Editing IBM PCjr Troubleshooting Guide

[[File:IBM PCjr (photo).jpg|thumb|right|300px|IBM PCjr. Source: Wikimedia Commons.]]
This guide covers diagnostic procedures for the IBM PCjr (4860). The PCjr has a smaller and less rigorous POST than the 5150/5160/5170, and some of the most common PCjr complaints are software/timing issues rather than hardware failures.

== Power-On Audio Beep Codes ==

The PCjr BIOS uses the same beep coding scheme as the 5150 family. Beeps are produced by the PC speaker, mixed in with the SN76496 audio.

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''IBM PCjr power-on audio codes'''
! Pattern !! Meaning
|-
| (no beep) || No power, no system clock, bank-0 RAM failure, dead CPU or dead motherboard
|-
| 1 short || POST OK
|-
| 2 short || Numeric error on screen
|-
| 1 long, 1 short || Motherboard failure
|-
| 1 long, 2 short || Display adapter failure (video circuitry)
|-
| Continuous || PSU or motherboard fault &mdash; check external brick voltage and motherboard tantalums
|}

== Numeric POST Codes ==

The PCjr ROM POST is shorter than the AT's; the BIOS reports a small set of codes on the screen if the failure occurs after the video has initialised.

{| class="wikitable styled-table" style="width:100%; text-align:center;"
!Code!!Meaning
|-
| 101 || System board error
|-
| 102 || ROM checksum error &mdash; BIOS, BASIC, or one of the installed cartridges
|-
| 110 || Motherboard parity error (note: PCjr does not implement DRAM parity check; this code is reserved on the PCjr)
|-
| 201 || Memory test failure &mdash; the bytes preceding the "201" identify the failing segment / bit
|-
| 301 || Keyboard error (also seen when the IR receiver does not receive a valid wake-up burst within the keyboard test window)
|-
| 401 || Display adapter test failure (built-in video)
|-
| 501 || Display alternate test failure (RGB / composite output)
|-
| 601 || Diskette / drive test failure (4860-067 only)
|-
| 1301 || Internal modem test failure (when the IBM PCjr Internal Modem sidecar is present)
|}

== Common Symptoms ==

=== System will not power up ===

* Verify the wall outlet, the AC mains lead, and the external power brick output. The PCjr's brick supplies regulated DC at the system unit's DIN connector; check the pinout in the IBM PCjr Technical Reference.
* Verify there is no short on the brick output. A failed tantalum on the motherboard's +5 V line will pull the brick to fold-back protection and the system will not power up.
* Remove all sidecars and cartridges. Re-test. A faulty sidecar (especially a memory or modem sidecar with a failed tantalum) is a common cause of "PCjr will not start."

=== Keyboard not responding ===

* Replace the 4&times;AA cells in the keyboard. Low cells reduce IR range.
* Move the keyboard within 10 ft / 3 m of the system unit IR receiver and direct it towards the system unit.
* Reduce ambient IR interference: close blinds against sunlight, turn off plasma TVs and CCFL fluorescent fittings near the system.
* If the keyboard still fails, fit the optional '''keyboard cord''' between the keyboard and the system unit. If the cord works, the fault is in the IR transmitter or receiver. If the cord also fails, the fault is in the keyboard scan matrix or the system unit's keyboard controller.
* The PCjr Technical Reference documents a self-test mode that lights one LED for each pressed key &mdash; useful for confirming key matrix faults.

=== Keystrokes lost during disk access ===

This is a '''known PCjr limitation, not a fault'''. The PCjr has no DMA controller, so floppy reads/writes consume all CPU time. The IR keyboard interrupt cannot be serviced during disk access and the keystroke is dropped. The Tandy 1000 has the same limitation in its earliest revision. Workarounds: avoid typing during disk access; use a directly-cabled keyboard with hardware that buffers keystrokes; install a third-party PCjr DMA-style accelerator (some sidecars added a DMA controller).

