Osborne Vixen Troubleshooting Guide

This guide provides systematic troubleshooting for the Osborne Vixen portable computer. As one of the last CP/M luggables, the Vixen’s reliability depends on healthy power, video, disk, and logic subsystems. This page details common failure symptoms, diagnostic steps, and component-level remedies.

Preliminary & Power-up Checks

Begin by confirming the Vixen’s power supply and startup sequence, as many issues stem from simple electrical faults.

Visual & Basic Checks

• Remove the case top and inspect for burnt components, corrosion, or leaking capacitors.
• Check for loose or oxidised connectors, especially at the power input and disk drives.
• Ensure all ribbon cables are firmly seated.

Power Supply & Voltage Checks

Test Point	Expected Voltage	Notes
Mainboard +5V rail	+5.0 V DC (±5%)	Logic supply—measure at RAM/CPU Vcc pins
Mainboard +12V rail	+12 V DC (±10%)	Disk drives, CRT, some logic
CRT board	+12 V DC, +180 V DC	For video and CRT anode (observe safety)
Floppy drive connector	+5 V / +12 V DC	Both rails required for operation

• If voltages are low or missing, check the internal fuses and PSU output.
• Inspect for cracked solder joints at the power jack and switch.

Power-up Symptoms Table

Symptom	Likely Cause	Action
No lights, no fan, no CRT glow	Blown fuse, dead PSU, broken switch	Test/replace fuse, check switch continuity, recap PSU
Fan runs, CRT dark, no beeps	CRT supply failure, video board fault	Check CRT voltages, inspect video board components
Power LED on, no disk activity	Logic board not running, CPU/RAM fault	Proceed to logic-level checks, test CPU reset/clock

Display & Chime Diagnostics

The Osborne Vixen has a built-in monochrome CRT. Video faults are common and often easy to localise.

CRT & Video Faults

Symptom	Probable Cause	Remedy
CRT glows, but no text or border	Video signal missing, logic board not running	Check CPU, ROM, RAM, and video output ICs
Horizontal line only	Vertical deflection failure	Inspect/replace vertical output IC, check for cold joints
Vertical line only	Horizontal deflection failure	Check horizontal driver transistor, flyback transformer
Dim or flickering display	Ageing CRT, weak PSU, bad capacitors	Adjust brightness, recap video board, check PSU
Garbled or missing characters	Video RAM or character ROM fault	Test/replace video RAM/ROM, reseat socketed chips

Diagnostic Steps

1. Observe CRT at power-on: does it glow, display raster, or show garbage?
2. Listen for relay click or faint high-voltage whine (indicates CRT HV present).
3. Adjust brightness/contrast—if raster appears, logic board is likely at fault.
4. If no glow, test CRT heater voltage and inspect video board for failed parts.

Floppy Disk & Storage Subsystem Failures

The Vixen uses standard 5.25" double-density drives. Disk faults are frequent due to drive age and connector issues.

Disk System Fault Table

Symptom	Likely Cause	Action
"BOOT ERROR" or no boot from disk	Bad disk, dirty heads, drive failure	Clean heads, try known-good disk, check drive power
Drive spins but no access	Ribbon cable loose, controller fault	Reseat cables, test controller ICs (e.g. WD1793)
"NO DISK" or "DISK ERROR" messages	Faulty drive select logic, bad ROM	Check drive select jumpers, test/replace ROM
Disk not ejected	Mechanical jam, failed eject solenoid	Inspect/repair drive mechanism

Floppy Drive Diagnostic Steps

1. Try booting from a known-good CP/M disk.
2. Listen for drive spin-up and head movement.
3. Swap drive ribbon cables; if problem moves, suspect drive.
4. If both drives fail, check disk controller IC (WD1793 or equivalent) and associated logic.

Memory & ROM Faults

RAM and ROM failures are a leading cause of "dead" or unstable Vixens.

Memory/ROM Fault Table

Symptom	Probable Cause	Action
Blank screen, no beep	RAM or ROM failure, CPU not running	Test/replace RAM (4164/41256), check ROM sockets
Random characters, system crashes	Marginal RAM, bad address/data bus	Swap RAM chips, logic probe bus lines
Consistent boot errors	Corrupt ROM, socket corrosion	Reseat/replace ROMs, clean sockets

RAM/ROM Diagnostic Steps

1. Power off, remove and reseat all socketed RAM and ROM chips.
2. If available, substitute with known-good chips.
3. Use logic probe to check for activity on address/data lines.
4. If possible, run a RAM test utility from disk.

Connector & Socket Issues

Ageing sockets and connectors are a frequent source of intermittent faults.

• Inspect all IC sockets for corrosion or looseness—reseat or replace as needed.
• Clean edge connectors (keyboard, disk, CRT) with isopropyl alcohol.
• Check for cracked solder joints at the power, video, and disk connectors.

Component-level Tests

Clock & Reset Circuitry

• Confirm CPU clock (typically 4 MHz) with an oscilloscope or logic probe.
• Check RESET line: should pulse low on power-up, then go high.
• If CPU is not running, check reset capacitor and associated logic (e.g., 555 timer or discrete components).

Voltage Test Points

Location	Expected Voltage	Notes
CPU Vcc pin	+5 V DC	Main logic supply
Floppy drive connector	+5 V, +12 V DC	Both required for operation
CRT board anode	+180 V DC	High voltage—use caution!

Logic IC Substitution

• Swap socketed logic chips (74LS, 4000 series) one at a time with known-good parts.
• For soldered ICs, piggy-back a good chip to test for improvement (not for MOS/ROMs).

Audio & I/O Failures

The Vixen has a simple beeper for sound and serial/parallel ports for I/O.

Audio Faults

• No beep at power-on: check beeper, drive transistor, and CPU port output.
• Replace beeper or transistor if silent.

I/O Port Issues

• Serial/parallel ports dead: check for broken traces, failed driver ICs (e.g., 1488/1489).
• Inspect port solder joints and connector pins for damage.

Related Pages

• Osborne 1 Troubleshooting Guide
• CRT Discharge Procedure