Macintosh Plus Capacitor Replacement Guide

The Macintosh Plus logic board rarely needs recapping: its electrolytics are axial and radial through-hole parts, not the surface-mount capacitors that leak on later Macintoshes, and Recap-a-Mac says most Plus problems start on the analog board. That board, which Apple calls the power/sweep board, holds the mains switch-mode supply and the CRT sweep circuits. Apple's service manuals for the Plus are module-level and give no component values; the values below come from the parts lists in Larry Pina's Macintosh Repair & Upgrade Secrets, checked against Console5's capacitor-kit lists, which are community lists read from boards.

Visual inspection

• A healthy electrolytic has a flat top. A bulging top, or bulging at the base, means the part has failed. On C1 the base is often buried in hot glue.
• Brown or white crust at the leads is leaked electrolyte; look for corrosion or discoloured board under and around the part.
• The RIFA metallised-paper safety capacitors crack and blacken with age.

See Capacitor Failure Symptoms.

Logic board

The Plus logic board, 820-0174, has six electrolytics.

Logic board 820-0174

Reference	Capacitance	Voltage
C5	33 µF	16 V
C6	33 µF	16 V
C7	33 µF	16 V
C12	33 µF	16 V
C13	33 µF	16 V
C14	1 µF	50 V

Console5's community list gives the designators. Recap-a-Mac's photographic guide, made from an 820-0174-D board, marks the same six positions on the silkscreen: 33 µF 16 V at C5, C6, C7, C12 and C13 and 1 µF 50 V at C14. The 128K and 512K logic board, 630-0101, has a different set: C5 and C20 at 33 µF 16 V and C22 at 4.7 µF 35 V.

Macintosh Plus logic board capacitor locations

The logic board carries no battery. The 4.5 V PRAM battery sits in the compartment on the back of the case, in a holder on the power/sweep board, and reaches the logic board through pin 11 of J4.

Power/sweep board

The 128K, 512K and Plus have no separate power supply: the one board behind the CRT carries the mains input, the switch-mode supply, the horizontal and vertical sweep and the video amplifier. Apple sold it in two versions, 661-0462 for 110 V and 661-76214 for 220 V; the 128K/512K Service Source lists 661-76214 as the power/sweep board for those machines. Apple's 1994 Module Identification manual also lists 661-0461, the 110 V board with a beige power switch instead of a platinum one, and says 661-0461 and 661-0462 replaced the earlier 661-76153. Pina publishes separate parts lists for the U.S. and International boards, and the two use the same designators for different parts. Console5's list for board 630-0102 matches Pina's U.S. list position for position, and its list for 630-0108 matches his International list; Mac84 describes 630-0102 as the US/120 V board. Read the 630- number on your board before using either table.

U.S. board (630-0102)

U.S. power/sweep board, 630-0102 (Pina, Appendix C; Console5)

Reference	Capacitance	Voltage	Rated temperature	Type (Pina)
C1	3.9 µF	25 V (35 V alternative)	85 °C	Non-polar, HF; see below
C2	4700 µF	16 V	85 °C	Radial
C3	220 µF	16 V	105 °C	Radial
C5	47 µF	10 V	85 °C	Axial
C6	22 µF	50 V	85 °C	Axial
C9	10 µF	160 V	85 °C	Axial
C11	33 µF	16 V	85 °C	Radial
C13	33 µF	16 V	85 °C	Radial
C14	22 µF	50 V	85 °C	Radial
C24	2200 µF	10 V or 16 V	85 °C	Radial
C25	1000 µF	16 V	105 °C	Radial
C26	1000 µF	16 V	105 °C	Radial
C29	2200 µF	10 V	105 °C	Radial
C30	220 µF	16 V	105 °C	Radial
C31	1000 µF	16 V	105 °C	Radial
C33	4700 pF	250 V	-	RIFA safety capacitor
C35	100 µF	200 V	85 °C	Radial
C36	100 µF	200 V	85 °C	Radial
C37	4700 pF	250 V	-	RIFA safety capacitor
C38	0.1 µF	250 V	-	RIFA safety capacitor
C39	470 µF	10 V	85 °C	Radial
C43	470 µF	10 V	85 °C	Radial

Values, voltages and temperatures are Pina's originals; he also lists acceptable 25 V parts for C25, C26 and C31, and 16 V parts for C24 and C29. Console5's kit list has the same values, gives C38 as 275 V, and notes that C3 and C30 are 25 V on some revisions and that early boards have a series resistor on the negative lead of C6. Console5 says C33 and C37 can stay where they are ceramic discs; RIFA parts in those positions should be replaced. The board Mac84 photographed, from a Macintosh 512K, had higher-rated parts fitted at C1 (50 V), C24 and C29 (16 V) and C25, C26 and C31 (35 V). Fit the rating on your own board or higher.

