Macintosh 128K/512K Capacitor Replacement Guide

A Macintosh 128K with the case removed. The power/sweep board is on the right, beside the CRT; this one is marked "INTERNATIONAL".

Recapping a Macintosh 128K or 512K is mostly work on the power/sweep board. The logic board carries three electrolytic capacitors; the power/sweep board behind the CRT carries about twenty, runs hot in a case with no fan, and is where most faults on these machines start. The 128K and 512K use the same logic board and power/sweep board part numbers.

Safety

Apple rates these machines at 105-125 V AC, 50-60 Hz, 60 W maximum.

Capacitor inspection

Look for bulging tops, dried electrolyte around the leads, and corrosion or board staining near a capacitor's base. On C1 the base is often hidden under hot glue. See Capacitor Failure Symptoms for the failure modes.

Logic board

The 128K and 512K logic board, Apple part 630-0101, has three electrolytic capacitors.

Logic board 630-0101

Reference	Capacitance	Voltage
C5	33 µF	16 V
C20	33 µF	16 V
C22	4.7 µF	35 V

Recap-a-Mac says of the similar Mac Plus logic board that it rarely needs recapping, because its axial electrolytics are not prone to the leakage of the surface-mount parts in later Macs. Replace them during a full overhaul; they are not urgent.

The logic board has no battery. PRAM is backed by a user-replaceable 4.5 V battery reached through a compartment cover on the back of the case; its holder is on the power/sweep board, and Apple has the battery moved across when a power/sweep board is exchanged.

Power/sweep board

The 128K, 512K and Plus have no separate power supply. Apple calls the single board behind the CRT the power/sweep board, and the 630-0102 board is silkscreened "MACINTOSH SWEEP/POWER SUPPLY". It carries the mains input, the switch-mode supply, the horizontal and vertical sweep circuits and the video output. Two revisions exist and their capacitor lists are not interchangeable; the part number is printed along the left-hand edge of the board. Larry Pina's parts lists identify 630-0102 as the U.S. board and 630-0108 as the International board, with different components under the same designators. His Appendix C (U.S.) and Appendix D (International) agree with the two tables below.

The same designator can be a different kind of part on the two boards. C13 is a 33 µF 16 V electrolytic on the U.S. board and a 0.01 µF 1.5 kV ceramic disc on the International one; C25 is 1000 µF 16 V on the U.S. board and a 0.022 µF 400 V film capacitor on the International one; C34 is a 330 pF 1 kV ceramic disc on the U.S. board and a 100 µF 200 V electrolytic on the International one. Pina's warning is "Be sure to look in the right appendix!"

Board 630-0102 (U.S.)

Power/sweep board 630-0102

Reference	Capacitance	Voltage (Console5 list)	Voltage (fitted, Mac84)	Notes
C1	3.9 µF (3.3 µF on later boards)	25 V	50 V	Non-polarised, low ESR; see below
C2	4700 µF	16 V	16 V
C3	220 µF	16 V	16 V	25 V on some revisions
C5	47 µF	10 V	10 V
C6	22 µF	50 V	50 V	Early boards have a series resistor on the negative lead
C9	10 µF	160 V	160 V
C11	33 µF	16 V	16 V
C13	33 µF	16 V	16 V
C14	22 µF	50 V	50 V
C24	2200 µF	10 V	16 V	Runs hot
C25	1000 µF	16 V	35 V	Runs hot
C26	1000 µF	16 V	35 V	Runs hot
C29	2200 µF	10 V	16 V	Runs hot
C30	220 µF	16 V	16 V	25 V on some revisions
C31	1000 µF	16 V	35 V	Runs hot
C33	4700 pF	250 V	not listed	Y-class safety; replace if RIFA or metallised paper; leave if ceramic disc
C35	100 µF	200 V	200 V
C36	100 µF	200 V	200 V
C37	4700 pF	250 V	not listed	As C33
C38	0.1 µF	275 V	not listed	X-class safety; RIFA or metallised paper
C39	470 µF	10 V	10 V
C43	470 µF	10 V	10 V

Console5's TechWiki publishes the reference-by-reference list, and Mac84 photographed and labelled the original parts on a 630-0102 board from a Macintosh 512K. The designators and capacitances agree. The voltage ratings differ at C24-C31, where the parts fitted at the factory are rated higher than those in Console5's list. Fit the rating printed on your own board, or higher.

Board 630-0108 (International)

Power/sweep board 630-0108

Reference	Capacitance	Voltage	Notes
C1	3.9 µF	25 V	Non-polarised; see below
C2	4700 µF	16 V
C3	220 µF	16 V
C5	47 µF	10 V
C6	22 µF	50 V
C10	33 µF	16 V
C11	10 µF	160 V
C12	22 µF	50 V
C18	33 µF	16 V
C24	2200 µF	16 V
C27	2200 µF	10 V	16 V on later builds
C28	1000 µF	16 V
C30	2200 µF	10 V	16 V on later builds
C31	220 µF	16 V
C32	2200 µF	16 V
C33	4700 pF	250 V	Y-class safety; replace if RIFA; leave if ceramic disc
C34	100 µF	200 V
C35	100 µF	200 V
C36	4700 pF	250 V	As C33
C37	0.47 µF	250 V	X-class safety; sometimes RIFA; replace if so
C38	100 µF	200 V
C39	100 µF	200 V
C42	470 µF	10 V
C45	470 µF	10 V

This list is from Console5's TechWiki, and agrees with Pina's Appendix D; no photographed board has been checked against it.