=== Cartridge does not work ===

* Power off, remove and reinsert the cartridge.
* Clean the cartridge edge connector and the slot fingers with a soft eraser or contact cleaner.
* Verify the cartridge is one of the two PCjr cartridge formats (32 KB or 64 KB ROM with proper /CS routing).
* Some third-party cartridges (especially homebrew SD-card cartridges) require very precise chip-select timing; if the cartridge does not work on one slot, try the other.
* If the system boots normally to BASIC without any cartridge but hangs at POST with the cartridge fitted, the cartridge ROM may have a corrupt header (the PCjr BIOS scans cartridges at well-known offsets during POST).

=== Sidecar fault ===

* Unlatch all sidecars and boot. If the system POSTs, re-add sidecars one at a time.
* Verify each sidecar's edge connector is clean and seats fully.
* A modem sidecar or a memory sidecar with a failed tantalum on its internal regulator will pull the system rails down. Replace the cap (see [[IBM PCjr Capacitor Replacement Guide]]).
* If multiple sidecars are fitted and the PCjr now resets randomly, current draw may exceed what the standard external brick can supply. Fit the IBM PCjr Power Expansion Attachment.

=== Video noise / wavy display ===

* Verify the composite or RGB cable is fully seated and not running parallel to mains cables.
* Verify the external power brick is supplying clean DC; ripple from aged electrolytics in the brick produces noise on the video output.
* The PCjr TV Connector (RF modulator) is inherently noisier than the direct RGB output to the IBM 4863 display; use RGB if available.

=== "PCjr cliff" software incompatibility ===

Some PC-compatible software written before late 1984 fails to run on the PCjr because of differences in the memory map (video RAM placement) and the absence of DMA. This is a software compatibility issue, not a hardware fault. A common symptom is the program loading and then immediately exiting to DOS, or hanging at a blue/black screen. Use software that explicitly supports the PCjr or use a sidecar that remaps video memory (Racore PC Drive, Tecmar Captain Jr.).

== No-Beep / No-Video Diagnostics ==

# Verify the external AC brick is producing DC at the system unit connector.
# Verify the system unit's +5 V rail comes up (probe on the motherboard).
# Pull all sidecars and cartridges. Re-test.
# If still dead, swap the keyboard (or fit the cord) to rule out the keyboard receiver.
# If still dead, reseat the BIOS ROM and the CPU.
# Inspect motherboard tantalum capacitors for shorts (see [[IBM PCjr Capacitor Replacement Guide]]).
# Bank-0 RAM failure on the PCjr motherboard produces a 'dead' system. The PCjr Technical Reference identifies the bank-0 chips by silkscreen.

== Cassette Port ==

The PCjr cassette port is functional and supported by Cassette BASIC in ROM. Most surviving units have never had it used. If you wish to use it, see the PCjr Technical Reference for the pinout and PCjr BASIC's CSAVE / CLOAD commands.

== Battery / RTC ==

The PCjr has no battery-backed CMOS and no RTC. There is no battery to replace and no SETUP to run. If the date and time are wrong on every boot, that is normal &mdash; set them manually from DOS (DATE and TIME commands) or via a sidecar that adds an RTC.

== Internal Modem (4860-067) ==

If the optional IBM PCjr Internal Modem sidecar is fitted and the system reports a 1301 error:
* Check that the sidecar latches fully.
* Verify telephone-line connectivity (RJ-11 phone jack to wall and to the modem).
* Some modems require a specific country code jumper; verify against the modem documentation.

== ⚠️ Power supply (the PCjr's weak point) ==

The PCjr is otherwise robust, but its '''internal power-supply board''' is under-engineered and often fails. When its capacitor fails it can spread '''black soot over every internal component'''. The board was rated 33 W originally and 45 W later, and even the 45 W version struggles to run more than about three sidecar expansions (the original supplies only 0.4 A at 5 V on the sidecar connector). The external transformer runs hot and draws power even when the machine is off. On a dead PCjr, check the internal power board first; many owners fit a modern replacement ("jrPSU" board or an ATX conversion).<ref name="pcjr">[https://oldsilicon.com/personal-computers/ibm-pcjr/ OldSilicon — IBM PCjr]; [https://www.retrotronics.org/home-page/jrpsu/ retrotronics — PCjr Power Supply (jrPSU)]; [https://www.brutman.com/PCjr/pcjr_faq.html IBM PCjr FAQ], Brutman; and [https://hackaday.com/2018/08/10/ibm-pcjr-revived-by-an-atx-power-supply-and-many-false-starts/ Hackaday]. Source for the internal power-board failure (capacitor failure spreading soot), the underpowered 33W/45W ratings and sidecar current limits, the transformer heat, and the harmless green choke-coil wax.</ref>

