2004Monitors forum
Focus tube identification and CRT replacement for IIIa monitors
22 years old. Gear and prices may have changed since.
What the thread found
- The focus tube in IIIa monitors is typically a NL-7235 made by National Electronics or Mu.
- Replacement CRT tubes can be sourced from Chelmer Valve or Richardson Electronics.
- Poor voltage regulation to the heater causes premature tube failure, resulting in focus drift when switching from standby to operating mode.
- Fresh battery power causes heater voltage to spike temporarily, making the tube hotter than normal and shifting focus characteristics until the circuit stabilizes.
- A focus tube regulator board using a low-dropout voltage regulator to maintain heater voltage below 12V can prevent focus drift caused by inconsistent heater voltage.
Depends on
- The availability of the custom regulator board is unclear from the discussion.
- The specific cause of tube failure mentioned applies to the heater voltage regulation issue and may not account for all CRT failures.