Editing IBM PC (5150) Troubleshooting Guide

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This guide provides systematic, component-level troubleshooting for the '''IBM 5150'''. It covers POST beep codes, power-up failures, parity errors, video problems, keyboard 301 errors, and floppy faults. 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.

== Preliminary & Power-up Checks ==

The 5150 has '''no on-board POST code output port''' — a hardware POST card plugged into an ISA slot will not show meaningful codes. Use the audio beep and the on-screen error number instead.

=== POST sequence summary ===
On a healthy 5150, power-up produces:
# A single short beep after about five seconds.
# The memory count appears in the top-left of the display.
# The machine attempts to boot from the floppy in drive A, then falls back to [[IBM Cassette BASIC|Cassette BASIC]] if no boot disk is present.

=== Beep codes ===
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''IBM 5150 POST audio beep codes'''
! Pattern !! Meaning
|-
| 1 short beep || POST OK
|-
| ''No beep, no video'' || PSU, CPU, clock, reset, or bank-0 RAM failure
|-
| 1 long + 1 short || System board failure
|-
| 1 long + 2 short || Video adapter failure (or, on the 10/27/82 BIOS, a corrupted HDD ROM at C8000)
|-
| 1 long + 3 short || EGA/VGA card failure (where supported)
|-
| Continuous short beeps || PSU fault
|-
| Repeating short beep cycles || RAM failure
|}

{| class="wikitable styled-table" style="width:100%;"
|+'''Power-up symptom table'''
! Symptom !! Probable cause !! Action
|-
| No fan, no LED, no beep || Dead PSU; blown fuse; rear-panel switch || Test mains; check fuse; measure PSU rails with motherboard disconnected
|-
| Fan runs, no beep, no video || Bank-0 RAM, BIOS ROM (U33), or CPU || Reseat U33 and bank-0 RAM; probe clock at 8088 pin 19; try a known-good 8088
|-
| Fan runs, "131" error displays || &minus;5 V rail is missing (cassette I/O test fails) || Recap/replace PSU; do '''not''' use an ATX PSU without &minus;5 V
|-
| Tantalum cap audibly pops, PSU latches off || Short-circuit tantalum on the motherboard or an ISA card || Remove all cards; if PSU then runs, fault is on a card; otherwise see [https://www.minuszerodegrees.net/5150_5160/MDC/tantalum_short_plus_5volts.htm minuszerodegrees.net's +5 V short-circuit guide] or [https://minuszerodegrees.net/5150_5160/MDC/tantalum_short_plus_12volts.htm +12 V guide]
|}

=== No beep, no video ===
On a 5150, "dead machine" is most often one of: bank-0 RAM, the BIOS ROM, the CPU, or a missing &minus;5 V rail.

# Confirm '''+5 V''', '''+12 V''', '''&minus;5 V''' and '''&minus;12 V''' at the P8/P9 motherboard connector with the machine running. The '''&minus;5 V rail is required''' — a 16KB-64KB board with no &minus;5 V will silently fail the first 16 KB RAM test and halt with no audible or visual indication.
# Reseat '''U33 (BIOS)''', '''U29-U32 (Cassette BASIC)''', the '''8088 CPU''', and all of bank-0 RAM.
# Probe the '''8088 clock (pin 19)''' and the '''reset (pin 21)'''. If clock is missing, suspect the 8284 clock generator or the 14.31818 MHz crystal. If reset is stuck low, suspect the 8284 or the Power Good signal.
# Piggyback a known-good RAM chip on each bank-0 RAM position in turn.

== Display & Video Diagnostics ==

The 5150 has no on-board video — a faulty video card is the most likely cause of a no-video symptom even when the machine is otherwise alive.

=== No video, but POST beep is normal ===
* Confirm the video adapter card (MDA or CGA) is firmly seated.
* Check the monitor cable and the monitor itself.
* If both [[IBM Monochrome Display Adapter|MDA]] and [[IBM Color Graphics Adapter|CGA]] are fitted, SW1 bits select which is the boot adapter — see the IBM 5150 Technical Reference for the exact bit layout.

