Osborne Vixen Troubleshooting Guide

Osborne Vixen system board

Fault-finding on the Osborne Vixen (Osborne 4). Osborne's field service fault trees cover only the Osborne 1 and the Osborne Executive; none for the Vixen is hosted here. What survives is Osborne's Osborne 4 Theory of Operation Manual of August 1985, which describes how the production boards work, and the March 1983 engineering drawings, which list an earlier design of the boards. Do not apply the Osborne 1 or Executive supply limits to the Vixen; they belong to different supplies.

Safety

The Vixen contains a CRT and a mains power supply. The CRT can hold a dangerous charge after the machine is switched off and unplugged; see CRT Discharge Procedure.

How the system board works

From the 1985 Theory of Operation Manual:

• A crystal-controlled oscillator runs at 23.9616 MHz. It is divided to a 12 MHz dot clock, the 4 MHz CPU clock, a 2 MHz clock for the serial controller and a 1 MHz clock for the disk controller. A dead oscillator stops everything.
• At power-up, C8 charges through R30. Two inverters, 8M (a 74LS14 Schmitt trigger) and 6C, hold the Z80A in reset until C8 reaches the threshold of 8M. The reset switch S1 discharges C8 through R29, and diode CR3 discharges it quickly if the power drops. A machine that never comes out of reset points at this circuit.
• The monitor program is in a single 2732 at 4C. It initialises the hardware, boots from the floppy and handles the serial, disk, parallel and 60 Hz display interrupts.
• Main memory is eight 4164 64K×1 DRAMs: 60K for general use and 4K for the screen buffer. One device per data bit, so a memory test that reports a single stuck bit names the chip.
• The display controller copies each text line from DRAM into a 256-byte line buffer made of two 2114 static RAMs at 2F and 2H, and generates characters from a 2716 ROM at 1J. A display that is wrong in a consistent pattern while the machine otherwise runs points at the line buffer, the character generator or the display control logic rather than main memory.
• The composite video output at the phono jack J4 sits at about 3.8 V with video present, about 0.6 V with no video, and falls to 0 V during sync pulses. R27 sets the CRT brightness.
• The floppy disk controller is a 1797.

Where the parts are

The 1985 manual uses grid positions with the number first (4C, 1J). The 1983 lists of materials use the letter first and describe an earlier layout, with three 2732 EPROMs and a 1793 controller where the 1985 board has one ROM and a 1797. Use the table that matches your board.

Vixen device positions

Function	1985 manual (revision C)	1983 main logic list (revision 2.1)
CPU	Z80A	Z80A at C4
Monitor ROM	One 2732 at 4C	Three 2732 at F11, G7, H3
Main memory	Eight 4164	Eight 4164 at F1 to F4, G1 to G4
Video RAM	Two 2114 line-buffer RAMs at 2F and 2H	Two 2114 at E5, E6
Character generator	2716 at 1J
Disk controller	1797	1793 at H12
Keyboard and I/O	8155	8155 at H7
Timebase	23.9616 MHz oscillator	23.9616 MHz crystal at Y1
Reset	C8, R29, R30, CR3, 8M, 6C, S1	S1

The I/O board, in both versions, carries an 8155 whose two ports form the parallel interface, programmable for IEEE-488 or Centronics, and an 8251A USART for the RS-232-C port. In the 1983 I/O list these are at C7 and C3, with a 1488 driver at A1, 1489A receivers at A2 and A3, and the connectors at J1 (25-pin RS-232) and J2 (24-pin IEEE-488).

Working through faults

Nothing at all

1. Check the mains lead, the fuse and the power switch.
2. Measure the supply outputs. No published limits for the Vixen supply are hosted here, so record the readings. Osborne's drawing of the supply labels its outputs +12V1, +12V2, +5V and −12V, with a crowbar SCR and zener on +5 V.
3. Measure again with the logic board unplugged. A supply that recovers when unloaded is being pulled down by a short downstream. Osborne gives this loaded and unloaded comparison in its Executive procedure.
4. With power good, check for the 23.9616 MHz timebase and the 4 MHz CPU clock, then that the CPU comes out of reset.

Power on, screen dark

1. Turn brightness and contrast up.
2. Check the video harness at both ends.
3. Connect a monitor to the composite output J4. If the external picture is good, the fault is in the internal monitor or its harness.
4. The 1983 monitor drawing names a Kaga monitor with V-LIN, V-HEIGHT, V-HOLD and VIDEO CENTER adjustments; the hosted schematic set also includes a Zenith monitor drawing. Identify the monitor fitted before adjusting it.

Drives

1. Try a known-good diskette in each drive.
2. Clean the heads without disturbing alignment.
3. Swap the drives. A fault that follows the drive is the drive; one that stays is the cable, the 1797 or its support logic.
4. Check the drive jumpers. The 1983 drive sub-assembly drawing allows MPI or ALPS drives: on MPI drives, jumper pins E and F on both, the second set of pins on drive A and the fourth set on drive B; on ALPS drives, the first and second sets on drive A and the first and third sets on drive B.

The 1797 reads the Osborne 1 single- and double-density formats and IBM PC (CP/M-86), DEC VT180, Xerox 821 and Cromemco formats. Table 9-1 gives 100K per diskette (92K for CP/M) in Osborne single density and 200K (185K) in Osborne double density. See Floppy Drive Repair for general drive work.

Serial and parallel ports

The parallel port is software-programmable as IEEE-488 or Centronics, so check the configuration before suspecting the 8155 or its buffers.

Capacitors

The 1983 main logic list has five aluminium electrolytics, C1 and C2 at 47 µF 10 V, C3 and C4 at 68 µF 10 V and C7 at 10 µF 10 V; the I/O list adds C1 at 22 µF 16 V. The schematic in the same package differs at C2 to C6, putting C2 and C4 on the −12 V and +12 V inputs, and the revision C board renumbers its capacitors (its reset capacitor is C8); see the Osborne Vixen Capacitor Replacement Guide.

Related pages

• Osborne Vixen
• Osborne Vixen Maintenance Guide
• Osborne Vixen Capacitor Replacement Guide
• Osborne Vixen Engineering Drawings Mar83
• Osborne 1 Troubleshooting Guide
• Osborne Executive Troubleshooting Guide
• CRT Discharge Procedure