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CRT safety & high-voltage discharge

Read this before any work on an arcade monitor or CRT. A tube stores a lethal high voltage even after being unplugged for days. This page covers discharge, hot-chassis and mechanical risks.

Lethal hazard

The anode of an arcade CRT sits at ~20-30 kV. The tube acts like a large capacitor that holds its charge when off. The chassis's primary electrolytics (often ~400 V) stay charged too. Never put your hands in a chassis without discharging, and never work alone if you are untrained.

Discharging the anode (before any work)

  1. Unplug from mains.
  2. Get a clip lead connected to the tube's ground return (the ground braid / the outer aquadag coating — not a floating chassis).
  3. Slide an insulated-handle screwdriver under the anode cap (the rubber suction cup) until it touches the metal clip beneath it.
  4. Wait for a possible "crack." Repeat — the charge can rebuild (dielectric effect). Some leave the discharge lead in place during work.

Discharge to the RIGHT point

Discharge to the tube ground, not a random point: on a hot-chassis the "chassis" may be at mains potential. Use the HV ground return.

Hot-chassis & isolation transformer

  • Many arcade chassis are "hot": the circuit ground is tied to mains (no galvanic isolation). Touching ground and earth = shock.
  • Always put an isolation transformer between mains and the monitor/cabinet when working on it powered. It breaks the earth reference and makes a hand-to-earth path non-lethal (but stay careful).
  • Measurement: a scope on a hot-chassis without isolation can short through the probe ground → isolation mandatory.

Powered adjustments (rules)

  • One hand in your pocket (avoids arm-to-arm current through the heart).
  • Ceramic/plastic alignment tools for adjustments (coils, transformers).
  • Locate G2/Screen and Focus on the flyback (pots). Brightness/sharpness — see the specific chassis page.
  • Don't bridge things at random; keep your free hand away from the chassis.

Mechanical risk (implosion)

  • The CRT is evacuated: a knock on the neck (fragile) or the screen can cause an implosion (flying glass). Handle with goggles, gloves, and never by the neck.
  • Transport: protect the neck base (the RGB board plugs onto it).

Degauss, purity, convergence (reminder)

  • Degauss: the coil erases magnetisation (colour blotches) at power-up; a persistent blotch = manual degauss.
  • Purity / convergence: neck-magnet and coil adjustments — see the chassis manual (specific values, not invented here).

See also

Sources & attribution

  • Synthesis of established CRT safety practice (arcade/TV); precise adjustments = chassis manual.

Review & corrections

How this space is used

Spotted a wrong value, an outdated procedure, a chip reference that does not match your board? Say so here, with what you observed (model, board revision, serial number, measurement). Every report is cross-checked against a source before anything changes — an unverifiable correction is published as “reported by …, not cross-checked” rather than silently applied.

Reading is open to everyone; posting requires signing in with Discord. Reports from the wiki's declared reviewers are handled first; anyone else's are read too, but go through a human before anything is changed.