C1

C1 is in the horizontal sweep circuit and carries large high-frequency currents. It is non-polarised, and it is the part most likely to have failed in a compact Mac that has never been opened.

Larry Pina lists it as 3.9 µF NP 25/35 V HF and recommends a 3.9 µF NP 100 V HF part to stop the fault recurring. His U.S. parts list gives the position as 3.9 µF 25 V 85 °C HF non-polarised aluminium radial, with substitutes up to a 3.9 µF 250 V metallised polypropylene part. Thomas Lee calls the original the wrong type of part: its ESR rises as it heats, which heats it further. He recommends a non-polarised low-loss part rated 50 V or more, or four or five 1 µF 50 V ceramic capacitors in parallel.

The failure has a recognisable sequence: poor horizontal linearity (the left of the screen wider than the right), then bright diagonal lines a few minutes after switch-on, then a darkening picture and smoke. Switch off at the first sign, and check the yoke plug J1, which is often burned.

Parts that run hot

Thomas Lee names rectifiers CR20 and CR21 and filter capacitors C24-C26 and C29-C31 on the 630-0102 board as the weak points. He suggests an MBR1045 Schottky diode (10 A, 45 V) for CR20 and an MUR410 fast-recovery diode (4 A, 100 V, 35 ns) for CR21. CR22, on the lightly loaded −12 V rail, has no history of trouble. CR20, CR21 and the crowbar SCR Q8 are heatsinked.

Connectors

Power/sweep board 630-0102 connectors

Connector	Carries
J1 (4-way)	Horizontal yoke, ground, vertical yoke low, vertical yoke high
J4 (11-way, to the logic board)	Video, horizontal sync, sound, vertical sync, +5 V, ground, −12 V, ground, +12 V, +4.5 V battery
J2 (9-way, to the CRT socket board)	G1 bias, not connected, focus, brightness, ground, ground, CRT +12 V, CRT cathode

Thomas Lee calls cracked joints at J1 the most common fault on the board. They give an intermittent, wobbly, missing or narrow raster, or a single vertical line down the centre, and slapping the side of the case often makes the symptom come and go. Reflow all four J1 pins with the connector halves mated, so the heat does not distort the plastic.

Replacement procedure

1. Discharge the CRT; see CRT Discharge Procedure.
2. Confirm that the primary-side capacitors have discharged before touching that part of the board.
3. Photograph the board, showing each capacitor's orientation.
4. Desolder with a temperature-controlled iron and wick or a pump. Do not lever a capacitor out while only one lead is molten.
5. Clean the pads and check them for corrosion.
6. Fit the new parts with the stripe (negative lead) the right way round. C1 must be replaced with a non-polarised part.
7. Inspect for bridges and cold joints under magnification.
8. Check each rail for shorts before applying power.
9. Set the voltages as below.

Tools are listed on Recommended Tools. Match or exceed the voltage rating of each part removed. Replace metallised-paper (RIFA) safety capacitors; ceramic-disc parts in the same positions can stay.

Voltage adjustment

Apple's procedure for the 128K and 512K (the Plus has the same procedure in its own manual):

1. Set the meter to its 20 V DC range.
2. Connect Apple's voltage test cable, part number 077-0135, to the external drive port. Keep the hoods on the test plugs: shorting them together will damage the computer. Without the cable, Thomas Lee gives the DB-19 pins as +5 V on pin 6, +12 V on pins 7 and 8 and −12 V on pin 5, with ground on the threaded standoffs either side of the connector.
3. Switch on and adjust the regulator on the power/sweep board so that +12 V reads 11.90-12.75 V.
4. Switch off, move to the +5 V lead, switch on and check for 4.85-5.15 V.
5. Repeat both checks. If the board cannot be adjusted so that both are in tolerance, Apple replaces the board.

The adjustment is trimmer R56, just above the speaker. The supply regulates the +12 V rail and the other rails follow it, so turning R56 to correct a low +5 V reading caused by a dirty connector pushes +12 V over specification and can do permanent damage. Clean the contacts of connector J4 first; Thomas Lee reports that this alone cures most cases of the "boots, bongs, resets, boots again" loop. Turning R56 clockwise, viewed from the cardboard-insulator side of the board, lowers the output; turned too far the other way, the overvoltage crowbar fires.

The "whup-whup-whup" noise

A 12 A crowbar SCR, Q8, shorts the +12 V rail if it rises about 15% over specification, forcing the supply into current limiting. The supply retries periodically, which gives the rhythmic "whup-whup-whup" of a faulty compact Mac. To rule out a misadjusted R56, turn it fully clockwise (viewed from the insulator side) for the lowest setting; if the noise continues, the fault is elsewhere.

Module part numbers

Apple's exploded view gives the power/sweep board as 661-76214, the logic board as 661-06236, the CRT and yoke assembly as 076-0103, the insulator shield as 725-0011 and the chassis as 805-0766.

Related pages

• Macintosh Repair & Upgrade Secrets: Larry Pina's book, with the U.S. and International analogue-board parts lists
• The Dead Mac Scrolls: the same author's symptom-to-component reference
• Macintosh 128K/512K Service Source: Apple's service manual
• Macintosh 128K
• Macintosh 128K/512K Troubleshooting
• Macintosh Plus Capacitor Replacement Guide
• Capacitor Failure Symptoms
• Sad Mac Error Codes