Editing IBM PCjr Maintenance Guide

This guide documents preventive maintenance procedures for the IBM PCjr (4860). The PCjr is mechanically and electrically simpler than the AT or even the 5150, but has a small number of PCjr-specific maintenance points (no battery-backed CMOS, the infrared keyboard receiver, the sidecar latch).

== Regular Cleaning ==

* '''Power off and unplug''' the system unit and its external AC brick before opening.
* Remove the system unit cover. The PCjr cover slides forward and lifts off after one screw at the rear is removed.
* Use a soft brush and low-pressure compressed air to clear dust from the motherboard, the floppy drive (4860-067 only), the cartridge slot fingers and the sidecar connector.
* Avoid blowing dust into the floppy drive head, the keyboard IR receiver lens (at the front of the system unit) or the cartridge slot contacts. Hold the floppy drive spindle by hand if compressed air is used near the drive.
* If sidecars are fitted, unlatch and remove them before cleaning, and clean the sidecar connector on both the system unit and each sidecar before reseating.

The cartridge slots on the front of the system unit accumulate dust and oxide. Clean them with a soft eraser or a deoxidising contact cleaner, applied sparingly with a foam swab. Do not use cotton-tipped swabs &mdash; stray fibres in the connector are a known cause of intermittent cartridge faults.

== Keyboard Maintenance ==

The infrared keyboard contains four standard alkaline cells (typically 4&times;AA). Replace the cells when typing becomes erratic or the IR transmission distance shortens.

* The IR transmitter at the front-left of the keyboard fires an encoded burst on each keypress.
* The IR receiver on the front-right of the system unit decodes the burst.
* Clean the IR transmitter lens and the IR receiver window with a dry microfibre cloth; do not use solvents directly on the plastic lenses.
* If the keyboard becomes unreliable, IBM also supplied a '''keyboard cord''' that plugs into the keyboard and the system unit (bypassing the IR link entirely). Using the cord is a reliable diagnostic step.

Strong direct sunlight, certain office fluorescent lighting and proximity to other IR remotes (TV, VCR) interfere with the keyboard receiver. The PCjr keyboard cannot reliably share a room with another wireless PCjr keyboard.

The original "chiclet" keyboard variant is widely considered uncomfortable for any sustained typing. IBM offered a free upgrade to the revised typewriter-style keyboard for owners after mid-1984; many surviving PCjrs are fitted with the revised keyboard. Both keyboards use the same 62-key layout and IR encoding.

== Floppy Drive (4860-067) ==

* The PCjr ships with a '''360 KB''' 5.25" half-height drive. Most surviving units are Mitsubishi, Tandon or Qume drives.
* Use '''DSDD''' (double-sided, double-density) 5.25" media only. '''Do not use HD media''' in a 360 KB drive &mdash; the high-coercivity HD media takes a poor magnetisation from the 360 KB drive's write current.
* Clean the heads periodically with a head-cleaning diskette or with isopropyl alcohol on a foam swab (drive off, by hand-rotating the spindle).
* The PCjr's lack of a DMA controller means floppy reads/writes are done by the CPU in tight PIO loops, which steals all CPU time during floppy access. Keystrokes from the IR keyboard arrive during this period but the keyboard interrupt handler does not run, so they are '''lost'''. This is normal PCjr behaviour, not a fault.

== Cartridge Slots ==

* Two ROM cartridge slots on the front of the system unit. Each slot accepts a cartridge of up to 64 KB.
* Cartridges should only be inserted or removed with the '''system unit powered off'''. Hot-swapping a cartridge can cause the system to crash or, in rare cases, damage the cartridge or motherboard chip select logic.
* Edge contacts oxidise over time. Inspect and clean the cartridge edge connector and the slot fingers periodically with a soft eraser or a deoxidising contact cleaner.

== Sidecar Maintenance ==

Sidecars latch to the right side of the system unit through a 60-pin proprietary connector. Periodically:
* Unlatch each sidecar and remove it.
* Inspect the connector on the sidecar and on the system unit for bent pins or oxidation.
* Clean contacts with a deoxidising spray (sparingly) and a foam swab.
* Re-latch firmly. A loose sidecar produces intermittent operation and may corrupt RAM contents.
* If multiple sidecars are chained, the PCjr Power Expansion Attachment is sometimes required to supply the additional current. Without it, current-hungry sidecars (modem, hard drive interface) can pull the system rails low enough to crash the machine.

== PSU (External AC Brick) ==

The PCjr uses an external power brick that supplies regulated DC to the system unit through a multi-pin DIN connector at the rear. Replacement bricks are scarce; many surviving units have aging electrolytic capacitors inside.

* If the system reports random parity-like memory crashes, video noise or sidecars dropping out, verify the DC voltage at the system unit connector with a multimeter (referring to the PCjr Technical Reference for the pinout).
* If the brick is faulty, the bench-friendly fix is to fit modern replacement electrolytics inside the brick. See [[IBM PCjr Capacitor Replacement Guide]] for the procedure.

== CMOS / RTC ==

The PCjr does '''not''' have a battery-backed CMOS or real-time clock. There is no CMOS to lose; there is no SETUP screen to run; the date and time must be set manually at every boot through DOS or via cartridge software. This makes the PCjr immune to the 161/162/163 CMOS battery faults that plague the 5170.

== Connector Care ==

The plastic spacers between back-panel connectors are thin and brittle; they break off readily if a cable is pulled out at an angle. Always disconnect cables straight back from the connector. Pins on the keyboard cord and the printer cable bend easily &mdash; check pin alignment before pushing a connector home.

== Capacitor Health ==

Tantalum capacitor failure on the PCjr motherboard follows the same pattern as on the 5150/5160/5170. The PCjr is less prone than the 5170 because the board carries fewer rails and fewer tantalum capacitors, but failure does occur. Full procedure: [[IBM PCjr Capacitor Replacement Guide]].

== Recommended Tools ==

* Philips #2 screwdriver, T15 Torx for some service-replaced parts.
* Anti-static strap connected to chassis ground.
* Digital multimeter.
* Compressed air can.
* Lint-free wipes and isopropyl alcohol.
* Spare 4&times;AA alkaline cells for the keyboard.
* Soldering iron with a fine tip for capacitor work.
* Foam swabs and contact cleaner spray.

== Related Pages ==

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

== References ==

* IBM, ''IBM PCjr Technical Reference''. Source for motherboard chip layout, sidecar pinout and cartridge interface.
* [https://www.brutman.com/PCjr/ Mike Brutman's PCjr resource]. Reference for keyboard handling and sidecar operation.
* [https://minuszerodegrees.net/4860/doco/4860_documentation.htm IBM 4860 (PCjr) &mdash; Documentation pointers], minuszerodegrees.net.

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[[Category:IBM]]
[[Category:Maintenance Guides]]