Half-digit neon bulb 25/15 mm long

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Paolo Cravero

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Oct 30, 2017, 5:41:34 PM10/30/17
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Fellow neonixier's,
I have acquired a (Weston) Schlumberger 1240 3-1/2 digit multimeter made roughly in 1970/1971 mainly because of its compact design, suitable for a desktop numeric indicator (clock, thermometer).

I had the feeling the half-digit was broken, and I was correct. I initially suspected the 2N5179 NPN driver because other lamps are faint as well, but after removing the neon bulb I could confirm it was bad (even if it still contains gas).

The result of removing the board from the case caused a damage in a way that the multimeter is not reading the input but rather displaying the voltage across a slowly (dis)charging capacitor. If this fault sounds familiar, please provide some troubleshooting directions. BTW, I have no diagram: the closest I could get is the 1242 model that is slightly newer and has better precision. And Nixie drivers are the hardly-documented CuL9960 (and CuL9959 and CuL9958), but I can live with them. Unless I can trace the fault, the clock will require direct interfacing with the unknown logic.

Anyway, back to the neon bulb. It is 25 mm long, the "1" digit is 13-15 mm long (compatible with 5855 Nixie symbol height). Picture attached. Would someone suggest if this model is still around and its name?

Paolo

neon-bulb-digit1.jpg
schlumberger-1240-front.jpg

gregebert

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Oct 30, 2017, 8:25:21 PM10/30/17
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If I had to guess, one or more electrolytic caps has probably dried-up and is no longer working as a capacitor. Probably a good idea to replace all of them with fresh ones. Keep an eye out for corrosion on the PCB from leaky electrolytics, and be aware that some probably contain PCBs (not printed circuit boards, but highly toxic Poly-Chlorinated Bi-phenyls).

I always got a laugh about cost-cutting by using a neon bulb instead of a nixie tube for the leading '1' digit.

I bought 2 Fluke 8000 DMMs several years ago (they are LED, not nixie) because they were very reliable when I was a technician. But they have accuracy issues, and some of the pushbuttons dont work very well. I'll probably give them away as-is to anyone willing to pay shipping costs. Some things dont improve with age....

Paolo Cravero

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Oct 31, 2017, 5:03:37 AM10/31/17
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Funny thing is that it worked when I first powered it after opening the box it was shipped into. That's how I confirmed the bulb was not lighting up by measuring 12 V: I got only units and decimals when varying from 9V to 12V (20V range). 

Once the circuit was out it started misbehaving. No matter what I put in input, it just randomizes numbers. Well that could be entertaining nevertheless! No signs of leaks or fat capacitors. It has been serviced and a couple of traces rebuilt. I will check traces or bad joints in case something got damaged while on my desk. I attach two pictures of the digital section for the curious ones (I can share other internals too).

Still, I can't fit a full size Nixie in that space: I need the bulb or arrange something with LEDs.

Paolo



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Tomasz Kowalczyk

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Oct 31, 2017, 10:51:01 AM10/31/17
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I think producers of neon bulbs decided to make searches extra hard (datasheets with no dimensions or dimensions not tied to bulb name, using same name for different bulbs etc). But I think I've found something quite similar in size:
The digit will be most likely shorter, but I couldn't find anything closer.

John Rehwinkel

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Oct 31, 2017, 11:22:03 AM10/31/17
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Still, I can't fit a full size Nixie in that space: I need the bulb or arrange something with LEDs.

gregebert

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Oct 31, 2017, 11:58:13 AM10/31/17
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From the photo, I see what looks to be a blue electrolytic capacitor at the top (photo is clipped). It's a long-shot, but that's almost 50 years old so I wonder if it dried-out. I had the same thing happen to me with a Heathkit that was 30 years old: small electrolytic cap dried-out, became a short-circuit, and blew-out the fuse (actually, it was a resistor that was put there to behave as a fuse).

If you have a scope, look for AC voltages across an electrolytic; that's usually a clue the cap is defective because it's unable to do any filtering. After that, look for burn-spots or corroded traces (I didn't see any on the photos). Wiggling potentiometers [I saw at least 1 in the photo] might be worth trying, but that is risky because you dont know what they do and you will probably affect the calibration. Over time, I think the wiper contact can get dirty so wiggling it might help clear-away contaminants that alter the resistance to the wiper. That pot looked like a high-quality Bourns multi-trim, so you may want to leave it alone; the cheap 3/4 turn units are the ones I consider suspect.

