IBM PCjr Troubleshooting Guide: Difference between revisions

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[[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.

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* 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 ==

== Related Pages ==

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+[[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.
@@ Line 117: / Line 118: @@
 * 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 ==