Macintosh Plus General Maintenance

A Macintosh Plus with the rear housing removed: CRT, power/sweep board (right) and floppy drive (photo: Cody Logan, CC BY-SA 4.0)

The Macintosh Plus is a compact Macintosh with a 9-inch CRT, a combined power supply and video sweep board, and a logic board that slides out from the rear. Maintenance on it is mostly cleaning, the PRAM battery, reseating connectors and ROMs, and Apple's voltage adjustment. The component-level faults below are taken from The Dead Mac Scrolls, whose "Early Macs" chapter covers the analog (power/sweep) and logic boards of the 128K, 512K, 512Ke and Plus.

The Plus contains a CRT and a high-voltage flyback circuit. Read CRT Discharge Procedure before opening the case. Apple's take-apart instructions discharge the CRT to the chassis ground lug and remove the anode cap before anything else is disconnected, and say never to wear a grounding wrist strap until the CRT has been discharged.

Regular cleaning

Internal cleaning

1. Unplug the machine, remove the battery compartment cover and any programmer's (reset/interrupt) switch, remove the case screws (one is inside the battery compartment) with a Torx screwdriver, and prise the rear housing off with a case-cracking (pull-apart) tool.
2. Discharge the CRT.
3. Lift dust from the logic board, the power/sweep board and the CRT neck with compressed air or an anti-static brush.
4. Clean sticky grime with isopropyl alcohol (90 % or stronger) and a soft brush. A board that has been washed must be completely dry before it is powered.

Case and screen

Clean the case with a damp microfibre cloth and mild soap; avoid abrasives. Spray glass cleaner onto the cloth, never onto the screen. Isopropyl alcohol lifts adhesive residue and nicotine staining from the plastics.

Keyboard and mouse

Remove the mouse ball and clean the rollers. Blow debris out from under the keys, and take keycaps off only for a deeper clean. The Plus keyboard (M0110A) connects to an RJ-11 socket; The Dead Mac Scrolls records an M0110A killed by a telephone cable used in its place (the 8048 controller under the space bar failed) and intermittent keyboards caused by bent contacts in the logic board's RJ-11 jack.

PRAM battery

Apple's specification gives the Plus parameter RAM as a CMOS custom chip holding 256 bytes, backed by a 4.5 V user-replaceable battery. The cell is an alkaline 4.5 V type: The Dead Mac Scrolls names the Eveready 523, and Low End Mac lists it as PX21 (Eveready 523, ANSI 1306AP, IEC 3LR50).

The battery sits in a compartment on the back of the case, but its holder is part of the power/sweep board: Apple's replacement note tells the technician to move the battery and contrast knob from a defective power/sweep board to the exchange board, and The Dead Mac Scrolls places the battery at location B1 on the analog board. A leaking cell therefore damages the power/sweep board around the holder.

• Alkaline leakage is neutralised with white vinegar and then rinsed with isopropyl alcohol. The lithium cells in later compacts such as the Macintosh SE need bicarbonate instead.
• Take the battery out of a machine that is going into storage. Without it the Plus runs normally but forgets the date, time and control panel settings when switched off.
• Replace it with a fresh 4.5 V alkaline cell of the same type.
• If leakage has reached the board, clean it and check continuity along any trace under the affected area.

The Dead Mac Scrolls gives the battery as the cause of a clock that does not keep time. See also Battery Explosion, Capacitor or Corrosion Damage.

Connector and socket corrosion

Oxidised contacts cause faults that come and go with temperature or a knock. Check:

• the logic board cable between the logic board and the power/sweep board;
• the yoke cable and the neck (CRT socket) cable on the power/sweep board;
• the floppy drive cable;
• the two socketed ROMs at D6 and D8, and the four SIMMs.

Apple's symptom chart lists "Connect logic board cable" as the first cure for a clicking or chirping sound. Clean the contacts and reseat. For a Sad Mac, see Sad Mac Error Codes and the Sad Mac Decoder.

