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pcravero · 2nd Nov 2017, 9:46 am

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

2nd Nov 2017, 9:46 am	#1
pcravero Triode Join Date: Aug 2009 Location: Torino, Italy Posts: 47	(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 Attached Thumbnails