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Old 19th May 2021, 6:22 pm   #1
georgesgiralt
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Default Levell TM3A questions

Hello
Following some advice I got on Internet, I've got this AC millivoltmeter.
it is in very good shape, came quite cheap and complete in its leather case.
But it seems it had not had a lot of use the 20 last years...
The aluminium plug where the measuring wire plugs is oxidised and the power comes from a PP9 9V battery which is difficult to get and expensive in France.
As the measuring cord is fitted with big alligator clips, I wonder if I change the plug and wire to a BNC plug on the chassis and a new set of probes. I need your advice on this one.
And Levell had a switching power supply to power this unit. Off course I do not think I can get one but wonder if I can use a small current switching power supply to provide the 9V needed and do not risk to disturb the precision of the instrument which has a huge sensitivity ? The Chinese made switching modules seems not shielded nor heavily filtered on the low voltage side. What are your thoughts about this ?
I would very much like your comments and advices on the matter !
Have a very nice day and stay safe !
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Old 19th May 2021, 6:51 pm   #2
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Default Re: Levell TM3A questions

My personal opinion only - I would stick with batteries and use 6 x 1.5v AA cells in a holder. The unit only takes about 10mA and there would be no problem with supply cables or noise.
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Old 20th May 2021, 7:07 am   #3
georgesgiralt
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Default Re: Levell TM3A questions

Thank you George,
It was my thought too but as this device get not used that much I wonder if the remedy wont be worse than the problem. i fear battery leakage !
On the other hand I bet Levell did some research when they sold the switching power supply ! I'll start to look at a small module from China and do some testing. Won't cost much than the PP9... ;-) but devising the switching frequency of the original module or the Chinese one wont be easy...
I'm left with the corrosion on the plug.
All in all a very simple and neat device !
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Old 20th May 2021, 7:46 am   #4
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Default Re: Levell TM3A questions

With only 10mA needed, the benefit of using a switch-mode supply is slight.

A transformer, rectifier, reservoir and a linear regulator will give full isolation and very little noise.

Considering the period when Levell were active, are you sure their supply was a switcher? It would have been very difficult to make then, compared to a linear supply.

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Old 20th May 2021, 8:19 am   #5
georgesgiralt
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Default Re: Levell TM3A questions

Yes, David,
There is the schematics and PCB layout in one of the manual...
IMHO it was fine engineering at it's time.
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Old 20th May 2021, 10:21 am   #6
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Default Re: Levell TM3A questions

Very curious. Looks like a capacitor dropper followed by a Royer oscillator, and the output is only shunt regulated with the zener diodes. They must have been really trying hard to keep the size and weight down, and presumably the stray magnetic field from that little pot core is smaller than that from an iron transformer.

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Old 20th May 2021, 10:56 am   #7
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Default Re: Levell TM3A questions

The little inverter looks to do no regulation at all, and voltage stability is taken care of by the series resistor and zener diodes, which is even less efficient than a linear regulator.

As Chris says, they must have been desperate to save space.

Getting good safety isolation in a pot core is not easy, either.

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Old 20th May 2021, 11:28 am   #8
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Default Re: Levell TM3A questions

If you're worried about battery leakage, the AA battery holder can easily be fitted inside an old PP9 case. It's a common mod for cheaper running costs on vintage sets even here where PP9's are readily available.

I could provide one ready hollowed out (less weight) for cost of postage if you like.

(Won't be a "period" looking one, you'd need to print your own label for that!)
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