Macintosh Quadra 840AV Capacitor Replacement Guide

Replacing electrolytic capacitors (recapping) in your Macintosh Quadra 840AV is essential for long-term reliability. The surface-mount electrolytic capacitors used on the logic board can leak over time, causing corrosion and component failure.

Capacitor Inspection

Before recapping, inspect the logic board for signs of capacitor failure:

Bulging Tops – Capacitors with domed tops instead of flat are failing
Leaking Electrolyte – Brown or orange residue around capacitor bases
Corrosion or PCB Staining – Discoloration of the PCB near capacitors
Crystalline Deposits – White or yellowish crystite formations

Even capacitors that look fine externally may have leaked internally. Preventive recapping is recommended for any Quadra 840AV that hasn't been serviced.

Macintosh Quadra 840AV Capacitor List

Logic Board Capacitors

The Quadra 840AV logic board (820-0382) uses surface-mount electrolytic capacitors. The following list covers typical values found on the board:

**Logic Board Capacitor Specifications**
Location	Capacitance	Voltage	Type	Notes
Audio Section	10µF	16V	SMD Electrolytic	Multiple; critical for audio
Audio Section	47µF	16V	SMD Electrolytic	Audio coupling
Power Filtering	100µF	16V	SMD Electrolytic	Multiple locations
Power Filtering	220µF	16V	SMD Electrolytic	Main filter caps
Reset Circuit	10µF	16V	SMD Electrolytic	Near PRAM battery
Video Section	47µF	16V	SMD Electrolytic	Video I/O circuitry
DSP Section	10µF	16V	SMD Electrolytic	Near AT&T DSP 3210

Note: Exact capacitor locations and values should be verified against your specific board revision. The 840AV logic board went through revisions.

Power Supply Capacitors

The power supply contains larger through-hole electrolytic capacitors that may also need replacement:

**Power Supply Capacitor Examples**
Capacitance	Voltage	Type	Notes
1000µF	25V	Radial Electrolytic	Primary filter
470µF	25V	Radial Electrolytic	Secondary filter
220µF	16V	Radial Electrolytic	Output filtering
100µF	16V	Radial Electrolytic	Various locations
47µF	25V	Radial Electrolytic	Various locations

Warning: The power supply contains high-voltage capacitors. Discharge properly before servicing.

Capacitor Replacement Procedure

Required Tools

Temperature-controlled soldering iron (recommended: 350°C for SMD)
Fine tip for SMD work
Desoldering braid or solder wick
Flux (no-clean recommended)
Isopropyl alcohol (99%) and ESD-safe brushes
Multimeter
Magnification (loupe or microscope)
Anti-static mat and wrist strap
Safety glasses

Recommended Replacement Capacitors

Replace with high-quality capacitors:

SMD: Panasonic, Nichicon, Rubycon SMD electrolytics
Alternative: Polymer or tantalum capacitors (same value, check polarity)
Through-hole (PSU): Nichicon, Rubycon, Panasonic

Procedure

Prepare the workspace
- Use an anti-static mat
- Ensure good lighting and ventilation
- Have isopropyl alcohol ready for cleaning

Document capacitor positions
- Photograph the board before starting
- Note polarity markings on each capacitor

Remove old capacitors
- Apply flux to capacitor terminals
- Heat both terminals simultaneously if possible
- Gently lift the capacitor away
- Clean pads with solder wick and isopropyl alcohol

Clean the board
- Remove any leaked electrolyte with isopropyl alcohol
- Inspect traces for damage
- Check for lifted pads

Install new capacitors
- Apply fresh flux to pads
- Tin one pad with solder
- Position capacitor, observing correct polarity
- Tack one side, then solder the other
- Reflow first side if needed

Inspect and test
- Check all solder joints under magnification
- Verify no bridges or cold joints
- Test continuity where appropriate
- Clean flux residue with isopropyl alcohol

Voltage Adjustment After Recap

After recapping, verify power supply voltages:

**Target Voltage Ranges**
Rail	Target	Acceptable Range
+5V	5.0V	4.85V – 5.15V
+12V	12.0V	11.9V – 12.7V
-12V	-12.0V	-11.5V – -12.7V

Trace Repair

If capacitor leakage has damaged traces:

Identify damaged traces using multimeter continuity test
Scrape away damaged areas carefully
Jumper with fine wire (30 AWG recommended)
Secure jumper wires with UV-cure solder mask or epoxy
Document all repairs for future reference

Additional Tips

Work methodically – Replace capacitors one section at a time
Take photos – Document before, during, and after
Don't overheat – SMD pads can lift easily
Test incrementally – Power on and test after each section if possible
Keep spares – Order extra capacitors in case of damage
Consider tantalum – For critical areas, tantalum caps won't leak

Common Mistakes to Avoid

Incorrect polarity (will destroy new capacitors)
Excessive heat lifting pads
Incomplete cleaning of leaked electrolyte
Using low-quality replacement capacitors
Skipping power supply capacitors