[volt-nuts] HP 419 & Fluke 845

[Stan Katz]

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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