[volt-nuts] HP 419 & Fluke 845
jfor at quik.com
Sat Mar 27 00:25:22 UTC 2010
I vaguely remember reading that the time response (on to off time) of CdS
cells varied with input wavelength of the light, but can't currently find
a reference. It was in a paper I read in the late 60s, I think.
> 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
> 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
> In a hotel bathroom, I saw a wall-mounted hair dryer with a neon pilot
> 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.
>> I attempted to head off chopper failure by replacing my 740B's neon
>> 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.
>> 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
>> a string of relaxation oscillators. No go. Replaced with the original HP
>> style lamps, and no trouble since. From what I've read (Signalite
>> from around 1972), there was a difference between illumination NE2's,
>> control NE2's. Some differences would be due to lack of radioacitve gas
>> illumination neons, as these wouldn't be subject to total darkness, as
>> 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
>> emissive coatings on the electrodes. While coatings are good for
>> illumination, they're bad for stability. Perhaps my stock of lamps was
>> 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
>> years ago just happened to be stable enough, and bright enough, to
>> that HP tech to use them to replace factory lamps.
>> If anyone on this list has had recent, long term success, replacing the
>> lamps in HP419's, or HP740B's, please post the source for your lamps.
>> 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
>>> configuration. Its exact voltage is not important. What is important
>>> is a
>>> stable voltage over your measurement interval. Lithium cells are
>>> Also the NE-2 neon lamps are still made, readily available, and dirt
>>> 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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