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Old 2nd Nov 2017, 8:46 am   #1
pcravero
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Cool (Weston) Schlumberger 1240 multimeter

Hello.
I ran into a strange situation and I need help. I bought a working (Weston) Schlumberger 1240 multimeter. The first couple of times I powered it up it behaved normally, but I confimed that the most significant half-digit "1" was broken so that 12.45 V would read 2.45 V.

So I got the circuit board out of the case, noted the broken "1" neon bulb (will need a replacement: 25mm high, circa 15mm symbol height, 5-6 mm diameter) and ... it doesn't measure anymore!

In Voltage scales it shows the value of a phantom voltage (an internal integrator circuit with open input, I'd say) cycling through the full scale range, one step per second or so. In current scales the output is purely random numbers (it makes a good eye-catcher since it keeps on moving!).

I checked voltage rails on the 'scope and they look good (50mVpk ripple). So it's probably not the capacitors, also because ... would they "die" simply removing the board from the case? It was shipped to me 500km and well shaked

I found the manual of the later 1242 model (from BAMA), which works on the same principles but does show some differences, like more "common" parts. For example the Nixie driving logic is based on 7441 vs CuL9960 of my 1240.

Yesterday I tried to remove the only socketed part: the 8-pin circular IC whose marking reads 272408, maybe Fairchild like many other IC's in there. No datasheet to be found. The effect is that the display is now static, so that's where the action passes through. Next in the signal chain looks like to be the 7710393 IC (LM710)

Here I am, asking for help. Perhaps someone has a circuit diagram or a clue of something not-so-obvious that could have broken (input leads do reach the PCB, I checked that too). Or, why not, an easy fix!

Thanks for reading,
Paolo
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Old 2nd Nov 2017, 11:05 am   #2
MrBungle
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Default Re: (Weston) Schlumberger 1240 multimeter

I would still suspect the capacitors still. The older logic ICs in these are super-sensitive to spikes on the rails. If you have some suitable lying around you can tack them across the existing capacitors and see if it makes a difference.

I had a counter of similar construction by Heathkit (who are related to Schlumberger) and the thing had an internal oscillator that was broken resulting in a new one being built. Worth seeing if the internal oscillator and integrator/comparator are doing what they are supposed to as well.

See thread here: https://www.vintage-radio.net/forum/...d.php?t=136242
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Old 2nd Nov 2017, 3:22 pm   #3
pcravero
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Default Re: (Weston) Schlumberger 1240 multimeter

The Heathkit tip turns out to be very helpful MrBungle! They sold a 3 1/2 digit DVM that reminds very much the Schlumberger 1240: Heathkit IM-102. There is a manual and schematic online. The front panel layout differs, but functions are the same. It also mounts the same "obsolete" CuL chips I have in my 1240.

I want to make a clock out of the DVM but I'd like to keep the original circuit intact and - - possibly - fully functional.

I will report my findings.
Paolo
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Old 2nd Nov 2017, 4:12 pm   #4
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Default Re: (Weston) Schlumberger 1240 multimeter

Excellent - good luck
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Old 2nd Nov 2017, 4:19 pm   #5
The Philpott
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Default Re: (Weston) Schlumberger 1240 multimeter

Interesting project, especially if you can retain the original functions.

As a clock the display will of course have a 100% duty cycle, it might be appropriate to check that the ventilation is adequate for constant use..?
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Old 2nd Nov 2017, 4:59 pm   #6
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Default Re: (Weston) Schlumberger 1240 multimeter

Good point. The Fairchild nixie drivers get pretty hot. I'm surprised there weren't any dead ones in mine.
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Old 2nd Nov 2017, 5:06 pm   #7
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Default Re: (Weston) Schlumberger 1240 multimeter

Hi Paolo,
I have a (paper) manual for the 1240, 55 pages A5 size. Postage will be about €5.
PM you address if you want it.
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Old 3rd Nov 2017, 9:51 am   #8
pcravero
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Default Re: (Weston) Schlumberger 1240 multimeter

Quote:
Originally Posted by The Philpott View Post
Interesting project, especially if you can retain the original functions.

