Osborne 1 Troubleshooting Guide

Osborne 1 with the keyboard lowered

Fault-finding on the Osborne 1 and Osborne 1A. The fault trees below are Osborne's own, from chapter 7 of the Osborne Field Service Manual. They work at the level of field-replaceable units: each tree ends in "replace the logic board", "replace the power supply" or "replace or align the drive". For work below that level, the Osborne 1 Technical Manual gives the parts lists and schematics of the logic and disk electronics boards, and the Field Service Manual's appendices carry Astec's repair procedure for the power supply and Motorola's service data for the monitor.

Safety

• The CRT holds a dangerous charge after the machine is unplugged. Read CRT Discharge Procedure before working near the tube, its anode cap or the flyback.
• Several of Osborne's checks need mains power on. The manual prints the warning in capitals: "the following tests need AC power on. Use extreme caution."
• Osborne's precautions: do not remove or fit any component, or connect or disconnect any plug or cable, with the power on unless the procedure says so, and note every cable's orientation before disconnecting it.
• The Astec supply rectifies the mains directly; Astec's own bench procedure uses an isolation transformer.

Before you start

Osborne's first step is to decide whether the fault is in the computer at all. Disconnect the printer, external monitor and modem and try again. If the fault goes away, test the external device and its interface.

The sign-on message shows the monitor ROM revision in the box under "OSBORNE 1". Single-density machines with a ROM earlier than revision 1.3 have built-in diagnostics, reached by pressing Ctrl-D while the sign-on message is displayed.

Osborne's dealer diagnostic is the OCC-1 Field Service System Confidence Test diskette (version 2.04). It tests memory (refresh, galloping pattern, vertical bar and moving diagonal), the serial and IEEE-488 ports, both drives and the keyboard, and displays CRT test patterns. The port tests fail unless loopback plugs are fitted to the RS-232 and IEEE-488 connectors. The full confidence test takes about 40 minutes; the moving-diagonal memory test alone takes about 20. Xerox's procedure says the diskette "is not designed as a 'chip level' troubleshooting tool", though some tests isolate a chip.

DC input connector check

Osborne's trees all refer back to section 7.1.1.7: disconnect the DC input connector from the logic board and measure at the connector.

Osborne 1 DC input connector, Field Service Manual section 7.1.1.7

Pins	Wire	Output	Minimum	Maximum	Ripple
1 and 7	Red	+12 V	+11.40 V	+12.60 V	150 mV p-p
2 and 6	Yellow	+5 V	+4.75 V	+5.25 V	50 mV p-p
3 and 5	Orange	Common
4	Green	0 V

Measure pins 1 and 7 against pins 3 and 5, then pins 2 and 6 against pins 3 and 5. If both voltages are within limits, the AC and DC power systems are working. If both are missing, replace the AC power fuse. If one is missing or out of tolerance, replace the power supply and check again. The ripple limits matter: a rail can read correctly on a DC meter and still be out of specification.

Power problems

System does not power up

1. Switch the power off and on again. If the machine beeps and the drive lights flash but the screen stays blank, go to the CRT section.
2. Check the mains supply: 120 V or 220 V as applicable.
3. Switch off. Test the AC power fuse, or the circuit breaker on early machines. Replace the fuse or the AC power panel if necessary.
4. Check all cables for breaks and bad connections.
5. Check the AC thermal cutout with an ohmmeter. A working cutout reads as a short circuit.
6. With power on, check the DC connections at the power supply. Each connector is keyed.
7. Measure at the DC input connector as above.

Reduced screen display or slow-running disk

Measure at the DC input connector. Out of tolerance: replace the power supply. In tolerance: replace the video monitor. If the voltages are good with a new monitor fitted, replace the logic board.

Disk drive errors during applications

Measure at the DC input connector. Out of tolerance: replace the power supply. In tolerance: go to the disk drive section.

Inside the Astec power supply

The supply is an Astec AC8151. Its specification gives +5 V at 2.5 A, +12 V at 2.02 A and −12 V at 0.1 A, a maximum continuous output of 38 W, and a +5 V crowbar that trips between 5.80 and 6.80 V. The −12 V output is specified at −11.0 to −15.0 V with 150 mV p-p ripple. Astec's test load is 0.45 A on +5 V and 0.3 A on +12 V at minimum, so do not judge the supply on its open-circuit voltages.