=== Garbled text or random characters ===
* Suspect the video adapter's character ROM or VRAM.
* Cracked solder joints on the DE-9 output connector.

== Memory & RAM Faults ==

=== Parity error 201 / "PARITY CHECK 1" / "PARITY CHECK 2" ===
* '''PARITY CHECK 1''' — the error is in motherboard RAM.
* '''PARITY CHECK 2''' — the error is in expansion-card RAM.

The numbered error format conveys bank and bit position. Reseat the chip at the indicated position; if reseating does not fix it, replace that single chip. If the position moves around between resets, suspect DRAM refresh (the 8237 DMA controller generates refresh cycles; a bad 8237 produces random RAM errors).

=== Bank 0 RAM dead ===
A failed chip in bank 0 produces a silent "dead" motherboard with no beep — easily confused with a CPU or PSU fault.

== Keyboard & I/O Failures ==

=== 301 error (keyboard) ===
* The keyboard did not return the expected response within the timeout.
* Reseat the DIN-5 connector.
* Swap to a known-good Model F. If the suspect keyboard works on another 5150 or XT, the 5150's keyboard interface is at fault — check U36 (8255 PPI) and the 7406 keyboard data buffer.
* '''Model F variants''': the [[IBM Model F (83-key)|83-key XT-style Model F]] works on the 5150 and 5160. The 84-key AT-style Model F (P/N 6450200) and the 101-key Model M will '''not''' work without a protocol converter.

=== Cassette port silence ===
A POST 131 error on the 10/27/82 BIOS indicates the cassette I/O test failed, which is most often caused by a missing &minus;5 V rail rather than a real cassette problem. Verify &minus;5 V at the PSU before chasing cassette hardware.

=== Floppy faults ===
* '''Drive light stays on continuously''' — cable inserted backwards, or termination resistor missing on the last drive.
* '''Drive seeks, but read errors''' — head alignment, or the floppy itself is media-failed.
* '''POST 6xx errors''' — the floppy controller card. Reseat the [[IBM FDD Adapter|FDD Adapter]].

== Component-level Tests ==

=== Voltage test points ===

{| class="wikitable styled-table" style="width:80%;"
! Test point !! Expected !! Notes
|-
| 8088 pin 40 (Vcc) || +5 V || Main logic rail
|-
| 8088 pin 19 (CLK) || ~4.77 MHz square wave || From 8284 clock generator
|-
| 8088 pin 21 (RESET) || Low at power-on, high after Power Good ||
|-
| DRAM Vcc || +5 V ||
|-
| Power Good (P8) || +5 V, rises ~100-500 ms after +5 V is stable || A stuck-low Power Good keeps the CPU in reset
|}

== Expansion Card Diagnostics ==

=== Process of elimination ===
# Strip the machine to motherboard + PSU + RAM + video card + keyboard.
# Confirm POST passes. If yes, add cards back one at a time, powering down between each.
# The first card that prevents POST is the suspect.

=== 16-bit ISA cards ===
Some 16-bit ISA cards advertised as "8-bit slot compatible" do not work in the 5150's 8-bit slots. Some require reconfiguration via jumpers; some require the wider AT-class slot spacing.

== References ==

* IBM, ''IBM 5150 Technical Reference'' (6025005, August 1981) — contains schematics, BIOS listing, and DIP-switch reference.
* [https://minuszerodegrees.net/5150/post/5150%20-%20POST%20-%20Some%20errors.htm 5150 POST errors], minuszerodegrees.net.
* [https://minuszerodegrees.net/5150/problems/5150_known_problems_issues.htm 5150 Known Problems / Issues], minuszerodegrees.net.

[[Category:IBM]]
[[Category:Troubleshooting Guides]]