Macintosh Classic Capacitor Replacement Guide

This guide covers capacitor replacement on the Macintosh Classic — the logic board, which is specific to this machine, and the power/sweep board, which is shared with the Macintosh Classic II and is documented in full on that machine's page.

Before you start

Take the battery out first. The 1/2 AA cell in this family leaks and the electrolyte destroys traces, in some cases beyond repair. See Battery Explosion, Capacitor or Corrosion Damage.

Then follow the CRT Discharge Procedure. The power/sweep board carries mains on its primary side and the tube holds charge after the machine is unplugged.

Capacitor inspection

The surface-mount electrolytics on the logic board rarely bulge. Look instead for:

• Dark haloes or dried crust at the base of the cans.
• Dull or green solder joints on nearby IC pins.
• Thinned or missing traces, best seen under a loupe with light raking across the board.

Through-hole electrolytics on the power/sweep board do show the classic symptoms — domed tops, brown or white crust at the leads, staining on the board beneath them.

See Capacitor Failure Symptoms.

Logic board

Macintosh Classic logic board

Reference	Capacitance	Voltage	Quantity
C1 – C7	47 µF	16 V	7
C8	1 µF	50 V	1

Eight capacitors in total. From Recap-a-Mac's photographic guide, made from a physical board.

Note that this is not the Classic II's list. The Macintosh Classic II logic board is a different design with either 13 or 17 surface-mount electrolytics depending on revision; do not order a Classic II kit for a Classic.

Macintosh Classic logic board recapping guide. (Image: Recap-a-Mac)

Replacement parts

Recap-a-Mac uses solid tantalum at the same values on this board:

• 1 µF 50 V — KEMET T491B105K050AT
• 47 µF 16 V — KEMET T491D476K016AT

Solid tantalum has no liquid electrolyte and cannot leak, which is the failure mode that does the damage here. It fails short rather than open, so derate generously: a part rated well above the rail it sits on is far less likely to be pushed into failure by a switch-on surge. A 25 V part in the 47 µF positions is a common and sensible choice on a 5 V rail.

Power/sweep board

This board is shared with the Macintosh Classic II, and the full treatment — separate 120 V and 240 V capacitor lists, the board part numbers 630-0395, 630-0420, 630-0525 and 630-0560, the differences between Console5's and Recap-a-Mac's counts, the PP1 output adjustment, and the DIP ICs IF1, QP1 and JP1 that also fail on these boards — is on the Classic II capacitor guide. It is not duplicated here, because a second copy of a safety-critical table is a second copy to get out of step.

What you need to know before you go there:

• Read the part number off your board. 661-0597 and 661-0651 are the 110 V boards; 661-0599 and 661-0652 are the 220 V boards.
• If CP13 is a jumper rather than a capacitor, you have a 220–240 V board, and CP1 must be a 400 V part.
• Apple's Service Source Electrical specification lists only 100–120 V AC, 47–63 Hz, 100 W maximum — it documents the US machine. The parts list in the same document is what reveals that a 220 V board exists.

Replacement procedure

1. Remove the battery.
2. Discharge the CRT — see CRT Discharge Procedure.
3. Label every cable, then remove the logic board and, if you are working on it, the power/sweep board.
4. Desolder the old capacitors. On the logic board use hot air or a low-temperature technique; the pads lift easily on a board that has already been attacked by electrolyte.
5. Clean the board. Isopropyl alcohol and a soft brush, then inspect under magnification for thinned or broken traces near every capacitor position. Repair those before fitting anything new.
6. Fit the replacements with the correct polarity. Check twice on tantalum — a reversed tantalum fails short and takes the rail down with it.
7. Inspect the joints under magnification, trim the leads, and check for shorts across every rail with a meter before applying power.

After the recap

Apple publishes no rail voltages or tolerances for this machine, and no power-supply adjustment procedure. The Macintosh Classic / Classic II / Performa 200 Service Source hosted on this wiki gives only the mains side in its Electrical specification, and its Adjustments chapter covers video and yoke only — light meter setup, cutoff, brightness and contrast, centring, size and focus.

What you can check is that the rails sit inside the range the logic on the board is specified for. Five-volt logic is specified for a 5 V supply at ±5 per cent, which gives 4.75 V to 5.25 V. Treat anything outside that as a fault, and anything that sags under load or carries visible ripple as a fault regardless of what the DC reading says. Compare against a known-good example of the same model where you can.

Measure at the floppy drive or logic board power connector.

Recap-a-Mac reports that the output voltage on this board can be adjusted with the variable resistor PP1, which is not something Apple documents.

What not to do

• Do not "power the machine up periodically to keep the capacitors healthy". That advice circulates widely and it is the wrong way round for this machine: powering up a Classic with leaking surface-mount electrolytics drives electrolyte further into the board and turns a recap into a trace-repair job. Recap it, then use it. Regular use after that is fine.
• Do not fit a 250 V part at CP1 without confirming you have a 110 V board. See the warning at the top.
• Do not increase capacitance because bigger sounds better. On the power/sweep board it changes the inrush the rectifier sees.

Recommended tools

• Temperature-controlled soldering iron with a fine tip; hot air is easier on the logic board.
• Desoldering braid and a pump.
• Leaded solder, 0.5 mm.
• Isopropyl alcohol, a soft brush and a loupe.
• Multimeter.
• A CRT discharge tool — see Recommended Tools.

Related Pages

• Macintosh Classic · Macintosh Classic / Classic II General Maintenance · Macintosh Classic / Classic II Troubleshooting
• Macintosh Classic II Capacitor Replacement Guide — the shared power/sweep board, with both mains variants
• Macintosh Floppy Drive Maintenance · CRT Discharge Procedure · Capacitor Failure Symptoms · Battery Explosion, Capacitor or Corrosion Damage