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I'm restoring an HP 141T and it has a broken neon lamp which is used I believe as surge protection in the following configuration:

neon lamps

On the right is the CRT. I believe the only purpose for such lamps is so discharge a build up of charge between cathode and grid which may damage the cathode being that very close to the grid protecting also the bleeding resistor R86 which may blow up in case of too much charge build up.

The lamps used are NE-96 or 4AB which are of course not very readily available. The bulb has the following specifications:

NE-96 specifications

I deduce that this is designed so that a build up of (worst case) 450V is discharged through the bulbs instead of through R86.

What would be a suitable modern day replacement for the series of the three bulbs in this application?

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  • \$\begingroup\$ A 120V transient voltage suppressor (TVS) might be a suitable drop-in replacement, however I'm unsure how it's parasitic properties would compare with a neon bulb. Likely have more capacitance, which might affect the circuit operation. \$\endgroup\$
    – rdtsc
    Commented Apr 27, 2021 at 12:22
  • \$\begingroup\$ Capacitance may be an issue because there is a signal present at the cathode of the CRT: the blanking signal. \$\endgroup\$
    – cosenmarco
    Commented Apr 27, 2021 at 13:24
  • \$\begingroup\$ Anyway @rdtsc you put me on the right track. It seems to me I need a bidirectional TVS avalanche diode: has very low capacitance and there are available in many different voltages and power handling capabilities \$\endgroup\$
    – cosenmarco
    Commented Apr 27, 2021 at 14:11

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I feel the right answer here is (thanks @rdtsc) to use a TVS suppression diode. In this case I used BZW06-239B TVS Diode bipolar 600W 239V 280V 5% DO15.

I did put it across R86 and so far the CRT hasn't release any magic smoke ;)

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  • \$\begingroup\$ Thank you for following up. \$\endgroup\$
    – Bryan
    Commented Oct 3, 2022 at 16:51
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These sort of neon lamps are fairly generic. Something like this perhaps:

https://www.digikey.com/en/products/detail/visual-communications-company-vcc/A2A/3151670

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    \$\begingroup\$ They may be very generic but still have very different electrical characteristics. See here for example: tiffe.de/roehren/neon.pdf \$\endgroup\$
    – cosenmarco
    Commented Apr 27, 2021 at 12:46

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