Astec's own repair procedure for a supply with no output:

1. Check the fuse. If it has blown, replace it, but do not apply power until the cause is found.
2. Check the main primary-side parts: thermistor R1, bridge rectifier DB1, power transistor Q2, catch diode D3, turn-off transistor Q1, emitter resistor R11 and D1.
3. With the loads disconnected, measure with an ohmmeter from output common to each output to find shorted rectifiers or capacitors. If the +12 V output reads short, also check crowbar SCR1 and zener Z1.
4. Bring the supply up slowly on a variable transformer and check B+ at the positive terminal of DB1. With 190 V AC applied (230 V setting) it should be 250 to 300 V DC. If not, check the fuse, R1 and DB1, then R2 and D3, then the input capacitors C6 and C7.
5. Check that Q2 is switching (collector waveform on the case of the TO-3 package). If not, check Q2 for shorted junctions, then its base waveform; then check L3, Q1, D1 and the secondary-side parts Q3, D11, D12, D5 and L4.

Astec's manual assumes the 230 V setting and tells the reader to halve the figures for 115 V. The supply should start at about 80 to 120 V AC and regulate at 190 V AC. Capacitor values for the supply are on the Osborne 1 Capacitor Replacement Guide.

Disk drive problems

For a boot read error, drive not ready or BDOS errors, Osborne's first step is the drive alignment program: DRIVE TIMER, ALT TRACK 0 and 1, and HOME HEAD. Then check that the software suits the hardware, that every connector is seated and that no cable is damaged. Clean the head of the drive involved without disturbing its alignment or position, using a head cleaning diskette, 97 % isopropyl alcohol or head cleaning fluid only.

Boot read error, one drive

1. Try the other drive. If it boots and both drive lights are on, check that the terminator resistor is fitted in drive A only.
2. If the other drive boots, disconnect the internal monitor and connect an external one. If the drive then boots, replace the internal monitor.
3. If it still does not boot, swap the two drives, keeping the terminator in drive A.
4. If the error moves with the drive, replace or align that drive.
5. If it does not, replace the drive harness, then the logic board.

Boot read error, neither drive

Check the keyboard and its cable first. Then boot each drive with another diskette; if one boots, follow the one-drive sequence. If neither boots, disconnect the internal monitor and boot with an external one: if one or both then boot, replace the internal monitor. If neither boots on the external monitor, replace in turn the drive harness, the logic board, and then replace or align both drives.

Drive not ready

Clean the head and check every connector. If one drive boots, swap the drives. If the error stays put, replace the drive harness; if it moves with the drive, replace the logic board.

BDOS errors

The tree has the same structure: external-monitor test, drive swap with the terminator kept in drive A, then the drive, the harness or the logic board depending on whether the error follows the drive.

The two makes of drive, Siemens and MPI, have separate alignment procedures in sections 6.3 and 6.4 of the Field Service Manual. See also Floppy Drive Repair.

CRT malfunctions

Wavy screen

Check +12 V at the DC input connector. Out of tolerance: replace the power supply. Wavy only while a drive is running: replace that drive. Voltage good and drives working: replace the monitor.

Screen not centred

Adjust the video raster (section 6.6.4). If the display is tilted, replace the monitor.

Blank screen

1. Turn brightness and contrast fully clockwise.
2. Check the video harness connectors at the monitor and at the logic board.
3. Switch off and check the external video plug.
4. Switch off and, with the logic board video connector attached, replace the video plug.
5. Check +12 V and replace the power supply if it is out of tolerance. Then replace in turn the video harness, the monitor and the logic board.

Gaps in the sign-on message

Check +12 V and replace the power supply if it is out of tolerance. If the gaps remain, replace the logic board.

Flashing sign-on message

Osborne's note: do not confuse a flashing sign-on message with a vertical hold fault. Look for a stuck alphanumeric key, then replace the keyboard cable, then the keyboard.

The monitor adjustments (brightness and contrast, vertical size and linearity, focus, raster) are in section 6.6.0, and appendix 1 is Motorola's service manual for the MD1000-190 5-inch monitor, which runs from 12 V DC at 800 mA.

Keyboard malfunctions

For an individual key: run a keyboard test (Osborne names the SuperSoft diagnostics), disconnect and reconnect the keyboard cable, check the logic board keyboard connector for bent or broken pins, then replace the cable, the keyboard and the logic board in that order. For a complete keyboard failure, or a displayed character that does not match the key struck: replace the cable, then the keyboard, then the logic board.

Logic board problems

Continuous beep on power-up

Disconnect the keyboard. If the beep continues, replace the logic board. If it stops, replace the keyboard cable, and if a new cable does not cure it, the keyboard.

Scattered video display

Check the supply voltages. If they are within tolerance, replace the external video plug, then the logic board.

