Editing IBM 5120 Troubleshooting Guide (section)

== 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.