Electronics > Beginners

Neon lamp on analogic scope trigger pcb

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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:

--- Quote from: gkmaia on April 15, 2019, 03:29:21 am ---Thanks for all the answers.

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

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

tggzzz:

--- Quote from: gkmaia on April 15, 2019, 02:03:21 am ---
--- Quote from: tggzzz on April 14, 2019, 08:47:48 pm ---Show us the complete diagram, especially whatever the neons re connected to via P402.

--- End quote ---

There you go!

--- End quote ---

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.

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