Memory error during diagnostics

Check the supply voltages. If they are within tolerance, replace the logic board.

Where the main parts are

For component-level work on the multilayer logic board (assembly LM# 3A10063-06, in production from September 1982), Osborne's list of materials gives these positions:

Osborne 1 multilayer logic board, main devices

Device	Position
Z80A CPU (the Technical Manual names the NEC 780C, a Z80A equivalent)	UC11
1793 floppy disk controller (the Technical Manual names the Fujitsu 8877, a 1793 equivalent)	UB7
6821 PIA (two)	UC7, UC15
6850 ACIA (serial port)	UC4
4116 16K×1 DRAM (33 devices)	UA20 to UA27, UB20 to UB27, UC19 to UC27, UD20 to UD27
System ROM (programmed EPROM assembly 3A10206-00)	UD11
Character generator (programmed EPROM assembly 3A10256-00)	UA15
15.9744 MHz crystal	X1
Piezo audio alarm	X2
LM1458 op amp, LM3900 op amp, NE555 timer	UE3, UD4, UE1

The CPU clock is 4 MHz, divided from the 15.9744 MHz crystal by a 74LS161. Thirty-two of the 4116s are main memory; the thirty-third holds the dim video attribute. The earlier non-multilayer board uses different positions: Osborne's 1983 upgrade procedures put the monitor ROM at D12 and the Z80 in row C, columns 11 to 14. Schematics for that board, drawing 1A2011-00 revision E, are hosted as File:Osborne 1 Schematics 1A2011-00 Rev E.pdf.

RS-232 and IEEE-488 port problems

For no output to the printer or a scrambled printout, Osborne's sequence starts with configuration:

1. Check the Setup configuration of the diskette.
2. Check Install (WordStar only).
3. Check the connectors and interface wiring.
4. Use known-good software.
5. Use a different printer.
6. Check that the printer works.
7. Replace the logic board.

For a printer that drops characters, the sequence goes straight from step 3 to the logic board.

Double-density and SCREEN-PAC machines

Osborne's double-density table applies to machines that worked before the upgrade. For most symptoms it is: check that the double-density parts are installed correctly, replace the double-density board, replace its ribbon cable, then fall back to the ordinary tree. For gaps in the sign-on message or a continuous beep, the first step is to replace the revision 1.43 monitor ROM. Osborne's October 1983 double-density procedure ships a revision 1.44 monitor ROM in the kit and fits a revision 1.43 ROM at D12 only while the drives are aligned, so check which ROM is fitted.

The same procedure requires a logic board of revision G or later and disk drive boards of revision J or later, and gives the drive timer limit as 199.80 to 200.20 ms, set with R23 on the drive board. For drive errors during the confidence test, the Field Service Manual adds an op-amp check on the double-density board: oscilloscope channel 1 on auto trigger, 10 ms time base, 1 V/div, probe on the output of R8 with ground at C9, and adjust the board's potentiometer for 3.5 V ±0.5 V during the seek/write/read/verify part of the test.

Osborne's SCREEN-PAC table:

SCREEN-PAC troubleshooting, Field Service Manual

Symptom	Osborne's action
Continuous beep at switch-on	Switch off for 5 seconds and on again. Check the Z80, its connector and the 40-pin socket for bent or broken pins.
Scrambled logo at switch-on	Check the SETUP program for a logical screen size of 128.
Lines across the screen in 52- and 80-column modes	Check the 24-pin character generator for a secure fit and bent or broken pins.
No video	Check the power cable connections at the board and the power supply.
Only 52-column mode works	Check the power harness, then replace the SCREEN-PAC board.

Board and ROM revisions

By mid-1982 the bare main logic board had reached revision F (revisions B to F were in the field) and the loaded board had its own revision letters, A to N. A multilayer board that met FCC rules was in production from September 1982. There were four monitor ROM revisions (A, 1.2, 1.3, 1.4) and five BIOS revisions; Osborne upgraded version A ROMs and BIOS to 1.2 free of charge in November 1981. The disk drive interface board went through revisions A to J; Osborne then issued a new revision B under part number 1B10081 with a revised read filter and servo circuit, a top-adjust speed control and added tantalum and bypass capacitors.

Related pages

• Osborne 1
• Osborne 1 General Maintenance
• Osborne 1 Capacitor Replacement Guide
• Osborne 1 Technical Manual
• Osborne Field Service Manual (Revision 2.0)
• Osborne 1 / Executive Diagnostic Procedures Jan84
• Osborne Executive Troubleshooting Guide
• CRT Discharge Procedure
• Capacitor Failure Symptoms