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| What are these oscilloscope lightbulbs? |
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| chucrut:
Hello, I have an analog oscilloscope with no trace. I was checking the problem and found these two "lighbulbs" near the crt socket. I tested conductivity and they are not electrical conductive inside. They dont have visible filament. Any ideas what are this "lightbulbs"?. Maybe they are burned because they are not conductive inside?. Maybe they are something more sophisticate? Many thanks! Agustin |
| ataradov:
They are neon bulbs, they are supposed to be open. They specifically look like very classical NE-2 bulbs. They can have multiple uses in a circuit, so you would need to provide more context for where they are located on the board. Most likely they serve as some sort of protection. |
| chris_leyson:
They are neon lamps used to generate a fixed voltage drop in the CRT resistive biasing chain. Typical breakdown voltage might be 60V to 90V depending on the lamp used. Like ataradov said, it all depends how they are used in the circuit but as they are located near the CRT socket they are most likely used for protection. |
| David Hess:
In oscilloscope CRT circuits, the neon lamps usually serve as low leakage low capacitance shunt protection to prevent the grid to cathode and heater to cathode voltages from exceeding their maximum limits. Since the cathode voltage is in the range of -2000 to -3000 volts, this is pretty important. Often they will flash when power is applied or removed. Tube based oscilloscopes may use them for the same purpose at various points. In cheap designs, they might be used as voltage references. |
| james_s:
They were fairly common in CRT computer monitors and TVs too, for the same purpose of shunting HV to prevent arcs in the electron gun from damaging the electronics. They only ever light if an arc takes place. |
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