As a clock the display will of course have a 100% duty cycle, it might be appropriate to check that the ventilation is adequate for constant use..?
Well, "as is" there is no ventilation at all! The case is a single block of plastic and except for a small slit on the back panel, air cannot flow freely. As I will be running it on the desk while troubleshooting, I will check drivers temperature.

I have located the HV line going to the anodes and it can be interrupted easily if necessary to keep temperature down. Nixie clock builders like to use PIR sensors to detect presence/motion.

Meanwhile I have found a scan of "General Electric Glow Lamp Manual, 2nd Edition" at http://w5jgv.com/amateur_radio.htm#downloads that lists indicator bulbs. I might be looking for a NE-83 or NE-86, all variations of the NE-2. The document mentions radioactive doping on some of those lamps, so I'll check mine for possible weak residual activity too.

For the curious, building a clock with a voltmeter is straightforward: a modern microcontroller (Arduino and the like) generates a PWM proportional to the time of the day as expressed in Volts, and the DVM does the rest. And if the 8-bit resolution is not enough (256 steps), add a small D/A converter.
BTW, since this multimeter will count up to 1999, it can display 19:59 as a time and then loop to 8:00 pm.

Hopefully I will have time to continue troubleshooting this upcoming weekend.
Paolo
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Old 3rd Nov 2017, 10:08 am   #9
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Default Re: (Weston) Schlumberger 1240 multimeter

That’s a really good idea with the DAC. Very non invasive. I’d keep it a 12 hour clock for consistency
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Old 4th Nov 2017, 9:59 am   #10
pcravero
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Default Re: (Weston) Schlumberger 1240 multimeter

I had some time to poke around with the oscilloscope. It is a bit easier now that I know the TO99 IC on a socket (the only socketed component in there!) is an LM308 configured as dual slope integrator.

I attach a picture of what I got on the scope. Probe 1x. Input leads shorted, voltage range 2V. The dual slope keeps getting smaller and then goes negative. I don't like the bumps on the descending ramp, I think they should not be there.

In the 200V range I noticed that is senses correctly an input of 6V, either positive or negative. Some component injects a current in the input circuitry?! I will wait for the manual and diagram offered by Bill to do further analysis. The Heathkit IM-102 circuit is probably the same but the scans to be found online are low-res and incomplete.

On other open subjects of this troubleshooting. Nixie drivers CuL9960 do not get even warm. The neon bulb does not show beta/gamma radioactivity on my counter (as a practical reference, a lantern mantel is detected with about 50 CPM).

So this beauty goes stand-by until the manual arrives.

Paolo
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Old 11th Nov 2017, 12:29 pm   #11
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Default Re: (Weston) Schlumberger 1240 multimeter

Manual has arrived. I haven't compared the schematic diagram with 1242 or Heathkit devices because I want to get it working again first.

I recapped the PSU (3 electrolytics and one non polarized) and nothing has changed: it starts counting backwards. There's still a bit of ripple on 12V rails, about 50mV peak: it is worth trying to tame it any further? Or rectifying diodes?

I'm wondering what are made of the two long yellow capacitors (one is parallel to the rotary switch, another near an electrolytic - now replaced -) and if they're likely to fail. The yellow one near the rotary switch is part of the dual slope integrator, it is The Capacitor doing all the job.

Rather than unsoldering one lead to isolate the component, where possible I'd cut in half the reachable lead, measure and rejoin the lead with solder. The PCB doesn't seem to be too sensitive to re-heating, but better not take risks.

Paolo
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Old 13th Nov 2017, 10:10 pm   #12
pcravero
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Default Re: (Weston) Schlumberger 1240 multimeter

Gents, I FIXED IT!!!!!

It wasn't the PSU. It wasn't a component at all... it was a broken (!!) trace to ground in the network of the dual-slope integrator. Picture attached.

This damage occurred when/after I slid off the circuit from the case. I don't remember hitting anything. No sign of the fallen trace on my desk. Now I hope no more traces will simply fall with gravity! For posterity, I've taken photographs of the board how it looks now

Meanwhile I've even found out that Mouser stocks a neon bulb to replace the broken "1". A modern version of NE-2E.

Now the clock project can begin. Thank you all for your support!

Paolo
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