Neon Lamps
Neon lamps are used for at least three different things in Tek instruments:
- Indicator lamps, e.g., for power, beam position, uncalibrated status, or a decimal point
- Voltage clamp or stabilizer, similar to how a zener diode is used
- In relaxation oscillators, e.g. in 7000-series power supplies
Neon lamps fail more than most other components.
A common failure mode is for the leads to break off, often close to the glass bulb, due to corrosion and vibration.
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Broken lead (left) on NE-2 bulb in a 585 time base
Another failure mode is for the I-V characteristic to drift, especially after very long operation at DC:
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An NE2 tube from a 511A horizontal output stage. Operating voltage had drifted up to 102 V from nominal 55 V.
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The same tube connected in reverse. Operating voltage was 70 V.
Neon lamps are used as a voltage clamp in the CRT circuit for setting the CRT grid voltage in many Tek instruments.
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Neons as clamps in the 561A CRT circuit.
Another common use is for DC bias shifting, e.g. in 500-series timebase circuits, or the horizontal output of the 511. In such circuits, a 56 V Zener diode may be a possible replacement for the neon.
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545 timebase circuit
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511A sweep output amplifier. The manual specifies the voltage across each string of five NE-2 lamps connected in series should be 280±30 V.
An example for a neon based relaxation oscillator can be found in some 7000-series high-efficiency (switch-mode) power supplies to indicate remaining high voltage presence on the main primary filter capacitors:
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Neon flasher circuit in 7904 SMPS (DS1219)
When in doubt, replace the neon lamps. In a pinch, it may be possible to reattach broken leads after careful grinding of the glass.
Bruce Baur said:
I had to replace a neon that had a broken lead in a 511. It would oscillate. I had lots of neons and would put them on a curve tracer to check. All liked to oscillate. I would then take the neon and bang it on the table, trying to make the internal elements non parallel. I broke a few, but I finally got the internal elements non parallel and the low frequency oscillation stopped.
See also
- Neon lamp @ Wikipedia
- GE Glow Lamp Manual, 2nd Ed. 1965
- William G. Miller, Using and Understanding Miniature Neon Lamps (1969)
- Nixie tubes
- Voltage regulator tubes
- Curve trace of an NE-2 from IMSAI Guy @ YouTube
Types and data
| Type | Tek P/N | Voltage | Current | Notes | |
|---|---|---|---|---|---|
| Ignition | Operating | Nominal | |||
| NE-2 | 150-002, 150-0002-00 | 76 (60−90) V | 58 V | 0.3 mA | =~ modern type A1A |
| NE-2V | 150-030, 150-0030-00 | 76 (60−90) V | 58 (46-78) V | 0.6 mA | short length, formed tip for end-on viewing; dark compensated; =~ modern type A2B |
| NE-23 | 150-027, 150-0027-00 | 74 (60−90) V | 59 V | 0.3 mA | dark compensated, intended as voltage reference |
| A1D-T | 150-0035-00 | 0.3 mA | DS1219 in 7904 SMPS | ||
Please add
Dark compensation
Neon lamps require a higher voltage to ignite if operated in total dark, this is known as Dark effect. This was compensated in some types by a "mild radioactive additive" to help with ionization, typically a small amount of Krypton-85 (beta emitter, 10.8 years half life) added to the gas filling.
After 50 years, radiation from this source will have reduced to ~3% of the intensity at new. Modern NE-2 variants will likely not contain such an additive in the first place.