IBM PCjr Maintenance Guide

IBM PCjr system board

Routine care of the IBM PCjr (type 4860) centres on the cordless keyboard, the cartridge slots, the sidecar connector, the diskette drive and the Power Board inside the system unit. Fault diagnosis is in the IBM PCjr Troubleshooting Guide and capacitor work in the IBM PCjr Capacitor Replacement Guide.

Safety and the power supply

The PCjr's 33 W supply has two stages. The external unit is a transformer with a fused, extra-low-voltage AC output; the Power Board inside the system unit turns that into +5 V, +12 V and −6 V. The transformer carries mains voltage inside and has a non-resettable input fuse. Unplug it at the wall and at the system unit before opening the machine.

IBM's service manual warns that Power Board components can become hot when the system unit is on with the top cover off. The system unit has no cooling fan; the only fan is on the diskette drive.

See Battery Explosion, Capacitor or Corrosion Damage and Recommended Tools.

Cleaning

1. Unplug the transformer at the wall and at the system unit.
2. Remove any cartridges and sidecars.
3. Remove dust from the system board, cartridge slots and sidecar connector with a soft brush and low-pressure air.
4. Clean the cartridge slot contacts and the sidecar connector with contact cleaner on a foam swab.
5. Wipe the infrared windows on the keyboard and the system unit with a dry cloth.

Keyboard

The cordless keyboard takes four AA batteries. Its 80C48 microcontroller sleeps until a key is pressed, so the keyboard has no on/off switch. IBM treats the keyboard as a FRU with nothing repairable inside, and the batteries as the owner's responsibility; the battery replacement procedure is on p. 4-300-3 of the service manual.

The optional keyboard cord (part 8285983) carries the same data as the infrared link. Plugging it in disconnects the batteries and the keyboard's infrared transmitter, and the −CABLE CONNECT signal disables the system unit's infrared receiver. A keyboard that works on the cord but not over infrared has a fault in the infrared path, not in the keys. IBM's customer error code B means a keyboard problem, and its advice is to move away from bright light; Brutman also notes that the keyboard needs a clear line of sight to the front of the system unit.

IBM made two versions of the keyboard. The replacement for the original small-key keyboard is electronically identical, with bigger keys and the legends on the key tops.

Things that are not faults

• Three keys pressed together being ignored. The PCjr keyboard has phantom-key detection: an invalid combination of three or more keys makes it send hex 55 and discard all the keys pressed at that moment.
• Keystrokes lost during diskette access. The PCjr has no DMA controller, so the processor services the diskette drive directly; Brutman describes the resulting keyboard beeps and lost serial data.

Cartridge slots

Each cartridge is wired for a fixed address region (D0000, D8000, E0000 or E8000). Inserting or removing a cartridge resets the machine, and two cartridges that use the same region usually lock it up. Remove cartridges before running IBM's advanced diagnostics.

Sidecars and the power budget

A machine fitted with all the internal features has 400 mA of +5 V, and nothing on +12 V or −6 V, left for the I/O connector. Brutman reports that heavy or numerous sidecars cause erratic operation, and that IBM's power sidecar uses the same external transformer as the system unit. Add sidecars one at a time after any repair, so that a power-budget problem is not mistaken for a fault.

Diskette drive

The Model 67 has a 360 KB 5.25-inch drive, and the PCjr supports one drive only. High-density media cannot be used in it.

The drive is belt-driven. IBM's belt is part 8285998 and the drive fan assembly part 8285982. To fit a belt, turn the drive on its back and place the belt over both pulleys, small pulley first, with the smooth side against the pulleys.

IBM's speed adjustment (service manual pp. 4-600-16 to 4-600-18):

1. Switch off and remove the drive and its mounting bracket.
2. Jumper test point 9 to position 11 of resistor block RN1.
3. Reconnect the drive power cable, turn the drive on its back and switch on. The motor runs for as long as the jumper is fitted.
4. Under fluorescent light, watch the black strobe marks on the outer ring of the drive pulley and turn the speed variable resistor until they appear to stand still.
5. Switch off, remove the jumper, refit the bracket, adjust it (p. 4-600-10) and refit the drive.

A diskette test result ending in 98 sends IBM's procedure to this adjustment, with drive replacement if the speed cannot be set. To ship the drive, IBM specifies the generic shipping insert P/N 6447190 with about half an inch trimmed off, or its back tab removed, so the latch lever can close. General drive work is covered in Floppy Drive Repair.

Power Board checks

IBM's service limits, measured on the Power Board with the drive cables unplugged:

IBM PCjr Power Board checks (HMS pp. 3-020-8 to 3-020-10; SA38 pp. 2-7 to 2-8)

Measurement	Pins	Limits
Transformer output plug (unplugged from system unit)	1 to 3	15.0 to 30.0 V AC
Diskette-drive power connector, +12 V	1 to 2	+11.4 to +12.6 V
Diskette-drive power connector, +5 V	4 to 3	+4.7 to +5.3 V
Diskette-drive fan connector	2 to 1, and 2 to 3	+11.4 to +12.6 V

IBM's ratings for the outputs:

IBM PCjr DC outputs (Technical Reference p. 2-136)

Rail	Minimum load	Maximum load	Regulation
+5 V	1.5 A	3.6 A	±5 %
+12 V	0.04 A	1.2 A	±5 %
−6 V	0.0 A	0.025 A	±16 %

The −6 V output is present only when +5 V carries at least 1.5 A. The +5 V over-current protection resets after the fault is removed and the power has been off for at least 5 seconds; IBM does not mark the +12 V over-current protection as resettable. Over-voltage protection is by fuse F1.

Batteries

The only batteries in the PCjr are the keyboard's four AA cells. IBM's list of system-board functions includes no real-time clock or battery-backed memory. For cleaning up battery leakage see Battery Explosion, Capacitor or Corrosion Damage.

Connectors

IBM issued snap-in connector guides for the back cover to stop the system-board connector pins being broken. The kit is P/N 6447163, was a no-cost part, and needs the cables removed to fit. If the diagnostics need the service plug, remove the guides carefully and refit them afterwards.

Tools

IBM's diagnostics need a service plug, a POST-loop plug, a serial wrap plug and a parallel printer attachment wrap plug, with a Triplett Model 310 multimeter or equivalent. The service manual gives the plugs as parts 1503441, 1503442 and 1503443 and the parallel wrap plug as 8529228; IBM's 1989 service manual lists the first three as 6447196, 6447197 and 6447198. See also Recommended Tools.

Related pages

• IBM PCjr
• IBM PCjr Troubleshooting Guide
• IBM PCjr Capacitor Replacement Guide
• IBM PCjr Technical Reference
• IBM PC (5150) Maintenance Guide
• Floppy Drive Repair