EM Electronics Model N11 DC Nanovoltmeter

[TerraHertz]

Anyone interested in a teardown of this?
It's been sitting in storage since around 2001, and the batteries are not healthy. But it does seem to work.
I'm inquiring with the manufacturer about a manual and schematic, but am not very hopeful as when I tried in 2001 I had no luck, and their website seems to have barely changed since then. So, not exactly a dynamic, active company.

I don't suppose anyone here has found a manual and/or schematic for the N11?

[zlymex]

I'm very interested in the pre-amp part, which probably sealed in a metal box just like EM N2a

[TerraHertz]

It is. Looks like Mu-metal, which would make sense. Welded seams even.
But, quite difficult to get the box out, to open it, which is why I have not done it before.
All right... will do.

I'm very surprised the leads from the copper front panel terminals to the pre-amp box are so thin, unshielded, and look like plain pvc insulated. Short twist pitch, but they didn't seem to care about the lead length.

Edit: correction, it seems the input leads are silicone insulated. Feels too soft to be teflon.

By the way, are the batteries in your N2a the same kind? What are they? The charged voltage seems low for lead-acid, but likely that's just because they are ancient.

[zlymex]

That N2a belongs to a friend of mine. The batteries looks the same but he did not measure nor tear apart. What is the charge voltage of your batteries?


Edit: another friend of mine who also got similar nano-meter replied: each pack consist of 7 NiCad batteries, which is 8.4V, 1.2Ah nominal.

[SeanB]

So, in a pinch you can just use some PM9 batteries, while having the old ones repacked. Not as long a life and they are expensive as well.

[doktor pyta]

Dear volt nuts.
And now say a big thank you to Mickle T. who gave it to me:

http://www.mediafire.com/download/k9d37yi3kk0mqri/EM-N1a.rar
http://www.mediafire.com/download/hfk45rdb61h866o/EM-OEM-6045.rar

[lowimpedance]

These meters are a work of art.  Seems a pity to destroy one just because we are curious...

They are indeed works of art.

@ terahertz, Check the mains earth continuity from the mains in socket to the earth on the charger /supply PCB. It electrically goes through the potted transformer and a number of units here at work have failed safety testing due to an open cct inside this transformer. Must be some specific screening requirement I guess to route the earth in such a way so the mains can be left connected when in use ?. (I do not use them myself BTW).
 Also I do have manuals with ccts for a few different models including the N11, but that has allready been posted.

[TerraHertz]

That N2a belongs to a friend of mine. The batteries looks the same but he did not measure nor tear apart. What is the charge voltage of your batteries?

Under charge they both go over 10V. Idle-unloaded the negative side one is 9.4V (OK), the positive one is 6.6V and the case is bulged out a bit. I expect it's fairly ugly inside. The battery state via the meter says the plus side one is stuffed. It's right.

Edit: another friend of mine who also got similar nano-meter replied: each pack consist of 7 NiCad batteries, which is 8.4V, 1.2Ah nominal.

Excellent! So I can refurbish them, if the case seams can be split open without too much damage.

These meters are a work of art.  Seems a pity to destroy one just because we are curious...

Oi! A bit more confidence please. Who says I'm going to destroy it?

Progress pics below. And that's it for today, as I have to go out now.
The lid of the inner box is glued in. Hmm...

[TerraHertz]

As for the batteries, I couldn't see these labels till I removed from the N11. Now we know.

[zlymex]

Excellent tear down photos. Now I personally interested in how the low thermal EMF is achieved in the input stage, such as the binding posts, connection of wires to the binding posts, thermal/wind shield.

[TiN]

FE amp on EM A10 has copper rods with M3 thread for DUT connection. Similar shielding with mu-metal was present as well. FE assembly is potted so no further disassembly was done, as preamp still functiomal.

[TerraHertz]

Well, I really wanted to see the pre-amp, but as TiN says, it's not possible.

The lid of the innermost shielding box is held in only by some glue-like substance, a small bead of which can be seen around the join. Some of whatever it is has also been smeared on the rubber grommets, and it sticks well.

This small box holding the pre-amp is quite heavy, about the right weight for it being filled with something like potting compound. But I hadn't given up yet. What if the 'stuff' was something like hot melt glue, that could be softened at a safely low temperature to not harm the electronics and wiring?

So I shaved off some very small pieces of the compound from along the seam. Purpose: to see if they melted easily (hot melt), or singed like epoxy.

Gripping one of these little shavings in tweezers, first rough test was holding it close to a soldering iron. Nothing. Then actually against the iron (a Weller #7 tip, ie approx 700deg F.)  Wow, still nothing. In fact if I scrunched up the piece, when heated like this it would pop back into shape.

Conclusion: the box is solidly potted with some heat-resistant silicone compound, that is quite resilient. So not possible to disassemble without destroying it. *Now* I give up.
I guess the purpose was to prevent surface leakage, and maximize internal temperature stability. Not to mention keeping the circuit hidden, and making it impossible to service.

A shot of the rear side of the main terminals. The terminals and wire are all copper (don't know what alloy.) No lug is used, only the wire clamped directly to the posts & washers. Then the whole join is coated with some silicone sealer, that bonds well with the silicone insulation on the wires.

The controls ALL have insulating shafts, so there is no electrical connection to the outer case shell.

Also below are shots of the front and back of the two PCBs. EM Electronics kindly sent me the schematics for these, but not for the input amplifier. The schematics are in docx format and seem to have been converted (with errors) from some schematic format, but are readable. Nothing on the input amp though. I guess that's their secret.

Overall, this instrument is extremely awkward to disassemble and reassemble. Not just because no connectors are used, with all wires directly soldered to PCBs. Mostly it's screw placements - some are extremely difficult to access.

Next: refurbishing the batteries. Hopefully I can pack the old battery cases with some of the 18650 LiIon cells I have, with minimal changes to the power supply board.

[lowimpedance]

I'm very interested in the pre-amp part, which probably sealed in a metal box just like EM N2a

 Ok not to hijack the thread or give anyone an heart attack at the following pictures, the unit that provided the autopsy was completely knackered and also a very old non
'a' version, so in the interest of all wanting to know here is what is inside of the front end of an EM N2. Luckily its old enough that no glues were used on the mu metal box or leads into it.
 So two transistors epoxied to the copper plate, two polystyrene caps and one resitor plus of course the transformer all wrapped in some bog standard foam rubber.
Quite likely the later revision units have improved construction tecniques, model specific differences etc.

[Vgkid]

Thanks for the teardown. Them using that thick copper plate is interesting.  I wonder if it is for low frequency shielding, or to help maintain a thermal equilibrium. Or just a ground...

[TiN]

Thanks, lowimpedance!

All that epoxy and copper plate It's to provide long thermal constant and couple input transistors thermally. Down at nV-level everything has to be same temperature and stable, or those EMFs will shoot into sky like, no matter how good joints are. Those wires looks pretty standard to me though.

[Kleinstein]

With very sensitive wires going through a temperature gradient, one does not only keep the composition constant, but also keep the same degree of mechanical deformation. So one should be careful with bending these wires.

The wires might be pretty normal wires, but they are likely coming from the same spool and are possibly checked to be homogeneous, not to have local impurities or hidden deformations.

The main purpose the the copper is getting things isothermal, but it also works as a conductive shield, e.g. for 60 Hz residuals and especially higher frequencies.

[chinapp]

really amazing build,thanks for the teardown,how about the actual noise ?