Macintosh Plus General Maintenance

Macintosh Plus Interior

Regular maintenance keeps a Macintosh Plus working and, more importantly, keeps a failing part from taking the rest of the machine with it. This page covers cleaning, the PRAM battery, connector corrosion, the parts that actually fail, and Apple's own voltage check.

The Macintosh Plus contains a CRT. Read CRT Discharge Procedure before the case is opened; Apple's own service instruction is to discharge the CRT to the ground lug before disconnecting anything, and never to wear a grounding wriststrap until after the CRT has been discharged.

Regular cleaning

The Plus has no fan, so dust settles rather than being blown through. Periodic cleaning is worth doing.

Internal cleaning

• Unplug the machine and discharge the CRT before opening the case.
• Use compressed air or an anti-static brush to lift dust from the logic board, the power/sweep board and the CRT.
• Sticky grime responds to isopropyl alcohol (>90 %) and a soft brush.
• Heavily contaminated boards may be washed with distilled water and dried thoroughly. Do not reassemble anything that is not completely dry.

External case and CRT

• A damp microfibre cloth and mild soap is enough for the case; avoid abrasives.
• Spray glass cleaner onto the cloth, never onto the screen.
• Isopropyl alcohol lifts nicotine staining and adhesive residue.

Keyboard and mouse

• Remove the mouse ball and clean the rollers.
• Blow debris out from under the keys; remove keycaps only if a deeper clean is needed.

PRAM battery

The Macintosh Plus uses a 4.5 V user-replaceable battery — Apple's specification calls it exactly that — in a compartment on the rear of the case, backing 256 bytes of parameter memory in the CMOS clock chip. The cell is the alkaline 4.5 V lantern-style type sold as Eveready 523 or PX21.

This is an alkaline cell, not a lithium one, which matters for two reasons:

• Its leakage is alkaline, so it is neutralised with white vinegar followed by an isopropyl alcohol rinse. (Lithium cells in later compacts such as the Macintosh SE need bicarbonate instead — do not carry the vinegar habit across.)
• It sits in an external compartment, so leakage usually attacks the holder and its leads rather than the logic board — but check the wiring where it enters the case.

Practical handling:

• Remove the battery if the machine is being stored. The Plus runs perfectly well without one; it simply forgets the date and the control panel settings.
• Replace with a fresh 4.5 V alkaline cell of the same type. Do not substitute a lithium cell of a different voltage into the holder without understanding what the charging/isolation circuit on your board does.
• If leakage has reached the board, clean it out and check continuity across any trace that runs under the affected area.

See also Battery Explosion, Capacitor or Corrosion Damage.

Connector and socket corrosion

Ageing contacts oxidise and cause faults that come and go with temperature or a knock.

Areas worth checking:

• Logic board to power/sweep board cable
• Power and video harnesses
• CRT yoke and flyback pins
• Socketed ROM and RAM — reseating these cures a surprising number of Sad Mac and no-boot faults. See Sad Mac Error Codes and the Sad Mac Decoder for what the code on screen is telling you.

Contact cleaner and a reseat is usually all that is required.

What actually fails

Cracked solder joints on the power/sweep board

Thermal cycling cracks joints around the flyback transformer, the yoke connector and the regulator devices. The symptoms are a flickering, intermittent or absent display. Reflow the suspect joints with fresh solder rather than simply touching the iron to them.

Electrolytic capacitors

Degraded electrolytics show up as unstable voltages, a shrunken or dim raster, and audible whine. Values and positions are on the Macintosh Plus Capacitor Replacement Guide; general background is on Capacitor Failure Symptoms.

Flyback transformer

Arcing noise, a burnt smell or loss of video points at the flyback. Replacement units come from donor compact Macs of the same generation.

Serial port faults that look like software

Two well-documented Plus faults sit in one chip. If the machine resets a directly connected ImageWriter II on the printer port, or never appears on an AppleTalk network while otherwise behaving normally, The Dead Mac Scrolls puts the fault on the 26LS30 line driver at board location B3 on the Plus logic board.

Sound and mouse lock-ups

A machine that plays a simple beep correctly but freezes immediately after a complex sound, with the pointer jumping first, is a failed 6522 VIA at location D11.

Voltage check

Unlike the later compacts, the Macintosh Plus does have a published voltage procedure, and Apple gives real limits. Apple requires the check after replacing either the logic board or the power/sweep board.

Apple's procedure, in outline:

1. Set the meter to the 20 V range.
2. Connect the Macintosh voltage test cable, Apple part number 077-0135, to the external floppy drive port. Pull the insulating hoods over the plugs — shorting the test plugs together can damage the computer.
3. Clip the black lead between the meter's ground terminal and the chassis.
4. With the orange lead on the meter's input, switch on and adjust the voltage regulator on the power/sweep board to between 11.90 V and 12.75 V.
5. Switch off, move to the red lead, switch on, and adjust to between 4.85 V and 5.15 V.
6. Repeat both measurements to confirm they are still in tolerance. If the board cannot be adjusted so that both rails fall inside those windows, Apple's instruction is to replace the power/sweep board.

Two warnings that come with the procedure: the adjustment is made from the rear of the machine, so use a mirror to see the screen rather than reaching around a live CRT, and the machine is running with the case open while you do it.

The control is silkscreened Voltage on the board. On the 128K/512K-generation power/sweep board The Dead Mac Scrolls documents the same adjustment as variable resistor R56, measured at pin 6 of the power/video cable for 5.0 V DC. One control moves both rails, which is why Apple has you set +12 V first and then confirm +5 V.

If the rails will not sit still, look at the electrolytics, the solder joints around the regulator and the switching transistors before condemning the board.

Related pages

• Macintosh Plus
• Macintosh Plus Troubleshooting
• Macintosh Plus Capacitor Replacement Guide
• Macintosh Plus Memory Upgrade Guide
• CRT Discharge Procedure
• Recommended Tools