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Deep technical pre-PC IBM page with verified sources (Bitsavers MIM/MAP, Wikipedia, IBM Archives) — honest gap disclosure where IBM did not publish per-board cap values

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This guide documents fault diagnosis for the '''[[IBM 5120]] Computing System''' (also designated IBM 5110 Model 3). The 5120 inherits the PALM processor and APL / BASIC ROM environment from the [[IBM 5100]] and [[IBM 5110]], so much of the troubleshooting practice translates directly. The 5120's two built-in 8-inch floppy drives, larger 9-inch CRT, and detached keyboard add their own diagnostic surface.

== Reference Documents ==

* '''SY34-0192-0''' — IBM 5120 Maintenance Information Manual (MIM), December 1979 — the CE service manual, includes the diagnostic procedures.<ref>http://bitsavers.org/pdf/ibm/5120/</ref>
* '''SY34-0193-0''' — IBM 5120 Computing System Logic Manual, December 1979 — gate-level theory of operation, essential for board-level fault isolation.
* '''GH30-0232-1''' — Joint 5110 / 5120 Bibliography, September 1980.

== Initial Diagnosis Workflow ==

5120 power-on sequence:

# Power LED on the front panel illuminates.
# Fans spin (CRT and PSU cooling).
# PSU rails stabilise.
# Executable ROS self-test runs.
# Language interpreter loads from Language ROS (front-panel toggle selects APL or BASIC).
# Language banner ("BASIC READY" or "APL READY") appears on the 9-inch CRT.
# 5120 is ready for keyboard input.

If any of these does not occur, stop and diagnose at that stage.

=== Stage 1 — No Power ===

* Mains lead, switch, fuse on the 5120 PSU.
* Verify PSU rails at the planar power connector.
* If the front-panel power LED is dim, suspect 5VSB-equivalent regulator capacitor failure.

=== Stage 2 — Power But No Display ===

* '''No raster on 9-inch CRT''' — flyback or horizontal output failure. Check for HV at the CRT anode with an HV probe. No HV → flyback failure (rare but terminal without a donor).
* '''Raster but no characters''' — Executable ROS or video timing chain failure.
* '''Garbled characters''' — RAM (RWS) failure or Executable ROS bit error. Use Diagnostic ROS to verify.

=== Stage 3 — Banner Wrong / Garbled ===

* Language ROS bit error — reseat Language ROS.
* Toggle APL / BASIC — if one banner is clean and the other garbled, the failed ROM is isolated.

=== Stage 4 — System Won't Read / Write Floppies ===

* '''No drive activity at all''' — drive cable to planar unseated, or PSU spindle rail dead.
* '''Drive spins but won't read''' — head dirty; head alignment; drive belt slipping.
* '''Read OK but write produces corrupt diskettes''' — head alignment; write-protect sensor.
* '''CRC errors''' — belt aged, oxide shedding from old Diskette 2D media, head dirty.
* '''Won't load head''' — head load solenoid fatigued.

== Diagnostic ROS ==

Same keyboard sequence at power-on as the [[IBM 5100]] / [[IBM 5110]]; enters Diagnostic ROS for read / write access to RAM, video memory, PALM registers, interrupt vectors and the clock counter.<ref>https://en.wikipedia.org/wiki/IBM_5100</ref> The 5120 MIM SY34-0192 documents the Diagnostic ROS error codes specific to the 5120.

== Customer Acceptance Test ==

The 5120 CAT runs from a customer test diskette. It exercises:

* RAM (full pattern test).
* Both built-in 8-inch floppy drives (read / write / verify).
* The 9-inch CRT (cursor positioning and character set).
* The detached keyboard.
* The communications adapter (if fitted).
* The 5114 (if attached).

A test pattern of garbled characters at the same screen position across multiple tests usually indicates a specific bit position failure in a RAM bank.

== PALM Diagnosis ==

Same as the [[IBM 5100]] — PALM gate arrays are unobtainium. Reseat the edge connector before suspecting failure. PALM-specific check-stop codes in the MIM identify failed gate arrays.

== ROS / RWS Memory Faults ==

* '''Garbled banner''' — Language ROS bit error; reseat ROS modules.
* '''Refusal to complete Executable ROS self-test''' — Executable ROS bit error or PALM fault.
* '''RAM size mismatch on banner''' — RWS card failure; reseat each 16 KB module.

