[volt-nuts] HP 419 & Fluke 845

[Robert Atkinson]

Hi,NE-2 is now  a very generic number. Lots of variation in detail design and materials. Many discharge lamps and devices use radioactive isotopes to maintain a low level of ionisation. Krypton 85 was commonly used in the gas mix. It has a half life of 10 years, it's decay may be a factor in lamp failure. Tritium is used in modern devices. Nickel 63 or Cobalt 60 can be used in the electrodes (normally only one to save cost) and sometimes Radium 226 was used in older devices (I have a 1B29 T/R tube that is quite hot). Often the choice of isotope was down to what was cheapest at the time. Note that at low voltages some LEDs  won't work in the dark either! To use an LED in place of  a neon (AC) just use two back to back. CDS cells peak around 520nm so a high efficiency green LED (LMP-LM65 to keep it in the family) would be a good choice at 525nm.Robert G8RPI.
P.S. I'm a Geiger Counter Enthusiast as well as a time and Volt nut, whole different set of measurement issues!

--- On Fri, 26/3/10, J. Forster <jfor at quik.com> wrote:

From: J. Forster <jfor at quik.com>
Subject: Re: [volt-nuts] HP 419 & Fluke 845
To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
Date: Friday, 26 March, 2010, 3:11

Two different neons with the same part number seems very unlikely. More
likely is a change in production over time eliminating the radioactive
component.

In a hotel bathroom, I saw a wall-mounted hair dryer with a neon pilot light.

If the room was dark, the neon was out.
If you turned on the room lights, the pilot glowed.
A flashlight would also make the neon glow.

-John

=============




> I attempted to head off chopper failure by replacing my 740B's neon lamps
> with NE2H's (NE-2H was recommended by HP tech at now defunct Paramus,NJ
> repair center) from Digikey (about 1993). Worked for about an hour. Tried
> another set of lamps, choppers  still dead. I wasn't in the mode to
> troubleshoot, and figured I'll pre-age the rest of my stock for 24 hours
> in
> a string of relaxation oscillators. No go. Replaced with the original HP
> NE2
> style lamps, and no trouble since. From what I've read (Signalite catalog
> from  around 1972), there was a difference between illumination NE2's, and
> control NE2's. Some differences would be due to lack of radioacitve gas in
> illumination neons, as these wouldn't be subject to total darkness, as is
> the case in a chopper. Without radioactivity, NE2's suffer from "dark
> effect" with reduced light output. Also, it is better to have a lamp
> without
> emissive coatings on the electrodes. While coatings are good for constant
> illumination, they're bad for stability. Perhaps my stock of lamps was
> more
> prone to sputtering, which also would darken them. All China sourced NE2
> lamps I've come across, are strictly optimized for illumination. No one
> produces "control neons" anymore. Perhaps generic NE2's (NE-2H's) from 40
> years ago just happened to be stable enough, and bright enough, to permit
> that HP tech to use them to replace factory lamps.
>
> If anyone on this list has had recent, long term success, replacing the
> neon
> lamps in HP419's, or HP740B's, please post the source for your lamps.
>
> Stan
>
> On Thu, Mar 25, 2010 at 7:20 PM, Mark Sims <holrum at hotmail.com> wrote:
>
>>
>> The Fluke 845 does not use a mercury cell.
>>
>> In a lot of these type systems,  the mercury cell was used in a
>> ratiometric
>> configuration.  Its exact voltage is not important.   What is important
>> is a
>> stable voltage over your measurement interval.   Lithium cells are
>> usually
>> OK.
>>
>> Also the NE-2 neon lamps are still made,  readily available,  and dirt
>> cheap.
>>
>> Standard chicken soup for a twitchy 845 is to clean all the switch
>> contacts,   check the electrolytics,  check the chopper system.
>>
>>
>>
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