply from the two cells, the LCD bias and contrast circuit, keyboard faults and the memory module board, which differs between the PC-1250 and the PC-1251.[10] Kaiser identifies the 26-3590 as a PC-1250 and the 4 KB 26-3589 as a PC-1251.[11]

Kaiser gives the pinout of the 11-pin I/O connector, from pin 1 at the top of the machine: MT 2 input, ground, VCC, busy, data out, cassette out, cassette in, data in, acknowledge, I/O select 2 and I/O select 1. He expects 5 V TTL levels but advises testing with care.[11]

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References

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  1. ↑ Radio Shack, TRS-80 PC-3 Owner's Manual, catalogue number 26-3590. Hosted here as File:Radio Shack TRS-80 PC-3 Owner's Manual (26-3590).pdf (see TRS-80 PC-3 Documentation). Page numbers are the printed ones.
  2. ↑ 2.0 2.1 Owner's Manual, pp. 17-18, the reset button.
  3. ↑ Owner's Manual, pp. 207-208, Chapter 10, "Troubleshooting", machine operation.
  4. ↑ 4.0 4.1 Owner's Manual, pp. 19-22, battery replacement.
  5. ↑ Owner's Manual, p. 211, Chapter 11, maintenance.
  6. ↑ Owner's Manual, p. 90, "Errors" for the printer/cassette interface.
  7. ↑ Owner's Manual, pp. 213-214, Appendix A, error messages.
  8. ↑ 8.0 8.1 Owner's Manual, pp. 79-81, Chapter 7: power, recharging and connecting.
  9. ↑ Owner's Manual, p. 233, Appendix E, "Precautions" on PC-1 programs.
  10. ↑ Sharp Corporation, Service Manual, Model PC-1251, PC-1250, December 1982. Hosted here as File:Sharp PC-1251 Service Manual.pdf.
  11. ↑ 11.0 11.1 Cameron Kaiser, "Tandy Radio Shack Pocket Computer PC-1, PC-3, PC-8", Floodgap Retrobits, Tandy! Pocket! Mania! (retrieved 2026-10-21). PC-3 models and Sharp originals; 11-pin I/O connector pinout. Community source.


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