Electronics > Metrology

L&N 4385 Teardown


My first piece of L&N gear was a mundane shunt box(box of shunts  ;D ). Of the 2 that were ordered(both for $20), only 1 was 100% good. 
Spec wise this is really impressive, A low ohms semi-decade with steps from 0.01 Ohm to 2.0 ohms, but 0.02% accurate.

This shows the outside of the unit. Rather simple. The large grey knod w/ engraved alluminum skirt takes up most of the face. The top 4 binding post are the same ones in the bridge. In the bad unit, they are bent at an angle.

With the knob and binding posts the rest of the front is revealed, surprising how much that skirt takes up. It does look good.
I will dive into it tomorrow, It was too dark in the rest of my room, pics didn't look good.

Looks very nice and interesting :-+
Looking forward to see more

Teardown Time, some of these were done last night. With most done today, even with better light, it was still tricky.

First one is the terminals being labeled. This is set up like a standard decade switch, just in a 4 wire mode.

Now we can see the inner workings of the decade(I will always refer to it as that) Going clockwise we go from the low ohms folded sheet shunts to the the large diameter wire looped shunts. Until we get to the bifilar wire wound resistors.

Next is the schematic of how they are wired up. Grounds are connected to each other. The shunt positive connection is connected to the stationary contactor on the rotary switch. The potentiometer positive is connected to the moving contactor on the rotary switch.   

This shows the Highest amp ranges of the shunts. The large one is a 0.010 ohm resistor, connected to a smaller 0.01 ohm shunt. I figured they this shunt would have been physically larger, but the power dissipated will not be as much. Though a larger shunt would have helped keep things equal. The flat folded  wire shunt is responsible for the 3A current range. The value for this one is 0.03 Ohms.

Next is the Current shunts responsible for the middle range. The Most right(shortest) shunt is one with a resistance of 0.05 Ohms. The middle resistor(shorter one) is responsible for the 1.5A range, its value is 0.10 Ohms. The right most resistor(tallest) is responsible for the .30A range, its value is 0.30 Ohms. I'am wondering if these are of a different composition form the lower ohms range. Those are more copper colored, versus a brass color. Could be a different varnish, I can experiment on the bad one.

Finally we get to the resistors responsible for the 2 higher ranges. These are physically much smaller, as they need to dissipate the least amount of power The left most resistor is one responsible for the 150mA range, This one measures at 0.50 ohms. Lastly the remaining resistor on the right is used for the 75mA range. This one measures at 1.0 Ohms.
Updates for tomorrow, testing + methodology.

By any chance, does someone knows what type of varnish is used in this kind of instrument? I've seen one of exact same appearance used with mica sheet bobined resistors too.


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