Macintosh 128K RAM Chip Replacement

Columns G and F of a 128K logic board (630-0101). Rows 5 to 12 hold the sixteen RAM chips; the one at F10 is a different make from the rest. Photo: Dave Fischer, CC BY-SA 3.0, cropped.

The Macintosh 128K and the Macintosh 512K keep their memory in sixteen DRAM chips soldered to the logic board at grid positions F5 to F12 and G5 to G12: 4164 (64K) chips on the 128K and 41256 (256K) chips on the 512K. The Macintosh 512Ke is a 512K with the 800K drive and 128K ROMs, so its RAM is the same as the 512K's. When one chip fails the Mac stops at start-up with a Sad Mac whose code names the chip, and the repair is to take that chip off the board and fit a socket and a new chip.

The Macintosh Plus keeps its memory on SIMMs instead; see Macintosh Plus Memory Upgrade Guide.

Symptoms

• At start-up the Mac shows a Sad Mac with a six-digit code. The last four digits name the failing chip. The fault may appear out of the blue or straight after a RAM upgrade. For the meaning of the first two digits see Sad Mac Error Codes or the Sad Mac Decoder.
• Several failing chips give a code that is the sum of their single codes. Pina's example: xx0301 is xx0200 (G6) + xx0100 (G5) + xx0001 (F5).

Sad Mac code and RAM chip position (Pina, Table 10-3; The Dead Mac Scrolls, pp. 124-139)

Last four digits	Chip	Last four digits	Chip
0001	F5	0100	G5
0002	F6	0200	G6
0004	F7	0400	G7
0008	F8	0800	G8
0010	F9	1000	G9
0020	F10	2000	G10
0040	F11	4000	G11
0080	F12	8000	G12

Safety

The logic board comes out with the cover off, so discharge the CRT first. Apple warns that the machine contains high voltage and a high-vacuum picture tube, and says never to use a grounding wrist strap until after the CRT has been discharged. See CRT Discharge Procedure.

Tools and parts

• Replacement chip: 4164 (64K) for a 128K; 41256 (256K) for a 512K or 512Ke. The Dead Mac Scrolls lists a 41256 150 ns chip.
• A 16-pin DIP socket for each position; Pina strongly recommends them.
• Wire cutters, needle-nose pliers, a bench vice, a grounded 15 W soldering iron, a vacuum desoldering tool, a magnifying glass, aluminium foil and an antistatic wrist strap.
• Fast-drying, brightly coloured nail polish to mark the chip.

Procedure

Removing the logic board

Sliding the logic board out of its rails. The 128K/512K Service Source uses the same drawing (Logic Board p. 11). Apple Computer.

1. Remove the rear cover and discharge the CRT.
2. Disconnect the floppy drive cable and the logic board cable from the logic board.
3. Pull back on the top of the logic board and carefully slide it out of its rails.

Removing the failed chip

The early 128K board, 820-0086-C, with the 4164 RAM chips at F5 to F12 and G5 to G12. Pina, Figure 10-2, courtesy of Sophisticated Circuits.

1. Look up the chip position in the table above and find it on the board. The grid letters run down one side and the numbers along the bottom.
2. Mark the chip with nail polish so that you cannot cut the wrong one.

Cut each pin close to the chip body, then pull the pins out one at a time while heating them. Pina, Figure 10-4, courtesy of Sophisticated Circuits.

1. Cut all 16 pins with wire cutters, as close to the chip body as possible, and lift the body away.
2. Mount the board in a bench vice.
3. Heat each pin from the solder side with a grounded 15 W iron while pulling it out from the component side with needle-nose pliers. Pins 8 and 16 connect to the inner layers of the board and take longer to free.
4. Clear the solder from all 16 holes with the vacuum desoldering tool. Clean its O-ring often: solder build-up round the edge lets air past the seal and cuts the suction.

Faults to look for after desoldering: lifted pad, break between pad and trace, cut trace, solder bridge. Pina, Figure 10-6, courtesy of Sophisticated Circuits.

1. Examine every hole with a magnifying glass for lifted pads, breaks between pad and trace, cut traces and solder bridges. Pina warns that a lifted pad on this multilayer board is hard to repair, because the pads are joined to plated holes connected to the inner layers.

Fitting the socket and the new chip

1. Turn the board component side down. From underneath, insert a 16-pin DIP socket into the position, with its notched end towards the front edge of the board, away from the I/O connectors.
2. Solder pin 1, on the square pad, first; then pin 9 in the opposite corner; then the rest. Keep your fingers off the end being soldered.
3. Trim the new joints, if necessary, to the same length as the others on the board, so that they cannot short against the metal coating inside the case.
4. Lay the board solder side down on a sheet of aluminium foil and put on an antistatic wrist strap.
5. Press the new chip into the socket with its notch pointing away from the I/O connectors. Check that no pin has folded under.

Reassembly

1. Slide the logic board back into its rails and reconnect the disk drive cable and the power/video (logic board) cable.
2. Fit the rear cover loosely, leaving a small gap so that it comes off easily if the test fails.

Testing

• Switch on. A start-up tone and the disk icon with a blinking question mark mean the RAM test has passed.
• If a Sad Mac returns, decode it again. Pull the chips it names with an IC puller and check for bent-under, bent-out or broken pins; straighten them with a pin straightener or needle-nose pliers. A chip can also be damaged by static during the work; try another.
• Fit the cover screws once the machine starts normally.

Revision notes

• There are two 128K logic boards: early 820-0086-C and later 820-0141-A, identified by the part number stamped at grid G8 to G12. Pina's chip-replacement steps are the same for both; the boards differ in what a 512K upgrade needs.
• Pina says the earliest 820-0086-C boards lack a resistor network on the 6522 VIA, without which the Mac tends to lock up; fitting one is part of his 512K upgrade.

Related pages

• Macintosh 128K, Macintosh 512K, Macintosh 512Ke
• Macintosh 128K/512K Troubleshooting
• Macintosh 128K/512K General Maintenance
• Macintosh 128K/512K Service Source
• Sad Mac Error Codes
• CRT Discharge Procedure