IBM 5120 Troubleshooting Guide

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.
• 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:

1. Power LED on the front panel illuminates.
2. Fans spin (CRT and PSU cooling).
3. PSU rails stabilise.
4. Executable ROS self-test runs.
5. Language interpreter loads from Language ROS (front-panel toggle selects APL or BASIC).
6. Language banner ("BASIC READY" or "APL READY") appears on the 9-inch CRT.
7. 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. 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

1. Power on; observe LED, fan, display chain.
2. If no power, suspect PSU.
3. If power but no display, check CRT anode HV first, then Executable ROS / PALM.
4. If display but garbled banner, reseat ROS modules; try the other language.
5. Enter Diagnostic ROS (keyboard sequence at power-on); verify PALM, RAM, ROS.
6. Run the Customer Acceptance Test from diskette.
7. If diskette won't load, check drive belt, head, alignment.
8. 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

• Bitsavers — IBM 5120 documents. MIM SY34-0192, Logic Manual SY34-0193.
• IBM 5120 — Wikipedia.
• IBM PALM processor — Wikipedia.