[volt-nuts] HP 419 & Fluke 845

[J. Forster]

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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