Tomasz Kowalczyk

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Oct 31, 2017, 1:20:20 PM10/31/17
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There are three test points in there, two for voltage rails and one apparently for a time base. I suspect the power is OK if it does power up at all (unless all the logic chips needed for display do not use it and have separate voltage rails), what about the 5HZ point?
Also, if the reading is random, I suspect that the problem might be somewhere outside the boards - disconnected input socket or one of buttons not contacting.


W dniu wtorek, 31 października 2017 10:03:37 UTC+1 użytkownik Paolo Cravero napisał:
Funny thing is that it worked when I first powered it after opening the box it was shipped into. That's how I confirmed the bulb was not lighting up by measuring 12 V: I got only units and decimals when varying from 9V to 12V (20V range). 

Once the circuit was out it started misbehaving. No matter what I put in input, it just randomizes numbers. Well that could be entertaining nevertheless! No signs of leaks or fat capacitors. It has been serviced and a couple of traces rebuilt. I will check traces or bad joints in case something got damaged while on my desk. I attach two pictures of the digital section for the curious ones (I can share other internals too).

Still, I can't fit a full size Nixie in that space: I need the bulb or arrange something with LEDs.

Paolo


Il 31/Ott/2017 01:25, "gregebert" <greg...@hotmail.com> ha scritto:
If I had to guess, one or more electrolytic caps has probably dried-up and is no longer working as a capacitor. Probably a good idea to replace all of them with fresh ones. Keep an eye out for corrosion on the PCB from leaky electrolytics, and be aware that some probably contain PCBs (not printed circuit boards, but highly toxic Poly-Chlorinated Bi-phenyls).

I always got a laugh about cost-cutting by using a neon bulb instead of a nixie tube for the leading '1' digit.

I bought 2 Fluke 8000 DMMs several years ago (they are LED, not nixie) because they were very reliable when I was a technician. But they have accuracy issues, and some of the pushbuttons dont work very well. I'll probably give them away as-is to anyone willing to pay shipping costs. Some things dont improve with age....

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Paolo Cravero

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Oct 31, 2017, 2:49:30 PM10/31/17
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Hello.
Thank you Tomasz and John for the links to neon substitutes. I've tried to light it up touching what remains of the broken leg but there was no life. I can tell the bulb was pressed into position and leads forcedly bent.

Your replies inspire me to try troubleshooting a bit further. Voltage test points at +3.6V, +12V, -12V show a ripple within 50mVpk. The 5 Hz signal is present and periodic, but I can't visualise it properly on my analog scope because it's too slow. Nixies are powered with the output of a single diode rectifier, 220Vpk or so. Logic chips receive their 3.6V power supply.

I found a place near what should be an analog IC (the round one on a socket) that stops those random numbers if I touch a 6p8F capacitor lead with the scope probe. I am now going to gather part numbers, the analog IC being the hardest without removing it from the socket. I will take highres pictures too, to share and because it's easier to fly around the crowded PCB.

Paolo

PS: NeonJohn, the device is older than me.

Nicholas Stock

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Nov 2, 2017, 4:53:23 PM11/2/17
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Paolo Cravero

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Nov 2, 2017, 6:38:04 PM11/2/17
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Thank you Nicholas for the links: without S&H they already cost more than I paid for my unit! And I have the curious power cord ;-)

Meanwhile I asked for help on a forum and an innocent comment opened up new possibilities: Heathkit was linked to Schlumberger. In fact they sold the IM-102 DVM that is electrically very similar to Weston 1240, if not identical. And a manual/diagram can be found online.

Now I can troubleshoot the no-signal problem and give a name to the bulb. And, by the way, there are other devices that used the bulb as "1" so even more [documentation] sources (like Philips DVMs).

