Macintosh LC III Capacitor Replacement Guide
The Macintosh LC III logic board carries eleven surface-mount electrolytic capacitors that leak with age, and one of them, C22, is silkscreened the wrong way round and was fitted reversed at the factory. The board is 820-0650-A or 820-0650-B, and Apple's Performa 450 is the same machine sold to consumers.[1][2] Its capacitor set is not the same as the LC's or the LC II's.
Tools, the removal and fitting procedure, the power supply lists and the battery are common to the LC family and are on Macintosh LC Capacitor Replacement Guide.
C22: the reversed capacitor
[edit | edit source]C22 is the bulk capacitor for the −5 V rail. Both the factory placement and the silkscreen are wrong: fit the replacement the opposite way round to the silkscreen marking.
Doug Brown traced the circuit on a Performa 450: C22's positive pad connects to the −5 V pin of the power connector and its negative pad to ground, so with the machine running the capacitor sees 5 V of reverse bias. He found the equivalent capacitor on the LC, LC II and LC 475 boards wired the correct way round.[3] Console5's LC III list and Recap-a-Mac's LC III guide now carry the same warning.[1][4] Recap-a-Mac added it after years of leaving it out, and says it has not heard of the reversed capacitor causing a fault.[4]
The three bulk capacitors beside the power connector are C19 on +5 V, C21 on +12 V and C22 on −5 V.[3]
Why the orientation matters with tantalums
[edit | edit source]The original aluminium electrolytic was rated 16 V and saw 5 V of reverse bias, and the −5 V rail is needed only for the RS-422 serial ports, so the mistake went unnoticed; Apple issued no recall.[3] Tantalum capacitors tolerate reverse voltage far less well. Doug Brown reports owners who fitted a tantalum in the silkscreened orientation finding the −5 V rail at about −2.3 V with the capacitor drawing about 1.3 A, and one tantalum catching fire in a machine with an aftermarket supply that could deliver that current. Apple's supply is rated for only 75 mA on −5 V.[3]
Checking it
[edit | edit source]- With the machine unplugged, set the meter to continuity. The pad for the capacitor's negative terminal should connect to the −5 V pin of the power connector, and the pad for the positive terminal to ground.[3]
- With the machine running and the capacitor fitted correctly, the meter reads about +5 V with the positive probe on the capacitor's positive terminal and the negative probe on its negative terminal.[3]
Macintosh LC III logic board (820-0650)
[edit | edit source]Eleven surface-mount electrolytic capacitors. The designators are Console5's; the totals by value (five 10 µF, five 47 µF and one 100 µF) match Recap-a-Mac's photograph, and Doug Brown replaced eleven capacitors on his board.[1][4][3]
| Designator | Value | Voltage | Notes |
|---|---|---|---|
| C2, C4, C5, C7, C18 | 10 µF | 16 V | |
| C8 | 47 µF | 16 V | |
| C13 | 100 µF | 6.3 V | |
| C19 | 47 µF | 16 V | Bulk capacitor, +5 V rail |
| C20 | 47 µF | 16 V | |
| C21 | 47 µF | 16 V | Bulk capacitor, +12 V rail |
| C22 | 47 µF | 16 V | Bulk capacitor, −5 V rail; fit opposite to the silkscreen |
On Doug Brown's board C21 had leaked worst, damaging the solder mask and the copper of the ground plane beneath it.[3] Check continuity around each position after removal.
Recap-a-Mac lists KEMET tantalum replacements: T491D476K016AT (47 µF 16 V), T491D107K006AT (100 µF 6.3 V) and T491B106K016AT (10 µF 16 V).[4]
Signs of failure
[edit | edit source]The usual family pattern applies: residue around the capacitor bases, then faults in sound, startup or video. If the serial ports misbehave on an LC III that has already been recapped, check C22's orientation first.[3] See Macintosh LC Capacitor Replacement Guide and Capacitor Failure Symptoms.
Power supply, battery and procedure
[edit | edit source]These are the same as the rest of the family and are on Macintosh LC Capacitor Replacement Guide: power supply 661-1733 and lithium battery 742-0011, replaced below 3.0 V. Apple rates the LC III at 30 W maximum, not including the monitor.[5]
Things that are not faults
[edit | edit source]- Mono sound. Apple specifies a monophonic 8-bit sound generator for the LC III, feeding both channels with the same signal; stereo output arrives with the LC 475.[5]
- An empty coprocessor socket. The board has a PLCC socket for a 68882; Apple's developer note says some configurations may have shipped with an FPU fitted, and Recap-a-Mac's board has the socket empty.[6][4]
Related pages
[edit | edit source]- Macintosh LC III
- Macintosh LC III+ Capacitor Replacement Guide
- Macintosh LC III General Maintenance
- Macintosh LC III Troubleshooting
- Macintosh LC Capacitor Replacement Guide
- Macintosh LC Series/ Quadra 605 Service Source
References
[edit | edit source]- ↑ 1.0 1.1 1.2 Macintosh LC Series, Console5 TechWiki—link(accessed 2026-09-30) Section "LC III Logic Board: 820-0650-A / 820-0650-B", with the note "C22 orientation is incorrect and should be reversed during service", citing Doug Brown. Secondary source.
- ↑ Apple Computer, Macintosh Performa 400 Series Service Source, File:Performa 400 series.pdf (see Macintosh Performa 400 Series Service Source), Specifications, PDF pp. 3-15.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 Doug Brown, "The capacitor that Apple soldered incorrectly at the factory", 26 November 2024. Continuity and voltage measurements on a Performa 450; comparison with LC, LC II and LC 475 boards; C19 on +5 V, C21 on +12 V and C22 on −5 V; the −5 V rail is only needed for the RS-422 serial ports; the supply is rated for 75 mA on −5 V; reports of a reversed tantalum giving −2.3 V and drawing 1.3 A, and of one catching fire with an aftermarket supply; C21 leaked badly on his board, damaging the solder mask and ground plane; all eleven capacitors replaced.
- ↑ 4.0 4.1 4.2 4.3 4.4 Macintosh LC III (Performa 450), Recap-a-Mac—link(accessed 2026-09-30) Annotated photograph of logic board 820-0650-B (c) 1992, legend 5 × 10 µF 16 V, 5 × 47 µF 16 V, 1 × 100 µF 6.3 V, with C22 marked "Original orientation incorrect. Replacement capacitor should be installed in the opposite polarity to the screenprinting"; section "The backwards cap"; KEMET replacements. Hosted here as File:Mac LC III Recap Guide.jpg. Secondary source.
- ↑ 5.0 5.1 Apple Computer, Macintosh LC Series/Quadra 605 Service Source, File:Quadra 605.lc series.pdf (see Macintosh LC Series/ Quadra 605 Service Source): Specifications, PDF pp. 6-16; Battery Verification, PDF p. 107; Exploded View, PDF p. 112.
- ↑ Apple Computer, Macintosh LC III Developer Note (1993), File:Macintosh LC III Developer Note.pdf (see Macintosh LC III Developer Note), chapter 1, pp. 2-3.