Macintosh Plus Memory Upgrade Guide

The Macintosh Plus ships with 1 MB of RAM in four 30-pin SIMMs and can be taken to 4 MB, which is the maximum the machine supports.

Memory specifications

• SIMM type: 30-pin, 150 ns or faster. Apple notes that the speed is given by the −xx suffix after the manufacturer's part number, so −15 is a 150 ns part.
• Eight RAM ICs per SIMM. A nine-chip SIMM will work, but the ninth device is not connected to the logic board.
• SIMMs go in rows of two. Row 1 is SIMMs 1 and 2, row 2 is SIMMs 3 and 4. A row must be either empty or full; one SIMM on its own is not a valid configuration.
• All the RAM ICs in a row must be the same size and the same speed.
• If the two rows differ, the larger SIMMs go in row 1.
• Maximum: 4 MB.

Macintosh Plus RAM configurations (Apple)

Total RAM	Bank A (row 1)	Bank B (row 2)	R8 "256K BIT" resistor
1 MB (stock)	2 × 256 KB	2 × 256 KB	fitted
2.5 MB	2 × 1 MB	2 × 256 KB	removed
4 MB	2 × 1 MB	2 × 1 MB	removed

Macintosh Plus memory layout: Bank A, Bank B and the RAM size resistors

The RAM size resistors

The Plus tells its general logic circuits what memory is fitted through two resistors in the area silkscreened RAM SIZE:

• R8, labelled 256K BIT — fitted only when every SIMM contains 256 Kbit DRAMs. If two or four SIMMs contain 1 Mbit DRAMs, R8 must be removed.
• R9, labelled ONE ROW — fitted only when just two SIMMs are installed. With four SIMMs fitted it must be removed.

Apple's method is to clip one end of the resistor and bend it away from the clipped lead rather than desoldering it, so that it can be tacked back later if the machine is returned to its original configuration.

Both of the upgrades in the table above use four SIMMs and at least two 1 Mbit modules, so both require R8 to be clipped. Do not clip R9 as well unless you are actually running only two SIMMs.

Tools

• Long T15 Torx screwdriver
• Case separator or plastic spudger
• Anti-static wriststrap
• Matching 30-pin SIMMs (see Recommended Tools)

Procedure

1. Prepare. Power off, unplug, work on a grounded surface.

2. Open the case. Remove the Torx screws — two at the bottom and two deep in the handle recess — and separate the halves. The machine contains a CRT; read CRT Discharge Procedure first, discharge the CRT before disconnecting anything, and do not fit a wriststrap until after it has been discharged.

3. Reach the logic board. The SIMM sockets are on the logic board towards the rear ports.

4. Remove the existing SIMMs. Ease the retaining clips outward — they are brittle — and tilt each module out.

5. Fit the new SIMMs, matching the notch to the socket keying, in complete rows.

6. Set the resistors as above before closing up.

7. Reassemble and verify. Boot and check the memory figure. If it is wrong, the usual causes are a partly seated module, a mismatched row, or a resistor still fitted.

Common problems

Symptom	Likely cause	Action
Sad Mac or beeps on startup	Wrong, faulty or partly seated SIMMs	Confirm 30-pin, 150 ns or faster, eight-chip, fully seated
Memory count unchanged after upgrade	RAM size resistor still fitted	Clip R8 when any 1 Mbit SIMMs are installed
Only half the memory seen	A row is half-populated, or the larger pair is in row 2	Fill rows completely; larger SIMMs in row 1
Random crashes	Mixed speeds within a row, or a failing module	One speed per row; swap in known-good modules

Sad Mac codes are decoded on Sad Mac Error Codes and with the Sad Mac Decoder.

Related pages

• Macintosh Plus
• Macintosh Plus General Maintenance
• Macintosh Plus Troubleshooting
• Apple Service Source