IBM PCjr Troubleshooting Guide

IBM PCjr with the PCjr Color Display. Photo: Rik Myslewski, Wikimedia Commons, CC0.

IBM's service procedure for the IBM PCjr (type 4860) starts from advanced POST, a longer power-on test run with a service plug fitted, and uses its beeps and error codes to pick a Problem Isolation Chart (PIC). The procedure and the error table are in IBM's PCjr Hardware Maintenance and Service manual and in chapter 2 of the Personal Computer Family Service Information Manual. IBM's diagnosis stops at field-replaceable units (FRUs); the IBM PCjr Capacitor Replacement Guide covers going further on the Power Board.

What you need

IBM lists a known-good display or television, a service plug, a POST-loop plug, a serial wrap plug, a parallel printer attachment wrap plug and a Triplett Model 310 multimeter or equivalent. The 1983 parts catalogue numbers the plugs 1503441 (service), 1503442 (POST loop), 1503443 (serial wrap) and 8529228 (parallel wrap); the 1989 manual gives 6447196, 6447197 and 6447198 for the first three.

Advanced POST

1. Switch off the system unit and the display.
2. Plug the transformer into a working, earthed outlet.
3. Fit all connectors securely.
4. Remove any cartridge, cassette or diskette.
5. If the keyboard is used without its cord, put it within 12 inches of the front of the system unit with nothing between its infrared emitter and the receiver.
6. Plug the service plug into the system unit's J connectors.
7. Set the display's brightness, contrast and volume to mid-range, and switch the display on.
8. Switch on the system unit.

Advanced POST takes up to a minute and shows a stable IBM logo with a 16-colour test pattern. A failure shows as no beep, two beeps or three beeps, a wrong screen, or an error message. To repeat it, switch off and wait 5 seconds before switching on again.

IBM PCjr advanced POST error table (SA38 Figure 2-9, p. 2-4)

POST result	Problem area	Probable cause
No beep; no image or wrong image	Power	System board or Power Board
One beep; no image or wrong image	Display	System board or 64KB Memory and Display Expansion
Two beeps; no image or wrong image	Power	System board or Power Board
Three beeps; no image or wrong image	Memory	64KB Memory and Display Expansion
Advanced POST, or part of it, loops	System board	System board
No beep; ERROR 0AXX	Memory	System board
No beep; ERROR 0BXX	Memory	64KB Memory and Display Expansion
No beep; ERROR 0CXX	Memory	64KB Memory and Display Expansion
No beep; ERROR 1YXX	Memory	System board
No beep; ERROR 2000	Keyboard	Keyboard
No beep; ERROR 21XX	Infrared	Infrared receiver
No beep; ERROR 22XX	Keyboard	Keyboard
No beep; ERROR 23XX	Cassette or serial	System board
No beep; ERROR 24XX	Modem	Internal modem
No beep; ERROR 25XX	Cartridge	(none given)
No beep; ERROR 26XX	Diskette drive	Diskette-drive adapter
No beep; ERROR 27XX	Cluster	Cluster adapter
No beep; ERROR 28XX	Speech	Speech adapter
Any other ERROR message	Service plug	Check the service plug is good and fitted, and repeat advanced POST. If the message repeats, replace the system board.

If an error code and an audio error appear together, IBM says to ignore the audio error and test the FRU the code points to.

Advanced tests

After advanced POST reaches the BASIC screen, hold Ctrl and Alt and press Ins. The screen blanks, the IBM logo appears, the diskette light flashes if a drive is fitted, one beep sounds, and the advanced-test menu appears. The menu shows a symbol for each part of the machine, with a test ID under it; the ID blinks when the machine senses that device's interface. The joystick and sound IDs always blink because their interfaces are on the system board. The customer-level menu looks similar but shows 6 under the joystick symbol where the advanced menu shows E.

• While the menu is shown, a memory test runs continuously. The number at the bottom of the screen is the memory segment being tested; if it stops and the * beside it becomes an error message, note the message and go to the memory PIC.
• Move to a test ID with Ins and start it with Enter. A * under the ID when the test ends means no failure was found.
• Fn then B stops a test; "FFFF" means the test was stopped before it finished.
• Ctrl-Alt-Del leaves the menu.

For the diskette test, IBM's PIC treats a result ending 97 as no drive sensed, and one ending 98 as a speed fault: adjust the drive speed (service manual p. 4-600-16, and see IBM PCjr Maintenance Guide) and replace the drive if the speed cannot be set.

