Macintosh Quadra 840AV General Maintenance

Macintosh Quadra 840AV

Maintenance guidance for the Macintosh Quadra 840AV, a 40 MHz 68040 machine with an AT&T DSP3210 digital signal processor, built in the same case and on the same power supply as the Quadra 800.

Opening the machine

Unplug the mains lead. Loosen the four captive cover screws, slide the cover forward about 1/2 inch and lift it off.

Drives slide out once their retaining clip is pressed and their cables are disconnected. Before refitting the CD-ROM drive, make sure the floppy drive cable is seated in its channel, or the CD-ROM drive can damage it. The power supply comes out after the chassis support brace and the logic board power cable: press the latch, slide the supply forward and pull it out.

To remove the logic board, disconnect the SCSI, logic board power, speaker, CD-ROM audio, floppy and LED cables, remove the power-on actuator and the board's screw, slide the board forward until the slot lines up with the tab, lift the latch at the rear and pivot the front of the board out, then disconnect the composite video input and output cables.

Regular cleaning

• Blow dust out of the power supply, fan and logic board with compressed air.
• Keep the vents clear. Apple's first cure for intermittent shutdowns is to clear them: the thermal protection circuit can shut the system down, and it should work again after 30 to 40 minutes.
• Clean the mouse; Apple's chart for a cursor that does not move includes inspecting the inside of the mouse for dirt.
• For yellowed plastics see Retrobrite.

The floppy drive is a 1.4 MB SuperDrive. Apple says to fit dust shields to SuperDrives in this machine, and replacement drives ship with one fitted. For cleaning and lubrication see Macintosh Floppy Drive Maintenance.

Capacitors

The logic board carries fifteen surface-mount aluminium electrolytic capacitors (thirteen 47 µF 16 V and two 10 µF 16 V), and its traces are exceptionally thin, so leaked electrolyte damages it easily. Each time the machine is open, look for a dark glossy halo around a capacitor or white or green deposits on its pads. Replacing them is covered on the Macintosh Quadra 840AV Capacitor Replacement Guide; see also Capacitor Failure Symptoms.

PRAM battery

The battery is a lithium cell in a latched holder on the logic board, and the Service Source's checks need the cover and logic board removed.

1. Set the voltmeter to the 10 V DC scale.
2. Hold the positive probe to the positive end of the cell (marked "+" on the logic board) and the negative probe to the negative end.
3. If the cell reads below 3.2 V, replace it: prise open the latch at the end of the holder with a small flat-blade screwdriver, lift off the cover and lift the cell out.

Apple's charts list a battery check for intermittent shutdowns. If the cell has leaked, remove it and clean the board before powering it again; see Battery Explosion, Capacitor or Corrosion Damage and Battery Refurbishment.

Logic board reset

Apple says that whenever a Quadra 840AV fails to power up, the logic board should be reset before any module is replaced:

1. Unplug the computer, remove the cover and the logic board.
2. Open the battery holder and remove the cell.
3. With the power supply cable disconnected from the logic board, press the computer's power-on button.
4. Wait 5 to 10 minutes, refit the cell the correct way round and reassemble.

The reset clears PRAM, so check the date, time and other settings afterwards.

Connectors and sockets

Clean with a contact cleaner such as DeoxIT D5 and let it dry before powering on:

• the four 72-pin DRAM SIMM sockets. The 840AV needs 60 ns SIMMs that support CAS-before-RAS refresh, with no more than eight chips per bank; Apple's chart sends a two-part error chord at start-up to the SIMMs;
• the VRAM SIMM sockets (80 ns or faster 256K VRAM SIMMs; four slots in two banks);
• the three NuBus slots;
• the ADB port, which supplies +5 V at up to 500 mA through a 1.25 A circuit breaker;
• the SCSI and floppy cables.

Power supply and power budgets

Quadra 840AV electrical specification

Item	Apple's figure
Input	100-240 V AC, 50-60 Hz single phase
Maximum power	200 W continuous
Each SCSI device	1.5 A at +5 V, 4.3 A at +12 V, maximum continuous
Each NuBus card	2.0 A at +5 V, 0.175 A at +12 V, 0.15 A at −12 V, maximum continuous

The Service Source has no voltage-adjustment procedure and no rail tolerance table. The supply is Apple part 661-1687, the same as the Quadra 800's; its capacitors are covered on the Macintosh Quadra 800 Capacitor Replacement Guide.

For clicking, chirping or thumping from the supply, Apple's first step is a load-isolation test rather than a new supply:

1. Remove all NuBus cards and disconnect every internal peripheral (hard drives, CD-ROM drive, floppy drive).
2. Power on. If the machine starts normally, reconnect one device or card at a time, powering on after each. The module that brings the symptom back is the faulty one.
3. If the noise persists with everything disconnected, replace the power supply, then the logic board.

Apple's charts also send "intermittent shutdowns from a cold boot" straight to the power supply.

SCSI

The bus takes up to eight devices including the computer, which is always ID 7. Built-in circuitry terminates the internal and external SCSI cables automatically; Apple's chart requires the device at the end of the internal cable to be the only terminated internal device. An internal CD-300 is jumpered to SCSI ID 2 on this machine.

Related pages

• Macintosh Quadra 840AV
• Macintosh Quadra 840AV Troubleshooting
• Macintosh Quadra 840AV Capacitor Replacement Guide
• Battery Explosion, Capacitor or Corrosion Damage
• Recommended Tools