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Difference between revisions of "Macintosh LC III"

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* [[Macintosh LC Series/Quadra 605 Service Source]]
* [[Macintosh LC Series/Quadra 605 Service Source]]
* [[Macintosh LC III Developer Note]]
=See Also=
=See Also=

Revision as of 10:59, 24 June 2020


The LC III is powered by a 25MHz Motorola 68030. Apple also released the LC III+ which is essentially the same system but the board was altered to increase the clock speed to 33MHz. The LC III does not include a math co-processor but a socket is available to install a Motorola 68882.

The Macintosh LC III computer is an enhanced version of the Macintosh LC II computer featuring a faster CPU, increased video display options, and a 32-bit processor-direct slot interface that is compatible with cards designed for the Macintosh LC and LC II. The Macintosh LC III comes equipped with 4 MB of RAM and supports up to 32 MB of additional RAM (36 MB total). Overall performance is similar to that of the Macintosh IIci.

General Information

At some point during the production run it seems that the case design was changed from the traditional LC front panel to what was used in the Macintosh LC 475.

The LC III replaces the Macintosh LC II and is the last of the Motorola 68030 based "pizza box" LCs, as the Macintosh LC 475 used the Motorola 68LC040.


The Macintosh LC III uses a Motorola 68030 microprocessor running at a clock rate of 25 MHz. The microprocessor accesses RAM and ROM using 32-bit data transfers. VRAM accesses use 16-bit transfers.


On-board video support for Apple 12-, 13-, and 16-inch RGB monitors, the Apple Portrait monitor, and VGA monitors. The interface is RS-343. Video data is stored in 512 KB of video RAM (VRAM) soldered to the main logic board. One SIMM socket is included for optional expansion to 768 KB of VRAM. The Macintosh LC III uses the same type of VRAM SIMM as the Macintosh LC and LC II, except that the Macintosh LC III requires 80 ns components.

The video circuits in the Macintosh LC III computer are similar to those of the Macintosh LC and LC II, but provide support for additional monitors and 16-bit color. All video timing is controlled by Sonora. Video data is stored in a dedicated 512 KB VRAM frame buffer on the main logic board, and a SIMM socket allows VRAM expansion to 768 KB. The Macintosh LC III requires 80 ns VRAM. Color modes up to 8 bits per pixel use a 256 x 24-bit CLUT which is provided by an enhanced version of the custom chip used in the LC and LC II. Monochrome modes also use the CLUT but drive the red, green, and blue inputs with the same signal. The Macintosh LC III supports a 640 x 400 video mode that allows users to display 16-bit color on 13-inch RGB and VGA monitors without having to purchase additional VRAM. Users can select this mode from the Options menu in the Monitors control panel.


The LC III supports LC PDS expansion cards from the earlier LCs, but it also supports specific LC III PDS cards made for the larger data path of this system. The physical expansion slot is larger on the LC III to support these cards.

The Macintosh LC III introduces a new 114-pin processor-direct slot (PDS) for expansion cards. The PDS connector is notched to allow installation of 96-pin cards designed for the Macintosh LC and LC II. For compatibility with older cards, the Macintosh LC III computer’s processor-direct slot offers both 16-bit and 32-bit bus master capability.


The Macintosh LC III computer’s main ROM is contained in two 40-pin, 256K x 16-bit ICs providing 1 MB of read-only memory. A total of 4 MB of address space is reserved for future ROM expansion. ROM accesses by the main processor typically require 5 clock cycles, which is equivalent to 2 wait states.


  • Like almost all Macs of this era, you must recap the board due to old leaking capacitors.
  • The LC power supplies are known to require capacitor or other repairs due to age.
  • An incompatible or faulty MC68882 co-processor will cause the system to not power on.
  • The Macintosh LC III uses a new floppy disk controller circuit, SWIM2, that is part of the Sonora chip. Because the SWIM2 controller is different from the original SWIM, software that accesses the controller directly will not work with the Macintosh LC III.


Capacitor replacement (logic board)

The SMD capacitors on the logic board will most likely have already started leaking electrolyte out the bottom, and failing to replace them in a timely manner will result in further leakage with possible damage to nearby components. See below list of capacitors to replace:

  • 5 x 47uf 16V
  • 5 x 10uf 16V
  • 1 x 100uf 6.3V

Capacitor replacement (power supply unit)

A dead LC PSU can typically be revived by replacing all of the aluminum capacitors and cleaning up any leakage. Any working LC power supply should also be recapped as the capacitors will likely already be leaking due to component age, and the unit will eventually fail. The LC III may use one of a few different PSU types so check the label on yours for the parts list below.

NOTE: these values may differ if there happens to be multiple revisions of a certain model.


  • 1 x 180uf 385v
  • 2 x 270uf 25v
  • 3 x 1000uf 10v
  • 1 x 8.2uf 50v
  • 1 x 56uf 25v
  • 1 x 47uf 25v
  • 1 x 270uf 10v

Dyna Comp

  • 1 x 180uf 400v
  • 1 x 2200uf 10v
  • 2 x 1000uf 10v
  • 1 x 330uf 35v
  • 1 x 33uf 35v
  • 1 x 10uf 35v
  • 3 x 82uf 35v


  • 1 x 180uf 400v
  • 1 x 47uf 35v
  • 3 x 1200uf 10v
  • 1 x 470uf 16v
  • 2 x 120uf 25v
  • 1 x 1200uf 16v


Quadra series.png


See Also

External Resources