IBM PC (5150) Troubleshooting Guide: Difference between revisions

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[[File:IBM PC (5150) (photo).jpg|thumb|right|300px|IBM PC (5150). Source: Wikimedia Commons.]]

This guide provides systematic, component-level troubleshooting for the '''IBM PC (5150)'''. It covers the POST audio beep codes, the complete numeric error code list, parity errors, video problems, keyboard 301 errors, floppy faults and known motherboard problems. The 5150 has no battery-backed CMOS — all hardware configuration is set through DIP switches SW1 and SW2 on the motherboard, so verify those before chasing a hardware fault.

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IBM PC/XT switching supplies also need a '''minimum load''' to start, so a bare supply on the bench may not run without a dummy load.<ref name="ibm_rifa">[https://www.minuszerodegrees.net/failure.htm minuszerodegrees.net — IBM failure symptoms]; [https://www.classic-computers.org.nz/blog/2010-11-04-restoring-an-IBM-xt.htm Repairing and Restoring an IBM XT]; and [https://retrorepairsandrefurbs.com/2025/05/15/1983-ibm-pc-5160-xt-power-supply-rebuild-modifications/ Adam's Vintage Computer Restorations]. Source for the RIFA mains-filter capacitor failing short (smoke) and the tantalum capacitors failing short and preventing the PSU from firing.</ref>

== POST codes and diagnostics (deep dive) ==

A 5150 that fails POST before the video is initialised looks completely dead — no picture and no beep — so treat a silent, pictureless machine as a power / planar / CPU / ROM fault, not a video fault.<ref name="post5150">[https://minuszerodegrees.net/5150/post/5150%20-%20POST%20-%20Detailed%20breakdown.htm IBM 5150 – POST – Detailed breakdown] and [https://minuszerodegrees.net/5150/post/5150%20-%20POST%20-%20Some%20errors.htm Some errors], MinusZeroDegrees; and [http://ps-2.kev009.com/pcpartnerinfo/ctstips/49fe.htm IBM PC and PS/2 – Error codes]. Source for the 1xx–6xx error-code families, the 201 memory decode (bank/bit), the 301 keyboard decode, and the dead-board behaviour when POST fails before the video step.</ref>

=== Beep summary ===

{| class="wikitable styled-table"

! Beep pattern !! Meaning

|-

| 1 short || POST passed (normal)

|-

| 2 short || POST error — read the numeric code on screen

|-

| Continuous, repeating, or no beep || Power supply, system board, or speaker

|-

| 1 long + 1 short || System board

|-

| 1 long + 2 short || Display adapter (MDA / CGA)

|}

=== Error-code families ===

The numeric codes are grouped by device; the leading digits identify the subsystem:

{| class="wikitable styled-table"

! Code !! Subsystem

|-

| 1xx || System board (planar)

|-

| 2xx || Memory (RAM)

|-

| 3xx || Keyboard

|-

| 4xx || Monochrome display adapter (MDA)

|-

| 5xx || Colour graphics adapter (CGA)

|-

| 6xx || Floppy drive / adapter

|}

=== 201 (memory) decode ===

A '''201''' is a RAM failure, and the four characters in front of it locate the bad chip: in the '''xyzz 201''' form, '''x = 0''' means the fault is on the planar, '''y''' is the failing bank (0–3) and '''zz''' identifies the failing bit or the parity chip. Use that to go straight to the offending DRAM rather than swapping a whole bank.<ref name="post5150">[https://minuszerodegrees.net/5150/post/5150%20-%20POST%20-%20Detailed%20breakdown.htm IBM 5150 – POST – Detailed breakdown] and [https://minuszerodegrees.net/5150/post/5150%20-%20POST%20-%20Some%20errors.htm Some errors], MinusZeroDegrees; and [http://ps-2.kev009.com/pcpartnerinfo/ctstips/49fe.htm IBM PC and PS/2 – Error codes]. Source for the 1xx–6xx error-code families, the 201 memory decode (bank/bit), the 301 keyboard decode, and the dead-board behaviour when POST fails before the video step.</ref>

=== 301 (keyboard) decode ===

A '''301''' means POST did not receive the AA self-test byte from the keyboard in time, or a stuck key was found; a stuck key is reported as its scan code, e.g. "301 xx". Check the keyboard and its cable before replacing the keyboard.<ref name="post5150">[https://minuszerodegrees.net/5150/post/5150%20-%20POST%20-%20Detailed%20breakdown.htm IBM 5150 – POST – Detailed breakdown] and [https://minuszerodegrees.net/5150/post/5150%20-%20POST%20-%20Some%20errors.htm Some errors], MinusZeroDegrees; and [http://ps-2.kev009.com/pcpartnerinfo/ctstips/49fe.htm IBM PC and PS/2 – Error codes]. Source for the 1xx–6xx error-code families, the 201 memory decode (bank/bit), the 301 keyboard decode, and the dead-board behaviour when POST fails before the video step.</ref>

