Neon lamp on analogic scope trigger pcb

[gkmaia]

What are these neon lamps for when placed on a trigger board of an analog scope?

[alsetalokin4017]

voltage regulation

[tggzzz]

Show us the complete diagram, especially whatever the neons re connected to via P402.

[Cliff Matthews]

OP may find help here: https://www.eevblog.com/forum/testgear/tektronix-2246-no-beam!/msg314786/#msg314786
(Tek's 2246 uses this limiter too)

[gkmaia]

Show us the complete diagram, especially whatever the neons re connected to via P402.

There you go!

[Cliff Matthews]

Same idea, maybe they copied Tek limiting that grid to ~130v. A lot more than just triggering going on there IMO.

[floobydust]

The two series-connected neon lamps form a gas discharge tube protecting the CRT and driver from any static or arc discharges.
They go directly across the cathode-to-control grid of the CRT. Nothing to do with the trigger section. The two clamp diodes D407, D408 prevent any +ve CRT grid bias and the neons limit -ve grid bias to ~180VDC.
They should never be glowing except for maybe a blip on power up/down.

[gkmaia]

Thanks for all the answers.

What is right way to test these lamps and make sure they are in working condition?

[tautech]

Thanks for all the answers.

What is right way to test these lamps and make sure they are in working condition?

They rarely are crook as they're only ON in an over-voltage event (read fault).
Typically neons conduct ~90V.

[tggzzz]

Show us the complete diagram, especially whatever the neons re connected to via P402.

There you go!

That's more complete information, especially the scope type - other people might have specific experience of a particular scope type.

That isn't the case here, but we can see the neons are a traditional way to protect the tube from excessive grid-cathode voltages. In functioning Tektronix scopes they flicker on/off for up to 10s after the power has been turned off.