Osborne 1 Capacitor Replacement Guide

Osborne 1 main logic board

Capacitor values for the Osborne 1 and Osborne 1A, taken from the manufacturers' own parts lists. The machine has four boards that carry electrolytics: the main logic board, the disk electronics board on each drive, the Astec switching power supply and the Motorola 5-inch monitor. Osborne's Technical Manual lists the first two. The Osborne Field Service Manual reproduces Astec's component list for the supply and Motorola's parts list for the monitor in its appendices.

Safety

The Osborne 1 has a built-in CRT. The tube and the monitor board can hold a dangerous charge after the machine is unplugged. Read CRT Discharge Procedure before working near the tube, its anode cap or the flyback.

The Astec supply rectifies the mains directly. Astec's repair manual says dangerously high voltages are present and calls for an isolation transformer of at least 500 VA for bench work; with 190 V AC applied, the DC at the bridge rectifier is 250 to 300 V. The two 100 µF 250 V input capacitors hold that charge after the supply is unplugged. Discharge them before touching the primary side.

Before recapping

Measure the supply at the logic board's DC input connector first, against Osborne's limits in the Osborne 1 Troubleshooting Guide. Restorers report the mains filter capacitors in the power supply as a common failure. They are paper capacitors; when one fails it burns, with a bang and smoke from the front of the case soon after switch-on. Two restorers describe this fault on the Osborne 1, and each replaced the undamaged filter capacitors of the same type at the same time.

Main logic board

Osborne's list for the multilayer main logic board (LM# 3A10063-06), the board that meets FCC rules and was in production from September 1982, has only two aluminium electrolytics. The rest of the board is decoupled with ceramics and dipped tantalums.

Multilayer main logic board (LM# 3A10063-06), electrolytic and tantalum capacitors

Ref	Value	Type	Osborne part no.
C12	22 µF 16 V	Aluminium	7P45604-00
C30	22 µF ±20 % 12 V	Aluminium, axial	7P46106-00
C28, C35, C61, C62, C74	1.0 µF 20 V	Dipped tantalum	7P45501-00 (alternate 7P45505-00)
C71, C72, C73	15 µF 15 V	Dipped tantalum	7P45502-00

A shorted tantalum loads the supply rail it sits on; check them with a meter if a rail on the logic board is pulled down.

The ceramics, for identification: 0.1 µF ±50 % 25 V at C4 to C11, C20 to C27, C39 to C46, C53 to C60, C63 to C70, C85 and C86 (42 parts); 0.01 µF ±30 % 25 V at C1, C14 to C16, C18, C19, C29, C31 to C34, C36, C38, C47 to C52, C75 to C82 and C84 (28 parts); 0.001 µF 50 V at C17; 180 pF at C13; 68 pF at C2; 33 pF at C3.

The Technical Manual gives no list of materials for the earlier main board. Its schematic, drawing 1A2011-00 revision E (January 1982), marks C12 as 22 µF, C28, C35, C61, C62 and C74 as 1.0 µF and C71, C72 and C73 as 15 µF, all polarised, without voltage ratings. On that drawing C30 is a 1000 pF capacitor across R11 in the reset circuit, not an electrolytic, so the 22 µF axial part at C30 belongs to the multilayer board. If C30 on your board is a small non-polarised capacitor, use the schematic values and read the voltage ratings off the parts fitted.

Disk electronics board

Each Siemens or MPI drive carries an Osborne disk electronics board. Revision J (LM# 3A03004-00, 22 June 1982) lists:

Disk electronics board, revision J

Ref	Value	Type	Osborne part no.
C2, C33	47 µF 10 V	Aluminium	7P45606-00
C34	22 µF 16 V	Aluminium, low profile	7P45604-00
C36	4.7 µF (no voltage printed)	Aluminium	7P45605-00
C17	2.2 µF ±10 % 6 V	Tantalum	7P45504-00
C27	0.33 µF ±20 % 6 V	Tantalum	7P45503-00
C21	68 nF ±20 %	Mylar	7P45609-00

The earlier revision F schematic of this board (drawing 1A3004-00/01, December 1981) has different polarised parts at four of these positions: C2, C33 and C34 are 15 µF 20 V, and C36 is 1.0 µF 25 V. C17 (2.2 µF) and C27 (330 nF) are the same as on revision J. Read the parts on your own board before ordering.

After revision J, Osborne shipped a new disk board under part number 1B10081, also called revision B, with a revised read filter and servo circuit, a top-adjust speed control and added tantalum and bypass capacitors. The Technical Manual gives no parts list for it, so the revision J table does not apply to that board.