Customer error codes

The owner's Guide to Operations has its own single-letter error codes. They are not advanced-diagnostic codes, and IBM says actual failures should be found with the advanced diagnostics.

PCjr customer error codes (SA38 p. 2-7)

Code	IBM's action
A, D, G, X	Have the system serviced
B	Keyboard problem: move away from bright light
C	Cassette problem: if the cassette is not in use, press Enter and continue
E	Modem problem: if the modem is not needed, press Enter to continue

No power

IBM's power PIC isolates the FRU by removal:

1. Switch off. Disconnect everything outside the system unit except the transformer cable and the service plug. Switch on and wait a minute. If there is now one beep, reconnect the devices one at a time, running advanced POST after each, until the failure returns. Attach the TV connector before the television.
2. Remove the 64KB Memory and Display Expansion and run advanced POST. If there is now one beep, fit a new expansion; if that gives one beep the old expansion was faulty, otherwise the system board is.
3. Disconnect the diskette-drive adapter, the drive power cable, the drive fan cable and the internal modem. If there is now one beep, reconnect them one at a time to find the faulty item.
4. Measure the transformer and the Power Board as in the table below.
5. If the voltages are correct, fit a new Power Board and wait a minute. One beep means the old Power Board was faulty; no beep means the system board is.

IBM PCjr power checks (HMS pp. 3-020-8 to 3-020-10)

Measurement	Pins	Limits	If outside limits
Transformer output plug, unplugged from the system unit	1 to 3	15.0 to 30.0 V AC	Replace transformer
Power Board diskette-drive power connector (drive cables unplugged)	1 to 2	+11.4 to +12.6 V	Replace Power Board
Power Board diskette-drive power connector	4 to 3	+4.7 to +5.3 V	Replace Power Board
Power Board diskette-drive fan connector	2 to 1, and 2 to 3	+11.4 to +12.6 V	Replace Power Board

The transformer delivers AC; the Power Board inside the system unit rectifies and regulates it. The +5 V over-current protection trips at 3.9 ± 0.25 A and resets only after the fault is removed and the power has been off for at least 5 seconds. The +12 V protection trips at 2.2 ± 0.9 A; IBM does not mark it as resettable. A shorted capacitor therefore keeps the machine dead. minuszerodegrees.net describes shorted tantalum capacitors overloading IBM 51xx supplies in the same way, often with no visible damage. One collector found a PCjr Power Board with a capacitor blown apart and soot over the inside of the machine.

The transformer has no switch of its own. Retrotronics measured about 4 W drawn at the outlet with the PCjr switched off, and about 34 W with the system unit idle at a DOS prompt.

Keyboard

• Fit the keyboard cord. It disconnects the batteries and the keyboard's infrared transmitter, and disables the system unit's infrared receiver. If the keyboard works on the cord and not over infrared, the fault is in the infrared path: ERROR 21XX points at the receiver.
• The cord's continuity, 2 × 3 connector to keyboard plug: A2 to 5, B2 to 2, B3 to 4.
• IBM treats the keyboard as a FRU with nothing repairable inside.
• Bright light interferes with the infrared link (customer code B), and the keyboard needs a clear line of sight to the front of the system unit.

Display

IBM's voltages at connector D on the system board:

PCjr display connector D voltages (SA38 Figure 2-22)

Pins	Low (V DC)	High (V DC)
A2 to A4	1.9	2.3
A2 to A5	1.8	2.3
A2 to A6	0.0	1.0
A2 to A7	1.7	2.3
A2 to B1	0.1	0.3
A2 to B3	0.3	0.5

The Color Display adapter cable wires its 9-pin connector to the system unit's 2 × 9 connector as 1–B9, 2–B5, 3–A5, 4–A7, 5–A4, 6–A6, 8–B3, 9–B1; pin 7 is not used. One beep with no image or a wrong image points at the system board or the 64KB Memory and Display Expansion.

Things that are not faults

• Keystrokes lost, with beeps, while the diskette drive is working. The PCjr has no DMA controller, so the processor services the drive itself.
• No −6 V on a lightly loaded Power Board. IBM's −6 V output needs at least 1.5 A on +5 V.
• Three keys pressed together being ignored. The keyboard's phantom-key detection discards invalid combinations of three or more keys.
• The machine resetting when a cartridge is inserted or removed. Two cartridges wired for the same address region usually lock the machine up.
• Erratic running with several sidecars fitted. IBM gives 400 mA of +5 V as the spare capacity of a fully configured machine.
• With no internal modem, the serial port is COM1 but uses the COM2 address and IRQ 3; fitting the modem makes the modem COM1.

Related pages

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