Osborne 1 General Maintenance

Osborne 1 main logic board

Care and routine checks for the Osborne 1 and Osborne 1A. Osborne's Field Service Manual states that "the Osborne 1 and 1A do not require scheduled preventive maintenance". It recommends cleaning the disk drive heads regularly, at a frequency set by the conditions of use, and cleaning the outside of the case with a mild soap and water solution. Anything more, such as periodic rail checks or recapping, is the owner's choice and not a manufacturer schedule.

Safety

The CRT can hold a dangerous charge after the machine is unplugged; see CRT Discharge Procedure. The Astec power supply rectifies the mains directly, and its input capacitors stay charged after the plug is pulled.

Cleaning

Case and keyboard

• Clean the case with a mild soap and water solution, as Osborne specifies.
• The keyboard cable detaches at the front bezel: spread the latches on the keyboard connector. The harness lines up with pin 1 of the keyboard connector; looking at the underside of the keyboard with the numeric row at the top, pin 1 is the upper right pin.

Inside the case

• Disconnect the mains and discharge the CRT first.
• The video knobs come off with a 1/20 inch Allen wrench before the bezel is removed.
• Osborne has the two screws above and below the handle labelled "for handle" when they are removed, and warns that an incorrect screw on reassembly may puncture the disk harness running behind the handle. The AC power panel screws carry the same warning.
• Remove dust with a soft brush or low-pressure air.

Disk drives

• Clean the heads with a head cleaning diskette, 97 % isopropyl alcohol or head cleaning fluid only, and do not disturb the drive's alignment or position.
• The drives are Siemens or MPI units, depending on build date, with the maker's electronics replaced by an Osborne board. The Field Service Manual has a separate alignment procedure for each make (sections 6.3.0 and 6.4.0).
• The terminator resistor belongs in drive A only. Both drive lights coming on together is the symptom of a terminator in the wrong place.

See Floppy Drive Repair for general drive work.

Power supply and voltage checks

The supply is an Astec switching unit. Osborne's Technical Manual rates it at +5 V 2.5 A and +12 V 2.05 A; Astec's own specification for the AC8151 gives +5 V 2.5 A, +12 V 2.02 A and −12 V 0.1 A. Its internal fuse is a T2A250V; replace it with the same type.

Mains voltage selection

Getting this wrong damages the supply. The method depends on the case:

• Earlier power supplies: a wire with a bright orange tag on the power board is moved from its pin to the adjoining one.
• Blue-case machines: open the fuse box door in the rear power well, remove the fuse, pull the fuse card with needle-nose pliers and turn it so that only the wanted voltage is visible. The Technical Manual lists the card markings as 100, 120, 220 and 240. The Field Service Manual's procedure for the Osborne 1A gives 120 and 220 and specifies a 1 A slow-blow 250 V (3AG) fuse for either setting.

Expected measurements

Disconnect the DC input connector from the logic board and measure at the connector.

Osborne 1 DC input connector

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, then pins 2 and 6, against pins 3 and 5. Both voltages within limits: the AC and DC systems are working. Both missing: replace the AC power fuse. One missing or out of tolerance: replace the power supply and check again.

The logic board takes only +12 V and +5 V. The supply's three DC connectors SK2, SK3 and SK4 also carry −12 V on pin P1 (P2 +12 V, P3 common, P4 +5 V).

Capacitors

Osborne's parts list for the multilayer main logic board, in production from September 1982, shows only two aluminium electrolytics, C12 and C30; on the schematic of the earlier board, C30 is a 1000 pF capacitor. The Astec supply and the Motorola monitor carry most of the machine's electrolytics. Values for all four boards are on the Osborne 1 Capacitor Replacement Guide.

Battery pack

Osborne sold an optional battery pack: a 12 V battery in a shoulder case, a DC-to-AC inverter that plugs into the computer's mains input, and a charger. Osborne gives about one hour of normal use per charge, with the inverter beeping when the battery is low, and a charge of 12 to 48 hours at room temperature. The manual warns against overcharging, and says to unplug the charger from the mains before connecting or disconnecting it at the battery.

Recommended tools

Osborne's list for disassembly, drive alignment and monitor adjustment (section 6.0.0):

• No. 2 Phillips screwdriver, 9 to 12 inches long
• 1/20 inch and 3/32 inch Allen wrenches
• Needle-nose pliers
• 6 inch flat-blade screwdriver
• Broad-end non-metallic adjustment tool for the monitor
• Dual-channel oscilloscope with external trigger, 50 MHz minimum, 200 mV sensitivity, three ×10 probes with ground clips
• Alignment diskette, Dysan 224 or equivalent
• Osborne alignment program diskette, part no. 2D01003-00
• Drive head cleaning kit

See also Recommended Tools.

Related pages

• Osborne 1
• Osborne 1 Troubleshooting Guide
• Osborne 1 Capacitor Replacement Guide
• CRT Discharge Procedure
• Floppy Drive Repair