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

 

look like mini 'neons' to me .. just two electrodes separated by about a mm , in a glass envelope containing low pressure Neon or perhaps some other gas ...

They never die , no filament to burn out ...Feed mains voltage through a 47k ohm resistor into them , and they should glow red.

 

They are neon lamps- used in some circuits as trigger or voltage regulating elements.

They have very non-linear I/V curves, making them very useful for certain applications.

 

Those Neon bulbs don't conduct electricity until they reach a certain voltage, called the "Ignition" point, at which time they conduct, making them useful in various ways. One way they can be used is to discharge a capacitor. When a cap voltage steadily climbs, at a certain point they conduct and discharge the cap, resetting the voltage. This can be useful in a timing circuit. In an oscilloscope it can be used to reset the time sweep; meaning the little dot that appears at the left edge of the screen and slowly (or rapidly - depending on the setting) moves across the screen. When it gets to the right side (or slightly off screen) the Neon bulb discharges the sweep capacitor and resets the little dot back to the left side to continue its journey across the screen again. Meanwhile a different circuit controls how high or low that dot moves vertically. Combine the two and you get useful information. But I'm betting you already know that.

 

In CRT displays we used spark gaps (sometimes gas filled) to protect circuitry in the case of arcing in the CRT. The arcs are brief but could be thousands of amps -sometimes into susceptible circuitry.

 

They never die , no filament to burn out

Actually, they do go bad. While working out of state and staying in hotels many of which have neon illuminated switches in certain areas, those bulbs flicker annoyingly. Had one so bad that I took the label off of my deodorant and cut it and wrapped it around the switch to block out the annoying flickering light. They DO go bad. Just guessing here, but I think when powered via DC voltage they go bad quicker because the surface of the electrode erodes when current flows in one direction all the time. I could be wrong, that's why I'm guessing. The reason for this assumption is that in an AC circuit, the eroding electrode deposits on the adjacent electrode. When current reverses the erosion reverses, depositing material back onto the first electrode. But that's just summation on my part.

 

On some Tektronix ) don't know about other brands) scopes neon bulbs were used to indicate the beam's position - especially useful when the beam was off the screen by a large amount.

Can you tell us the make and model of the scope?