Thomas Lee names C24-C26 and C29-C31 as weak spots: with no fan they run too hot, which shortens their life. He names rectifiers CR20 and CR21 as the other weak spots on this side of the supply.

International board (630-0108)

International power/sweep board, 630-0108 (Pina, Appendix D; Console5)

Reference	Capacitance	Voltage	Type (Pina)
C1	3.9 µF	25 V (35 V alternative)	Non-polar, HF; see below
C2	4700 µF	16 V	Radial
C3	220 µF	16 V	Radial, 105 °C
C5	47 µF	10 V	Axial
C6	22 µF	50 V	Axial
C10	33 µF	16 V	Radial
C11	10 µF	160 V	Radial
C12	22 µF	50 V	Axial
C18	33 µF	16 V	Axial
C24	2200 µF	16 V	Radial, 105 °C
C27	2200 µF	10 V	Radial, 105 °C
C28	1000 µF	16 V	Radial, 105 °C
C30	2200 µF	10 V	Radial, 105 °C
C31	220 µF	16 V	Radial, 105 °C
C32	2200 µF	16 V	Radial, 105 °C
C33	4700 pF	250 V	RIFA safety capacitor
C34	100 µF	200 V	Radial
C35	100 µF	200 V	Radial
C36	4700 pF	250 V	RIFA safety capacitor
C37	0.47 µF	250 V	RIFA safety capacitor
C38	100 µF	200 V	Radial
C39	100 µF	200 V	Radial
C42	470 µF	10 V	Radial
C45	470 µF	10 V	Radial

Pina lists 16 V alternatives for C27 and C30, and Console5 notes that later builds used 16 V there. Console5 notes that C33 and C36 may be ceramic discs on some builds, and that C37 is sometimes a RIFA part. On this board C9 is a 0.22 µF 100 V film capacitor, not an electrolytic.

C1

C1 is in the horizontal sweep circuit and carries large high-frequency currents. It is a non-polar part. Pina's lists give 3.9 µF non-polar electrolytics at 25 V or 35 V as original, and accept a 4.0 µF axial, a 3.9 µF 100 V non-polar electrolytic, or 3.9 µF metallised polyester or polypropylene film at 100 V or 250 V. In The Dead Mac Scrolls he recommends the 100 V part to stop the fault recurring. Lee finds 3.9 µF or 4.7 µF fitted, depending on model and date, and describes the original type as unsuitable: its ESR rises as it warms, which heats it further. He recommends a non-polar, low-ESR part rated at 50 V or more, or four or five 1 µF 50 V ceramic disc capacitors in parallel. Console5 notes that later boards may have a 3.3 µF 50 V part or an axial film capacitor here.

Pina describes the failure in stages: poor horizontal linearity (things on the left wider than on the right), then bright diagonal lines a few minutes after switch-on with the picture darkening, or a bright vertical line, and finally smoke. Switch off at the first sign.

Before recapping: common non-capacitor faults

Recap-a-Mac's triage for a Plus:

• Completely dead, no chime: almost certainly the analog board. Check the RIFA capacitors for cracking or blackening, the fuse, cracked solder joints (especially at the connectors), the resistors along the bottom of the board and the diodes, then recap.
• Chimes, but the screen is black: check the brightness control on the bottom front of the case, then look for cracked joints at the connectors.
• Boots, but the picture flickers or wobbles, or the machine restarts at random: cracked joints at the connectors, then check and adjust the output voltage; if both are good, recap.

