Norma D4000 precision multimeter

[david77]

I got another old bench meter, the 5 1/2 digit Norma D4000 precision multimeter.
It was advertised as not working on ebay so I placed a bid and waited and won the auction.
It arrived yesterday, a quick check showed it was really not working. After power on it showed no outside sign of life. Yay! Something to fix  .



I can't find anything on this meter, no specs, no manual and no schematics.
Norma was an Austrian (not Australian) T&M gear manufacturer that belongs to LEM since 1995. Apparently Siemens rebadged some of their gear and sold it as their own.

My unit was probably made in 1978.
It's fitted with what I assume to be a GPIB interface, nice but useless to me. It also does 4-wire resistance measurment which
is the main reason I bought it.

Let's crack her open:

Top:


Bottom:


GPIB Interface:


There's a LM399 voltage reference inside:


Power supply:


Oddly they decided to not use 3-pin regulators, instead they built seven separate discrete supply rails, each with opamp and power transistor...

I powered the meter up while it was open and noticed the heatsink getting hot very quickly. So at least there was some sign of life. I deduced it might be a shorted cap to blame for it not working.
The meter is made up of several daughter boards plugged into the main logic board. I pulled one after the other to see if the meter would come back to life and it did after I pulled the "Amps board" out. This board seems to be responsible for the amps range and nothing else, without it the ohms and voltage ranges worked perfectly.



I followed traces to find out wich were the power pins and then measured across them with an ohm meter to find a short. And there it was! A dead short between a positive rail and ground. I traced that to a 10µf tantalum, desoldered it and it really measured 0 ohms. Replaced it with a new one, plugged the board back in and the meter works again as it should.

There's quite some bodgery in there as well. This is what I found underneath the "Amps Board". They cut a big ground track, bridged it over to the other side of that header with a huge wire link. That link is also soldered onto two of the pins on that header. In mid air.
Also they obviously mixed up the two small tracks in front of the header, had to cut them and cross them with enamelled wires.



Here it is after repairs and cosmetic clean-up. Now I need to figure out where I'll put it...  .

[Lightages]

Good work. Glad you were able to fix it. Tantalums are known for self destructing.

How accurate is it after all these years?

[david77]

Thanks.
Hard to say how accurate it really is without having access to calibration equipment, but compared to my other meters it is practically spot on.

I've got a little home made "voltage/amps reference" (I know full well it is NOT a proper reference!) that I use to check out meters, it's built around the AD581 10V reference and provides voltages from 1 to 10V using a resistor divider chain and 10mA current through a 1k resistor. The resistors used are 1k 0,1% Vishay types. This is pretty much the best I can do with hobbyists means and without spending stupid amounts of cash.

According to this "reference" the meter is pretty much spot on on all ranges. Only the odd flicker from 0 to 1 on the last digit, certainly good enough for me  .

[david69]

Hi,
I would like to ask You more about hardware Norma D4000 multimeter,
I have broken D4000 wattmeter, there is missing components, and I would
compare with multimeter, Wattmeter is based on LD120+LD121 a/d convertor,
has " for me unknow " decoder for 7-segment LED`s digits and others ?CMOS?
IC`s... could I ask You for help with investigation missing parts? I think that
my wattmeter contents other missing pcb board?? Thank You for
answer
David