Common failure points

Cracked solder joints on the analog board

The Dead Mac Scrolls traces several intermittent display faults on the early-Mac analog board to cracked joints that tapping on the cabinet makes worse:

• display blinking to black, white lines flashing, or two jittering interference lines: resolder pin 1 (composite video) of J4;
• display collapsing to a vertical line: resolder pin 4 of the yoke connector J1 and the width coil L2 (the joint may be under a foam pad);
• display collapsing to a horizontal line or rolling: resolder pin 1 of J1, pin 5 of J4 and capacitor C5;
• occasional bright flash across the screen: resolder pin 1 of J2.

Horizontal sweep capacitor C1

Poor linearity (items on the left wider than on the right), followed by bright diagonal lines, darkening and smoke, or a bright vertical line and smoke, is capacitor C1 (3.9 µF non-polarised, 25/35 V) in the horizontal sweep circuit. The Dead Mac Scrolls recommends a 100 V part and checking the yoke plug J1 for burning. Other capacitor values are on the Macintosh Plus Capacitor Replacement Guide; general background is on Capacitor Failure Symptoms.

Flyback transformer and horizontal output transistor

A high-pitched whine, a herringbone pattern, an ozone smell or a burning smell points at the flyback transformer T1. The book gives Apple part 157-0026-B and a later replacement type 157-0042-C, and says to check the BU406 horizontal output transistor Q3 at the same time; a flyback replaced without Q3 can leave a wiggling right-hand edge. A resistance check across CR5 with the power off separates a shorted flyback from a fault elsewhere in the supply.

Serial port faults that look like software

A Plus that resets a directly connected ImageWriter II several times at start-up, reports "The printer is not responding" on the first print only, or does not appear on an AppleTalk network, has a failed 26LS30 line driver at location B3 on the logic board. The same ImageWriter II works on the modem port, and an ImageWriter I works on either port, which rules out the printer.

Sound and mouse lock-ups

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

External floppy and mouse faults from one filter

An internal drive that works while known-good external 800K drives report every disk as unreadable, or a mouse that selects everything it touches with the button up, is the RC filter network (Apple part 115-0002) at location A11 on the Plus logic board.

Voltage check

The Macintosh Plus has a published voltage procedure with limits. Apple requires the check after replacing the logic board or the power/sweep board, and after replacing the CRT.

1. Remove the cover. Set the meter to its 20 V range.
2. Connect the Macintosh voltage test cable, Apple part number 077-0135, to the external drive port. Keep the insulating hoods over the plugs: shorting the test plugs together can damage the computer.
3. Connect the black meter lead between the meter's ground terminal and the chassis.
4. Connect the orange test lead to the meter, switch on, and adjust the voltage regulator on the power/sweep board to between 11.90 V and 12.75 V.
5. Switch off, hood the orange plug, connect the red lead, switch on, and adjust to between 4.85 V and 5.15 V.
6. Repeat both measurements. If the board will not adjust so that both voltages are within tolerance, Apple's instruction is to replace the power/sweep board.

The adjustment is made from the rear of the machine with the case off and the CRT live; Apple says to watch the screen in a mirror and not to reach around the computer.

The regulator is a single trimmer, R56, labelled "Voltage". It sets the switching supply as a whole, so +5 V and +12 V move together; Apple adjusts it against each rail in turn and then rechecks both. The Dead Mac Scrolls sets it for 5.0 V DC between pin 6 of the power/video cable and chassis ground. Two consequences:

• Winding R56 up to cure a low +5 V reading caused by a dirty connector pushes +12 V out of tolerance. Clean and reseat the logic board cable first.
• If the voltage is set too high the machine gives no start-up sound, makes a low-pitched "flup flup" noise and shows a dark screen. Turn R56 fully anticlockwise, restart, and set it again.

Apple's own symptom chart gives "Perform voltage adjustment" as the first cure when a Plus beeps continuously and keeps trying to power up, and warns that replacing only the power/sweep board in that case may damage the new board. On a 128K analog board, The Dead Mac Scrolls traces a supply that cannot be adjusted up to 5.0 V to the overvoltage reference diode at CR18, a 1N5349B 12 V 5 W zener.

Related pages

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