CRT Discharge Procedure

CRT Inside a MM12 Monitor

Cathode Ray Tubes (CRTs), used in vintage Macintosh and other classic computers, can retain a dangerous high voltage charge for hours or even days after being powered off. To avoid electric shock or injury, proper discharge is essential before performing internal repairs.

⚠️ Safety Warning

Working with CRTs involves potentially lethal voltages (up to 25,000 volts). It will kill you if you don't safely discharge it. You should only discharge a CRT if you are confident and equipped to do so safely.

Minimum Safety Precautions

• Unplug the system and wait at least 30 minutes before starting.
• Use only insulated tools and wear rubber-soled shoes.
• Never work on a CRT in a damp or conductive environment.
• Discharge the CRT before touching or removing the analog board.

🧰 Tools Required

To safely discharge a CRT, you will need:

• Flat-head screwdriver – Preferably with a long, insulated handle.
• Alligator clip jumper wire – To ground the tool.
• Gloves – High-voltage insulating gloves recommended.
• Discharge wand (optional) – A purpose-built safe discharger for CRTs.

⚡ How to Build a Discharge Tool

A simple and safe CRT discharger can be made as follows:

1. Attach one end of the alligator clip wire to the metal shaft of a long flat-head screwdriver.
2. Attach the other end to a ground point on the CRT (see below).
3. Wrap the shaft in electrical tape to avoid accidental contact with bare metal.

🔌 Identifying the Ground Point

A safe ground point includes:

• The metal chassis of the CRT tube.
• The spring clamp grounding wire around the CRT funnel.
• The analog board’s metal shield or logic board ground plane.

🛠️ Discharge Procedure

CRT Anode Cap

Step-by-Step

1. Ensure the system is **unplugged** and left off for 30 minutes.
2. Attach your alligator clip to a grounded part of the CRT or chassis.
3. Hold the **screwdriver by the insulated handle only**.
4. Carefully slide the screwdriver tip under the rubber suction cup (anode cap) on the CRT.
5. You may hear a "pop" or crackle sound — this is normal.
6. Hold contact for at least **10 seconds** to allow the charge to dissipate.
7. Remove the anode cap by pinching and lifting it free from the CRT.

Repeat Discharge

Some CRTs can regain charge due to internal capacitance. After 10–15 minutes, it's a good idea to discharge the anode again before handling.

🧹 After Discharge Safety

• Always verify the CRT has been discharged before touching or removing the analog board.
• Keep your discharge tool handy during repair in case you need to re-discharge.
• After completing your work, reinstall the anode cap securely and avoid twisting it.