Greetings, 
Paolo

JohnK

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Nov 5, 2017, 12:14:36 AM11/5/17
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The web is full of fact and OPINION so be careful which content you choose to believe [we all know this].
AFAIK the usual electros that we encounter are not a PCB problem, but ARE various other problems. If standard electros have PCB content I definitely want to know - but I really avoid contact/association with any leakage anyway. I worry about the aroma that can be detected from certain brands. I have some small green electros from the 1990s that are stored in small sealable plastic bags with a small square of paper inside as a label. Many of the pieces of paper show brown stains where they have been in conact with the capacitors [particularly the seal around the legs]. They obviously do leak  - and who knows how much gas comes off too.
These particular ones I kept because there was a spate of these exploding during product testing. The very obnoxious "dead fish" smell given off bothered me. The management made poor attempts to garner information from the manufacturers. I kept them in case health issues arose.
 
[On the subject of outgassing:- last year we had twenty light bulbs/CFLs replaced free through a system forced on power companies by the government.
The Edison screw 10W bulbs were OK. The bayonet 10W bulbs produced an acrid chemical smell. The opaque white plastic of the base was the culprit. I had quite a bit of difficulty getting replacements out of that supplier. They may or may not have been in discussions with the lamp manufacturer/supplier BUT were exceedingly annoyed when I kept a single sample back from the exchange. That probably says something !
I made a couple of simple attempts to interest government and goverment utilities in the possibility of a danger. I would have to push quite hard for it to go anywhere. I currently have serious health issues that have put that on hold.]
 
 
 
Here is a nice document for those in Australia:-
 
John K
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jb-electronics

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Nov 9, 2017, 1:02:03 PM11/9/17
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Paolo Cravero

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Nov 9, 2017, 1:29:57 PM11/9/17
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On Thu, Nov 9, 2017 at 7:01 PM, jb-electronics wrote:

Here is a cheap one:

Thanks! Unfortunately I'm on the wrong side of the Atlantic and I've got plenty of those Nixies. I am waiting for the original manual of the instrument to arrive, then I will do proper troubleshooting on the signal chain. Meanwhile another device might pop up, in Europe at least. Tomasz, maybe some Mera-Lumel/Meratronik voltmeters mount a similar "1" digit?

Paolo

Tomasz Kowalczyk

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Nov 9, 2017, 9:13:56 PM11/9/17
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Unfortunately, I haven't seen this kind of approach in any Polish equipment - even the V54x voltmeters/multimeters used full sized Z566Ms for all digits even though the leftmost digit was showing only "1" or nothing*. I also haven't seen any long neon bulb in any of equipment.
*I can't remember if only "1"  was wired or "0" was wired too, but this tube definitely didn't have a 74141 for full drive and was mostly off during regular operation.

Nicholas Stock

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Nov 13, 2017, 11:08:37 AM11/13/17
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Not sure if this helps..

Chicago Miniature (now VCC) sells neons upto 1 inch bulb length....I've attached the datasheet.

(Pharma) Nick

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Neon Circuit Component Lamps-1064719.pdf

Paolo Cravero

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Nov 13, 2017, 11:43:50 AM11/13/17
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On Mon, Nov 13, 2017 at 5:08 PM, Nicholas Stock wrote:

Not sure if this helps..

Chicago Miniature (now VCC) sells neons upto 1 inch bulb length....I've attached the datasheet.

U know what? I spotted them 4 hours ago while searching on Mouser catalogue! They do seem to be a fair replacement. Sizes in datasheet are only in inches. That's curious.

BTW, I was on mouser looking after a far infrared thermopile/sensor to build a thermocamera myself (after the other thread in here). Melexis produces a 16x4 pixel sensor with I2C output at 33 EURO (MLX90621). Purchases above 50$ are exempted from the 20$ shipping cost, so I am considering to buy two FIRs and drop the neon bulbs in the cart. However Melexis has announced the MLX90640 32x24 pixels sensor that should be released in december 2017. Price has not been disclosed and AFAIK the release was postponed at least once.

I will wait a few weeks more to see what happens with Melexis while I try to fix the Schlumberger 1240 multimeter (I got the original manual!!): it reads either a slowly decreasing voltage or a totally random number depending on the range/measure setting. I've recapped the PSU and nothing has changed. I still see a 100 Hz signal overimposed to the longer run of the dual slope (output from the integrator). Now I'm following the two signal paths on the diagram to understand what causes the malfunction. As usual. hints appreciated.

Paolo

Nicholas Stock

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Nov 14, 2017, 2:14:47 PM11/14/17
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Probably used for the same purpose!

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