Astec AC8151 power supply

The supply is an Astec AC8151 switching unit, rated +5 V at 2.5 A, +12 V at 2.02 A and −12 V at 0.1 A. Astec's component code list AA11330 gives:

Astec AA11330 electrolytic capacitors

Ref	Value	Tolerance	Astec part no.
C6, C7	100 µF 250 V	±20 %	057-10120170
C8	220 µF 10 V	+50/−10 %	057-22120080
C15, C16, C17	1000 µF 25 V	+50/−10 %	057-10220040
C18, C19	330 µF 16 V	+100/−20 %	057-33120120
C20	470 µF 25 V	+50/−10 %	057-47120110
C21	2200 µF 16 V	+50/−10 %	057-22220020

C6 and C7 are the input capacitors that Astec's no-output procedure tells the technician to check last, after the fuse, thermistor R1, bridge DB1, R2 and D3.

The same list gives the non-electrolytic parts. C1 (0.01 µF) and C2 (0.1 µF) are metallised-paper capacitors rated 250 V AC; C3 and C4 are 4700 pF 400 V AC ceramics; C5 and C24 are 0.22 µF 250 V metallised polyester; C9 is 470 pF 2 kV ceramic; C10 and C11 are 0.01 µF 1 kV ceramic; C12 and C14 are 0.22 µF 100 V and C13 0.022 µF 50 V polyester. There is no C22 or C23 in the list.

C1 and C2 are the paper filter capacitors that restorers find burnt. On a UK machine, DOS Days found a third 0.01 µF 250 V AC filter capacitor at C11, where Astec's 1980 list gives a 1 kV ceramic, and replaced all three with X1-rated metallised polypropylene parts. Check what is fitted at C11 before ordering. If you replace C1 to C4 or C11, use mains-rated parts of the same class.

Motorola MD1000-190 monitor

Appendix 1 of the Field Service Manual is Motorola's service manual for the MD1000-190 series 5-inch monitor, which runs from 12 V DC at 800 mA. The Technical Manual describes an original monitor and a separate blue-case monitor (section 9.5), so confirm that your monitor board is a Motorola MD1000 before ordering from this list.

Motorola MD1000-190 electrolytic capacitors

Ref	Value	Motorola part no.
C208, C212	100 µF 16 V	23R29914A40
C210	1500 µF 25 V	23R29914A68
C312	22 µF 25 V	23R29910A05
C314	22 µF 100 V	23R29914B49
C318	1500 µF 25 V	23R29914A66
C320	22 µF 35 V	23R29914A77

C102, C311 and C319 differ between the MD1000-190 and MD1000-191 variants; the list gives both.

Procedure

1. Discharge the CRT (CRT Discharge Procedure) and unplug the machine.
2. Remove the logic board, monitor and power supply following chapter 6 of the Field Service Manual. Osborne labels the handle screws and the AC power panel screws separately; Osborne warns that an incorrect screw on reassembly may puncture the disk harness behind the handle.
3. Discharge C6 and C7 on the Astec board through a suitable resistor and confirm with a meter.
4. Photograph each board and note the polarity of every electrolytic before removing it.
5. Fit parts of the same capacitance and an equal or higher voltage rating that fit the board footprint.
6. Reassemble, then check the supply as below before reconnecting the logic board.

See Capacitor Failure Symptoms and Recommended Tools.

Checks after recapping

Astec's specification for the AC8151 gives these limits under all rated conditions of line and load:

Astec AC8151 output limits

Output	Minimum	Maximum	Ripple
+5 V	4.75 V	5.25 V	50 mV p-p
+12 V	11.40 V	12.60 V	150 mV p-p
−12 V	−11.0 V	−15.0 V	150 mV p-p

Astec specifies a minimum load of 0.45 A on +5 V and 0.3 A on +12 V for testing, so measure with the machine connected or with a load board, not on an open-circuit supply. The +5 V crowbar trips between 5.80 and 6.80 V. Osborne's own check at the logic-board DC input connector uses the same +5 V and +12 V limits; see Osborne 1 Troubleshooting Guide.

Related pages

• Osborne 1
• Osborne 1 General Maintenance
• Osborne 1 Troubleshooting Guide
• Osborne 1 Technical Manual
• Osborne Field Service Manual (Revision 2.0)
• Osborne Executive Capacitor Replacement Guide
• Osborne Vixen Capacitor Replacement Guide