== References ==

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 <templatestyles src="Template:StyledTable/styles.css" />
+[[File:IBM PC (5150) (photo).jpg|thumb|right|300px|IBM PC (5150). Source: Wikimedia Commons.]]
 This guide provides systematic, component-level troubleshooting for the '''IBM PC (5150)'''. It covers the POST audio beep codes, the complete numeric error code list, parity errors, video problems, keyboard 301 errors, floppy faults and known motherboard problems. The 5150 has no battery-backed CMOS — all hardware configuration is set through DIP switches SW1 and SW2 on the motherboard, so verify those before chasing a hardware fault.
@@ Line 435: / Line 436: @@
 IBM PC/XT switching supplies also need a '''minimum load''' to start, so a bare supply on the bench may not run without a dummy load.<ref name="ibm_rifa">[https://www.minuszerodegrees.net/failure.htm minuszerodegrees.net — IBM failure symptoms]; [https://www.classic-computers.org.nz/blog/2010-11-04-restoring-an-IBM-xt.htm Repairing and Restoring an IBM XT]; and [https://retrorepairsandrefurbs.com/2025/05/15/1983-ibm-pc-5160-xt-power-supply-rebuild-modifications/ Adam's Vintage Computer Restorations]. Source for the RIFA mains-filter capacitor failing short (smoke) and the tantalum capacitors failing short and preventing the PSU from firing.</ref>
+== POST codes and diagnostics (deep dive) ==
+A 5150 that fails POST before the video is initialised looks completely dead &mdash; no picture and no beep &mdash; so treat a silent, pictureless machine as a power / planar / CPU / ROM fault, not a video fault.<ref name="post5150">[https://minuszerodegrees.net/5150/post/5150%20-%20POST%20-%20Detailed%20breakdown.htm IBM 5150 &ndash; POST &ndash; Detailed breakdown] and [https://minuszerodegrees.net/5150/post/5150%20-%20POST%20-%20Some%20errors.htm Some errors], MinusZeroDegrees; and [http://ps-2.kev009.com/pcpartnerinfo/ctstips/49fe.htm IBM PC and PS/2 &ndash; Error codes]. Source for the 1xx&ndash;6xx error-code families, the 201 memory decode (bank/bit), the 301 keyboard decode, and the dead-board behaviour when POST fails before the video step.</ref>
+=== Beep summary ===
+{| class="wikitable styled-table"
+! Beep pattern !! Meaning
+|-
+| 1 short || POST passed (normal)
+|-
+| 2 short || POST error &mdash; read the numeric code on screen
+|-
+| Continuous, repeating, or no beep || Power supply, system board, or speaker
+|-
+| 1 long + 1 short || System board
+|-
+| 1 long + 2 short || Display adapter (MDA / CGA)
+|}
+=== Error-code families ===
+The numeric codes are grouped by device; the leading digits identify the subsystem:
+{| class="wikitable styled-table"
+! Code !! Subsystem
+|-
+| 1xx || System board (planar)
+|-
+| 2xx || Memory (RAM)
+|-
+| 3xx || Keyboard
+|-
+| 4xx || Monochrome display adapter (MDA)
+|-
+| 5xx || Colour graphics adapter (CGA)
+|-
+| 6xx || Floppy drive / adapter
+|}
+=== 201 (memory) decode ===
+A '''201''' is a RAM failure, and the four characters in front of it locate the bad chip: in the '''xyzz 201''' form, '''x&nbsp;=&nbsp;0''' means the fault is on the planar, '''y''' is the failing bank (0&ndash;3) and '''zz''' identifies the failing bit or the parity chip. Use that to go straight to the offending DRAM rather than swapping a whole bank.<ref name="post5150">[https://minuszerodegrees.net/5150/post/5150%20-%20POST%20-%20Detailed%20breakdown.htm IBM 5150 &ndash; POST &ndash; Detailed breakdown] and [https://minuszerodegrees.net/5150/post/5150%20-%20POST%20-%20Some%20errors.htm Some errors], MinusZeroDegrees; and [http://ps-2.kev009.com/pcpartnerinfo/ctstips/49fe.htm IBM PC and PS/2 &ndash; Error codes]. Source for the 1xx&ndash;6xx error-code families, the 201 memory decode (bank/bit), the 301 keyboard decode, and the dead-board behaviour when POST fails before the video step.</ref>
+=== 301 (keyboard) decode ===
+A '''301''' means POST did not receive the AA self-test byte from the keyboard in time, or a stuck key was found; a stuck key is reported as its scan code, e.g. "301&nbsp;xx". Check the keyboard and its cable before replacing the keyboard.<ref name="post5150">[https://minuszerodegrees.net/5150/post/5150%20-%20POST%20-%20Detailed%20breakdown.htm IBM 5150 &ndash; POST &ndash; Detailed breakdown] and [https://minuszerodegrees.net/5150/post/5150%20-%20POST%20-%20Some%20errors.htm Some errors], MinusZeroDegrees; and [http://ps-2.kev009.com/pcpartnerinfo/ctstips/49fe.htm IBM PC and PS/2 &ndash; Error codes]. Source for the 1xx&ndash;6xx error-code families, the 201 memory decode (bank/bit), the 301 keyboard decode, and the dead-board behaviour when POST fails before the video step.</ref>
 == References ==