Macintosh Quadra 840AV General Maintenance

The Quadra 840AV shares its enclosure with the Quadra 800

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

Opening the Machine

Apple's procedure: loosen the four captive cover screws, slide the cover forward about half an inch, and lift it off. The screws stay in the cover; there are no release latches to press.

Unplug the mains lead first. The power supply's primary side sits at mains potential and its bulk capacitors hold a charge after the machine is switched off.

Regular Cleaning

Internal

• Power off and unplug before opening.
• Blow dust off the logic board, power supply and fan with compressed air.
• Pay particular attention to the processor heatsink — the 40 MHz 68040 is the hottest-running part in the machine.
• Clean the fan blades with a soft brush and listen for bearing rumble while the machine runs.
• Look for capacitor leakage and corrosion every time you are inside. On this board that is the most valuable thing you can do — see below.

External

• Case: damp cloth and mild detergent; isopropyl alcohol on stubborn marks.
• Clear the ventilation grilles. Apple's troubleshooting lists blocked air vents as a cause of intermittent shutdowns, because the thermal protection circuit trips; the machine recovers after 30 to 40 minutes. That is normal behaviour, not a fault.
• Keep moisture out of the ports and vents.

For plastic yellowing see Retrobrite.

Keyboard and Mouse

• Remove keycaps with a puller for a deep clean; blow out the debris and wipe the caps with a mild soap solution.
• Clean an ADB mouse's ball and rollers with isopropyl alcohol.
• Check the ADB cable for damage at the strain relief.

Capacitor Health — the Main Event

The Quadra 840AV logic board carries surface-mount aluminium electrolytic capacitors that leak, and its traces are exceptionally thin. That combination is why this model, and the AV machines generally, have a reputation for being difficult to save once left alone. There are fifteen electrolytics: thirteen 47 µF 16 V and two 10 µF 16 V.

What to look for:

• Audio faults first — distortion, crackling, sound from one channel only, or none at all.
• Loss of AV functions — no audio input or output, failed video capture, GeoPort problems. Apple attributes these to the DSP3210, and the chip does fail, but on an unrecapped board corrosion around it is far more likely.
• Instability that changes with temperature or when the board is flexed.
• A dark glossy halo around a capacitor can, or crusty white-green deposits on the pads. Wipe with isopropyl alcohol and inspect under magnification — a leaking capacitor usually looks fine from above.

Do not wait for symptoms. See Macintosh Quadra 840AV Capacitor Replacement Guide for values, replacement parts and procedure, and Capacitor Failure Symptoms for what failure looks like elsewhere.

PRAM Battery

Apple's check: set a voltmeter to the 10 V DC range, measure across the cell, and replace it if it reads below 3.2 V. Measure it rather than replacing on a calendar — and note that this threshold is specific to this generation. Apple specifies 2.8 V for the Quadra 700, IIcx and IIci, and 3.0 V for the all-in-one Performa 5000 machines, which use a different battery entirely.

The cell sits in a holder with a latched cover. Prise the latch open with a small flat-blade screwdriver, lift the cover off, and lift the cell out; it is not soldered in. Watch the polarity going back in.

Apple lists a battery check among the first steps for intermittent shutdowns, because the cell is part of the soft-power circuit.

If the cell has leaked

On a board with traces this fine, a leaking cell is at least as destructive as a leaking capacitor:

• Remove it at once.
• Clean the residue with isopropyl alcohol and check continuity on every trace running out from under the holder.
• Expect the damage to extend further than it looks — corrosion travels under the solder mask.

See Battery Explosion, Capacitor or Corrosion Damage and Battery Refurbishment.

Logic Board Reset

Apple documents this and is explicit that it should be done before replacing any modules on a machine that will not power up:

1. Unplug the computer.
2. Remove the cover and the logic board, then 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 5–10 minutes, refit the cell the right way round, and reassemble.

This resets PRAM, so date, time and other settings will need re-entering. A machine that comes back after this had a latched-up soft-power circuit, not a failed supply.

Connectors and Sockets

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

• 72-pin DRAM SIMM sockets — oxidation here produces the two-part error chord at start-up that Apple's chart routes to SIMM verification.
• VRAM SIMM sockets — four slots in two banks.
• NuBus connectors, if cards are not being detected.
• The power supply connector on the logic board — look for discoloured or corroded pins.
• Internal and external SCSI connectors — check for bent pins.
• The floppy drive ribbon cable and its connectors.

Power Supply and Power Budget

Apple's electrical figures:

Item	Apple specification
Line voltage	100–240 V AC, self-configuring
Frequency	50–60 Hz, single phase
Maximum power	200 W continuous
Per SCSI device	1.5 A at +5 V, 4.3 A at +12 V maximum continuous
Per NuBus card	2.0 A at +5 V, 0.175 A at +12 V, 0.15 A at −12 V maximum continuous

The per-device budgets are worth knowing: a machine that only misbehaves with a particular drive or card fitted may simply be over its allowance rather than faulty.

Apple publishes no rail tolerance band and no power-supply adjustment procedure for this machine. There is no adjustment chapter in the Service Source and no potentiometer to turn. If a rail is out, the supply is faulty — there is nothing to trim. For capacitor values and repair notes for the supply itself (a Delta SMP-220DB, Apple P/N 614-0012, shared with the Quadra 800 and the Power Macintosh 8100/80), see Macintosh Quadra 840AV Capacitor Replacement Guide.

Isolating a noisy supply

Apple's own procedure for clicking, chirping or thumping from the supply is a load-isolation test rather than a straight replacement:

1. Remove all NuBus cards and disconnect every internal peripheral — hard drives, CD-ROM, floppy.
2. Power on. If the machine now starts normally, the supply is probably fine.
3. Reconnect one device or card at a time, powering on after each. The faulty module is the one that brings the symptom back.

Only if the noise persists with everything disconnected does Apple call for a new supply.

Thermal Management

The 40 MHz 68040 generates real heat. Check that:

• The processor heatsink is properly seated and its thermal compound has not dried out.
• The fan spins freely and quietly.
• The vents are clear — see the thermal shutdown note above.

Related Pages

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