== 8-Inch Floppy Drive Faults ==

* '''No drive activity''' — PSU spindle rail (typically 12 V or 24 V), or controller card to drive cable.
* '''Drive spins but won't read''' — head dirty, head alignment, belt slipping.
* '''Won't load head''' — solenoid fatigued or stuck.
* '''Write errors but no read errors''' — write-protect sensor; or head alignment offset from write to read.
* '''CRC errors''' — belt aged; oxide shedding from old media; head dirty.
* '''Drive A: works, Drive B: doesn't (or vice versa)''' — drive select jumper or cable; check the position of the terminating resistor pack at the end of the daisy-chain (only the last drive should have the terminator).

The 5120 MIM SY34-0192 has a per-symptom decision tree for the 8-inch floppy subsystem.

== CRT Faults ==

* '''No raster''' — flyback or horizontal output transistor failure.
* '''Raster but no characters''' — video timing chain (PALM video output).
* '''Dim raster''' — cathode emission loss; CRT end-of-life.
* '''Single horizontal line''' — vertical deflection failed.
* '''Single vertical line''' — horizontal deflection failed (usually accompanied by flyback failure).
* '''Bright spot''' — both deflections failed; '''immediately power off''' to avoid phosphor burn.
* '''Phosphor burn-in''' (static image on inactive screen) — CRT has been operated for too long with fixed display; replacement requires donor.

The 9-inch CRT carries higher anode voltage than the 5-inch CRT on the 5100 / 5110; flyback failures are correspondingly more dramatic.

== Keyboard Faults ==

* '''No keyboard response''' — cable unseated; clean the keyboard connector with deoxidising contact cleaner.
* '''Repeating characters / sticky keys''' — buckling-spring or capacitive switch contamination; remove and clean.
* '''Specific key dead''' — switch contact contamination.
* '''All keys produce wrong characters''' — keyboard scan controller failure.

== Communications and RS-232 Faults ==

If a Communications Adapter or RS-232 channel is fitted:

* Verify cable.
* Verify baud / parity / stop bits.
* Test with a known-good target.

== Common Field Symptoms and Resolutions ==

* '''Won't power on''' — mains lead, switch, fuse, PSU primary.
* '''Power LED but no display''' — PSU bulk filter caps; see [[IBM 5120 Capacitor Replacement Guide]].
* '''Banner appears but system hangs on keystroke''' — keyboard cable or PALM input handling.
* '''Garbled banner''' — Language ROS or socket oxidation.
* '''Drive A: reads but Drive B: doesn't''' — drive select jumper or cable terminator.
* '''Both drives spin but won't read''' — head clean, then alignment, then belt.
* '''Random reboots / hangs''' — bulk filter cap ESR rising; recap PSU.
* '''Smell of fish from PSU''' — RIFA X2 mains-suppression capacitor venting; replace immediately.
* '''CRT loses focus after warm-up''' — flyback transformer insulation breakdown; replacement requires donor.

== Diagnostic Workflow Summary ==

# Power on; observe LED, fan, display chain.
# If no power, suspect PSU.
# If power but no display, check CRT anode HV first, then Executable ROS / PALM.
# If display but garbled banner, reseat ROS modules; try the other language.
# Enter Diagnostic ROS (keyboard sequence at power-on); verify PALM, RAM, ROS.
# Run the Customer Acceptance Test from diskette.
# If diskette won't load, check drive belt, head, alignment.
# Cross-reference any check-stop code against MIM SY34-0192 and Logic Manual SY34-0193.

== Related Pages ==

* [[IBM 5120]]
* [[IBM 5120 Maintenance Guide]]
* [[IBM 5120 Capacitor Replacement Guide]]
* [[IBM 5110 Troubleshooting Guide]] — sibling machine
* [[IBM 5100 Troubleshooting Guide]] — PALM / ROS diagnosis translates directly

== References ==

* [http://bitsavers.org/pdf/ibm/5120/ Bitsavers — IBM 5120 documents]. MIM SY34-0192, Logic Manual SY34-0193.
* [https://en.wikipedia.org/wiki/IBM_5120 IBM 5120 — Wikipedia].
* [https://en.wikipedia.org/wiki/IBM_PALM_processor IBM PALM processor — Wikipedia].

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

IBM 5120 Troubleshooting Guide - Revision history

Josh: Deep technical pre-PC IBM page with verified sources (Bitsavers MIM/MAP, Wikipedia, IBM Archives) — honest gap disclosure where IBM did not publish per-board cap values