Lee calls cracked solder joints at the yoke connector J1 the most common fault on the analog board. They cause 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. Resolder all four J1 pins with fresh solder while the connector halves are still mated, or the heat will distort the plastic. Lee's pinout of the three analog board connectors, viewed from the component side:

Power/sweep board connectors (Thomas Lee, Figure 16)

Pin	J1 (yoke)	J4 (to logic board)	J2 (to CRT socket board)
1	Vertical yoke high	Video	G1 bias
2	Vertical yoke low	-	Not connected
3	Ground	Horizontal sync	Focus
4	Horizontal yoke	Sound	-
5		Vertical sync	Brightness
6		+5 V	Ground
7		Ground	Ground
8		−12 V	CRT +12 V
9		Ground	CRT cathode
10		+12 V
11		+4.5 V battery

Replacement procedure

1. Unplug the machine, remove the case and discharge the CRT; see CRT Discharge Procedure.
2. Measure across the 200 V capacitors to confirm they are discharged.
3. Photograph the board, recording the polarity of every electrolytic.
4. Remove each part cleanly with desoldering braid or a pump. Do not lever a part out on one molten lead.
5. Clean the pads and check for corrosion or lifted tracks.
6. Fit the new parts with the negative stripe matching the original. C1 is non-polar.
7. Inspect the joints under magnification for bridges and cold joints.
8. Check each rail for shorts before applying power.
9. Set the voltages as below.

Tools are listed on Recommended Tools.

Voltage adjustment

Apple requires the voltages to be checked whenever the logic board or the power/sweep board is replaced.

1. Set the meter to the 20 V DC range and connect Apple's voltage test cable, part 077-0135, to the external drive port. Keep the insulating hoods on the plugs not in use: shorting them together damages the computer.
2. Connect the black lead to the chassis and the orange lead to the meter. Switch on and set the voltage control on the power/sweep board for +12 V between 11.90 V and 12.75 V.
3. Switch off, change to the red lead, switch on and set +5 V between 4.85 V and 5.15 V.
4. Repeat both checks. If both cannot be brought into tolerance together, replace the board.

Source: Apple's Macintosh Plus Service Source. Apple's 1987 Technical Procedures for the Macintosh and Macintosh Plus give a tighter +12 V window, 11.90-12.3 V, adjusted to 12.3 V, and take the readings with no peripherals attached and the internal drive not running.

Without the test cable, Lee measures at the external floppy connector: +5 V on pin 6, +12 V on pins 7 and 8, −12 V on pin 5, with ground on the threaded standoffs either side. He sets +5 V to 5.00 V and then checks that +12 V is still between 11.9 V and 12.75 V, cold and warm.

The control is R56, just above the speaker. It is the supply's only voltage adjustment: the regulator senses the +12 V output through R35, R56 and R38, and the other outputs follow it through the transformer turns ratios, so +5 V and +12 V move together. If +5 V is low because of resistance in the J4 connector, turning R56 up to compensate drives +12 V over its limit. Lee's first step for a Plus that bongs, resets and bongs again is to clean the J4 contacts, which he says fixes nine cases in ten, and he gives the same cause for most failures to get both rails into range at once. Viewed from the side covered by the cardboard insulator, turning R56 clockwise lowers the output.

The "whup-whup-whup" noise

If the +12 V rail rises about 15 % over specification, a 12 A crowbar SCR, Q8, short-circuits it and forces the supply into current limiting. The supply keeps trying to recover, which makes a rhythmic "whup-whup-whup" sound. 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

From Apple's exploded view for the Macintosh Plus:

• Power/sweep board: 661-0462 (110 V), 661-76214 (220 V)
• Logic board: 661-0525
• CRT and yoke assembly: 076-0103; CRT socket cable 590-0160
• Front bezel: 810-0379 (810-0385 platinum)
• Rear housing: 630-5211 (630-5235 platinum)

Choosing replacements

• Match or exceed the voltage rating on the part you remove; where sources differ, the higher rating is safe.
• Use 105 °C parts where Pina's list specifies them (C3, C25, C26, C29, C30 and C31 on the U.S. board).
• Fit only a non-polar part at C1.
• Replace RIFA metallised-paper safety capacitors; ceramic discs in the same positions can stay.

Documentation

• Macintosh Repair & Upgrade Secrets: Appendix C, U.S. analog board parts list (p. 327); Appendix D, International analog board (p. 335).
• The Dead Mac Scrolls
• Macintosh Plus Service Source
• Apple Technical Procedures: Macintosh and Macintosh Plus (1987)

Related pages

• Macintosh Plus
• Macintosh Plus Troubleshooting
• Macintosh Plus General Maintenance
• Macintosh 128K/512K Capacitor Replacement Guide
• Capacitor Failure Symptoms