TRS-80 PC-2 Troubleshooting Guide

The PC-2 power supply circuit and adjusting method (Radio Shack service manual, p. 16).

Fault-finding for the TRS-80 PC-2 (catalogue number 26-3601) and its 26-3605 printer and cassette interface. The PC-2 is a Sharp PC-1500, and Sharp PC-1500 Troubleshooting Guide covers the computer in depth: preliminary checks, power supply diagnostics, keyboard and display faults and the error codes. Below are the procedures from Radio Shack's own service manual, and the repair guide for the 26-3605 plotter, which the Sharp pages do not cover. Routine care is on TRS-80 PC-2 General Maintenance.

All keys dead

Radio Shack notes that, rarely, strong electrical noise or a hard knock can leave every key dead, including BREAK/ON. To recover:

1. Hold down the BREAK/ON key.
2. While holding it, press the ALL RESET switch for about 15 seconds. Use the tip of a ball-point pen; not a pencil, whose tip may break off, and nothing sharp such as a needle.
3. Wait for the prompt "NEW0?:CHECK" to appear.
4. Press CL, type NEW0 and press ENTER.
5. If the prompt does not appear, repeat the procedure.

Do not use ALL RESET for anything else: it destroys the program, data and reserve contents.

Power supply adjustment

The regulator is a single resin-moulded module containing the stabiliser, temperature compensation and bleeder circuits. VGG is 4.7 V, VCC 4.7 V ±0.02 V and the LCD voltage VDISP 3.7 V ±0.01 V at 25 °C. If a repair has disturbed them, Radio Shack's adjusting method is:

1. Apply 7.0 ±0.2 V DC at point A (shown on the circuit).
2. Turn VR1 until VCC reads 4.7 ±0.02 V DC from ground.
3. Turn VR2 until VDISP reads 3.7 ±0.01 V DC from ground.
4. Make the adjustment at about 25 °C.

For faults in the supply itself (no display, adaptor or cells not working), see Sharp PC-1500 Troubleshooting Guide.

Replacing the keyboard cable

The flexible printed cable (FPC) between the key board and the main ("operation") board is held by a crimp fitting at the main board and soldered at the key board.

1. Remove the crimp fitting to free the cable from the main board.
2. At the key board, peel away the double-sided tape that holds the cable to the board.
3. Heat the soldered joint from the side with the iron while lifting the end of the cable lightly, and remove it.
4. Tin the new cable and the board evenly.
5. Hold the new cable in place with double-sided tape.
6. Line up the printed pattern of the cable with the board, slide the cable back by about 1 mm, and solder it.

26-3605 plotter repair guide

The service manual has a nine-part repair guide for the printer mechanism, organised as phenomenon, condition, cause, check and repair. The main entries are summarised below; the manual gives several causes for most of them.

26-3605 repair guide (summary)

Phenomenon	Causes the manual lists	Repair
Does not draw lines, although the motors and pen magnet work	Pens off, or out of ink	Fit the pens properly or replace them
Lateral lines cannot be drawn (X motor does not turn)	X motor lead broken; X idle gear deformed; bobbin gear unit deformed; foreign matter in the gears; battery below 4.5 V; slider not sliding freely	Replace the motor, gear or bobbin gear unit; clean; recharge
Lateral lines cannot be drawn (motor turns, slider does not move)	Drive wire off the bobbin gear, broken, or slipping; gear damaged	Restretch or replace the wire unit; tighten the screws; replace the gear
Insufficient lateral movement	Rubber roller scratched; stepped paper	Replace the rubber roller unit; use normal paper
Drawing done segment by segment	Electromagnet actuator detached, or no current to the electromagnet; gap between rubber roller and pen tip wrong (0.6 mm in pen-up); pen the wrong length (standard 23.3 mm, +0/−0.1 mm); low battery; rotary holder out of position	Replace or repair the electromagnet and its drive; adjust the gap; fit a correct pen; recharge; reset the holder
Colour does not change, or wrong colour selected	Electromagnet; foreign matter in the slider; colour-change click deformed; pen off its return spring; weak magnet or faulty reed switch in the colour detector	Clean; replace the click, rotary holder, slider unit or reed switch unit
Paper not fed (Y motor does not turn)	Y motor lead broken; Y idle gear deformed; rubber roller unit not turning freely; foreign matter between the gears; low battery	Replace the Y motor, Y idle gear or rubber roller unit; clean; recharge
Insufficient Y movement	Paper roll too heavy; Y gears damaged; paper feed gear one tooth out; worn rubber roller bearing; paper and roller slipping	Repair the roll paper guide; replace gears or the rubber roller unit; re-mesh the gear; wipe the roller clean
Character misalignment	Bobbin gear or paper feed gear one tooth out; play in the slider or rotary holder; wire spring fatigue	Re-mesh the gears; replace the slider, rotary holder or wire unit

A battery below 4.5 V appears as a cause under several headings; recharge the NiCd pack before stripping the mechanism. On p. 60, the remedy printed for a broken Y motor lead reads "Replace X-motor", which appears to be a misprint for the Y motor.

Kaiser reports that the 26-3605's gears tend to strip and break even under light load. Lubrication and screw-locking points are on TRS-80 PC-2 General Maintenance.

Interface adjustment

The 26-3605 circuit diagram carries one adjustment: set point A to 1.0 V with the 22 kΩ trimmer, with 4.81 to 4.85 V DC applied at point B, VCC at 3.9 to 4.1 V DC, and the temperature around 20 °C.

Related pages

• TRS-80 PC-2
• TRS-80 PC-2 General Maintenance
• TRS-80 PC-2 Capacitor Replacement Guide
• TRS-80 PC-2 Documentation
• Sharp PC-1500 Troubleshooting Guide