New neons for a Fluke 845AB

[Tony_G]

Hey All,

I just acquired a Fluke 845AB and it is fairly noisy - Dr. Frank suggested replacing the neons and from other threads, I see that people have had good experiences with Barthelme 00082305 ones.

I've looked around for these and can't seem to find any.

Does anyone have a source for them or have you successfully used a different model in your 845AB?

I'd like to try and keep the unit as original as possible so I'd prefer not to try the LED upgrades if I can swap out the neons.

Thanks,

TonyG

[Kosmic]

I did not try them, but they are cheap.

https://www.ebay.com/itm/134511250650

branadic wanted to use them but since the Barthelme 82305 were working fine, he never did.

https://www.eevblog.com/forum/metrology/teardown-fluke-845aabar-tweaks-and-mods-(and-repairs)/msg2833622/#msg2833622
https://www.eevblog.com/forum/metrology/teardown-fluke-845aabar-tweaks-and-mods-(and-repairs)/msg2597721/#msg2597721

[Kosmic]

The manual of the 845AR mention NE2U neon bulb.

[Bill158]

I used these in my 845AB a few years ago and they worked great.
I got them from Allied now RS.
Allied Stock #70597374      VCC Lamp; Neon; 95VAC (max.) 135 VDC ( max );  1.2 mA;  47K Ohms
I bought 25 of them thinking that I might have to match the striking voltages but they all were pretty well matched.  I gave 12 to a friend who also had a 845AB and he had good results also.
My 845AB showed a marked improvement in the overall noise (needle bounce ) on the 1 uV range after replacing the originals with these.  I seem to remember that I had to take care in aligning them in the assembly.  I can't remember exactly what I did but it was trivial.
Bill.

[floobydust]

Not an 845AB owner but I would say brightness is what matters. CdS cells are slow and that would dominate the switching times.
The service manual has:
DS101, DS102 - Lamp, Neon 105-125V, 0.7mA A9A-1, which is a vanilla NE-2E
Later it appears to be Fluke 3902-162602, NE-2U neon lamp - which is the high brightness 1.9mA lamp.

VCC and Chicago Miniature Lamp offer neon lamps. My experience is they are all made in china now so the quality is poor. Crooked electrodes, short stubby electrodes in a long glass package etc. per my pic is VCC A1A I got from Digi-Key as A1A-ND la junk
So the incentive to change to LED or opto-couplers is just that old school quality is no longer available.

[Tony_G]

Thanks, everyone for the comments - I got some time to at least get the covers off the neons to take a look at them:



They certainly look like they need to be replaced... Hopefully, I'll get to that soon.

TonyG

[floobydust]

I don't see any blackening of the inside glass. The glow should line up exactly with the light-pipe ends.
But the glow is usually along more of the electrode so they might have a problem. I read the radioactive isotope additive 1/2 life is 10-20 years so they simply age as well.
Useful info: Using and Understanding Miniature Neon Lamps (1969)

Are these 1" or 3/4" glass tubes? They look shorter. 1/2" would be A1C. Again, careful cheap makes are even shorter inside to accommodate a big glass nib at the end.

[Tony_G]

But the glow is usually along more of the electrode so they might have a problem.

That is why I think they look like they need to be replaced - The glow is aligned with the end of the pipe but I would expect the whole filament on that side to light up.

According to the manual and my quick measurements, they are A3C/NE-2U bulbs.

TonyG

[Tony_G]

I rummaged around in my parts bin and found some random NE2U bulbs I bought about 10 years ago for a different project. I just swapped one in to see what it looked like compared to the originals:



Much more of the filament is lit up but I'm not sure if it is brighter or if that is just an artifact of the angle I took the photo on.

TonyG

[Tony_G]

I went back and checked the image from Branadic's post (here) and these random NE2U's look pretty much like the ones he ordered from eBay (my one is on the left):



Anyway, time to change the other one and then get the meter back together to see if it addresses the noise aspect I was seeing.

TonyG

[Stray Electron]

The manual of the 845AR mention NE2U neon bulb.

   FYI. I think this has been discussed on here before and IIRC someone pointed out that the NE-2U lamp is different than the standard NE-2. IIRC the U lamp has a bit of some kind of Gamma Ray emitter inside of it that makes it ignite at a lower and more consistent voltage.

[Dr. Frank]

I rummaged around in my parts bin and found some random NE2U bulbs I bought about 10 years ago for a different project. I just swapped one in to see what it looked like compared to the originals:



Much more of the filament is lit up but I'm not sure if it is brighter or if that is just an artifact of the angle I took the photo on.

TonyG

Hello Tony,
the lower neon bulb is lit in-homogenously compared to the upper (new) one. The upper neon is assembled too short; The neon glow must be centered in front of the plastic rod.

That looks similar to my case, see here:
https://www.eevblog.com/forum/metrology/teardown-fluke-845aabar-tweaks-and-mods-(and-repairs)/msg2691363/#msg2691363

My old neons were flickering slightly, and were dark inside.
Anyhow, you can judge the performance by measuring the output monitor voltage.
I also replaced several capacitors and replaced the zener power supply by a proper LM317 circuit.

It might be necessary to carefully re-align the zero switch blades, and to wipe / polish its carbon (?) contacts by a few pulls of a piece of paper through the closed contacts.

Frank

[Tony_G]

Thanks, Dr. Frank, appreciate the guidance.

My goal here was really just to see what these random neons would look like in place compared to the existing ones (I bought a pack of the neons that Branadic referenced but they haven't arrived yet).

Anyway, I put that new one in place trying to reference the existing one for height as the light pipes moved outward without the casings. However, after your comment, I went back and measured the casing block to see where the pipe actually would sit relative to the neon and this new one is indeed too low. Thanks for the pointer there.

If I get time today, I'll replace them and set the neons to the right height before giving it a try - Interested to see what impact that has.

Thanks again for your insight, appreciate it.

TonyG