'''Not a fault:''' the input choke coil often oozes a '''green waxy filler''' after years of heat — that is just the coil's potting compound and is harmless, not a failure.<ref name="pcjr">[https://oldsilicon.com/personal-computers/ibm-pcjr/ OldSilicon — IBM PCjr]; [https://www.retrotronics.org/home-page/jrpsu/ retrotronics — PCjr Power Supply (jrPSU)]; [https://www.brutman.com/PCjr/pcjr_faq.html IBM PCjr FAQ], Brutman; and [https://hackaday.com/2018/08/10/ibm-pcjr-revived-by-an-atx-power-supply-and-many-false-starts/ Hackaday]. Source for the internal power-board failure (capacitor failure spreading soot), the underpowered 33W/45W ratings and sidecar current limits, the transformer heat, and the harmless green choke-coil wax.</ref>

== Infrared keyboard ==

The PCjr's cordless keyboard talks to the machine over '''infrared''' and runs on batteries. "Dead keyboard" faults are usually flat batteries, a blocked line-of-sight to the IR receiver, or bright ambient light/interference — fit fresh batteries and test in line of sight before suspecting the keyboard or receiver. A wired keyboard cord was also available and bypasses the IR link for testing.<ref name="pcjr">[https://oldsilicon.com/personal-computers/ibm-pcjr/ OldSilicon — IBM PCjr]; [https://www.retrotronics.org/home-page/jrpsu/ retrotronics — PCjr Power Supply (jrPSU)]; [https://www.brutman.com/PCjr/pcjr_faq.html IBM PCjr FAQ], Brutman; and [https://hackaday.com/2018/08/10/ibm-pcjr-revived-by-an-atx-power-supply-and-many-false-starts/ Hackaday]. Source for the internal power-board failure (capacitor failure spreading soot), the underpowered 33W/45W ratings and sidecar current limits, the transformer heat, and the harmless green choke-coil wax.</ref>

== Tantalum capacitors ==

As on other IBM boards of the era, '''tantalum capacitors''' on the planar can fail short with age and prevent start-up; look for a cracked or discoloured tantalum on a dead board.<ref name="pcjr">[https://oldsilicon.com/personal-computers/ibm-pcjr/ OldSilicon — IBM PCjr]; [https://www.retrotronics.org/home-page/jrpsu/ retrotronics — PCjr Power Supply (jrPSU)]; [https://www.brutman.com/PCjr/pcjr_faq.html IBM PCjr FAQ], Brutman; and [https://hackaday.com/2018/08/10/ibm-pcjr-revived-by-an-atx-power-supply-and-many-false-starts/ Hackaday]. Source for the internal power-board failure (capacitor failure spreading soot), the underpowered 33W/45W ratings and sidecar current limits, the transformer heat, and the harmless green choke-coil wax.</ref>

== References ==

<references />

== Related Pages ==

* [[IBM PCjr]]
* [[IBM PCjr Maintenance Guide]]
* [[IBM PCjr Capacitor Replacement Guide]]
* [[IBM PC (5150) Troubleshooting Guide]] &mdash; sibling business PC

== References ==

* [https://www.brutman.com/PCjr/pcjr_keyboard_handling.html IBM PCjr Keyboard Handling], Mike Brutman.
* [https://www.brutman.com/PCjr/pcjr_hardware.html IBM PCjr Hardware], Mike Brutman. Reference for the no-DMA limitation and sidecar bus behaviour.
* IBM, ''IBM PCjr Technical Reference''.
* [https://helppc.netcore2k.net/hardware/diagnostic-codes HelpPC &mdash; Diagnostic Codes].

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