Tear down of (and how to calibrate) a Maplin/Precision Gold N73B Multi Meter

The N73B! I've had this little meter for a while now, and I think I cost me about £15 when I bought it.

Reason for buying it: well it was cheap, I needed something I could throw in a tool box when I went trailing, and seem quite rugged;as you will no doubt see from the oil stains this thing has!

Now it claims from the Datasheet to be CAT III rated, though I've never really trusted that, even when testing the circuitry on the HAMEG! If it went pop, then so what, £15 and no funeral held! All I had to do for testing that scope was to not hold the damn thing whilst testing high voltages!

But this thing keeps on going and going and GOING! Dropped, bounced, kick across the floor, thrown into a toolbox so many times, stood on, its been thoroughly abused!  So I thought I'd treat it to a set of fresh batteries and a bit of a calibration!

Why calibrate such a shitty DMM? Well I was more interested in how far out it was, if at all. Meh, call it shits a giggles!

Attempt at calibrating

On my bench at work is a KEYSIGHT 34470A - which I think most of you will agree is a shit hot DMM! Its less than a year old from the factory, and came with a calibration certificate. So safe to say if this thing says something is 1.234567V, it is safe to assume it is exactly that (within it's own tolerances)!

Using this as the standard I gave it a quick comparison after putting in some fresh AAA's. I simply just hooked up a bench supply and compared the 34470A's readings to the N73B's:

Well that's no fun: seems to be reading with the N73B's spec! I expected it to be more than that out after the years of service! So calibrating this seems to be a bit pointless, it seemed to be reading within its own tolerances. Still I wonder: what if you wanted to calibrate this? And I couldn't resist taking it apart and looking inside!

From the outside, it's pretty hard to figure out where to start! But after some gentle prying on the probe clips, it came away to reveal a single screw. Once that screw had been undone, I had to peel the decal away from the lower half of the meter, to gain access to its internals:

With the upper case removed you can still operate the DMM, but you may have to push the PCB down so the battery terminals make good contact:

So you will notice the non-contact voltage detector antenna (INT2) above the 14-pin IC (U2) and the white LED next to the Crystal (X1), that's the built in torch (which can be handy) and directly below that, the black cylinder is the buzzer (BZ1) for button press sounds, auto-shutdown warning and continuity alert.

Anyway, trying to calibrate it: I found 2 trim pots. My guess was that one of these was bound to adjust the reading...

 VR1 looked promising! With some small adjustment I managed to alter the reading on the display:

Success! OK! So how far has my tweak put out the readings on other ranges?

5V? ok that's within spec!

10V? Yep, that's ok.

15V?yeh that's ok, still in spec.

20V? Good!

30V? Fine too!

So my adjustment hadn't knocked it out completely, so I thought I best not tweak it anymore. As a belt and braces I checked the current too:

Although at 20mA and 10mA it would be considered quite bad for a higher end meter to give that reading; its actually within spec, and at uA range is not far off either.

 Whether or not my adjustment of VR1 altered the current reading too, I don't know. But I just wanted to check the current still read right.

A closer look

Ok for those who want a nosey inside! With the upper half of the case off, the PCB with yellow front cover protecting the LCD just slides out of the bottom half.

 

The leads are soldered to the front facing side of the PCB before passing through the PCB itself through 2 holes. The green device is a PTC and the orange is a MOV.

And over the other side of the board is a poly fuse, rated to over 2A I think, markings are barely visible.

 6 self tapping screws hold the cover that holds the switch mechanism to the PCB and covers the LCD.

 Notice that all bare contacts are gold plated? That's a point awarded for robustness of design! Don't want your contacts getting grotty now!

LCD backlighting is taken care of by 2x 3mm white LED's above the LCD.

Nice thick insulation on the lead wires...

Put a ruler against it to measure the clearance between the solder joints and at near 13mm, I think that's wide enough for 1000V rating(considering its 5mm minimum)

Overall

With regards to calibration - is it really worth sending this off to get calibrated, when its highly likely you'll be quoted the price you paid for it (if not: more)?

NO, its not worth it!

Is it worth calibrating it yourself? If you have access to a pretty hot DMM to compare? Sure why not, depends how attached you are?

With regards to its build: Its a tough old thing! Mines been properly abused and given the calibration is set with those SMD, single turn trim pots, that didn't even appear to have any adhesive on them to hold them in position, I was expecting it to be more than it was out! 

It also appears to be exactly as specified! Usually these cheap meters give specs on the case that are total bull! But this appears to be exactly what it says on the tin! So it looks like it would have handled some of the high voltage in the HAMEG!

For a 3.5 digit meter, that you can throw in a toolbox, and you know won't break if you do, its worth the money!

Should you get one? Ahh go on!

Now before this turns into a review, I'll leave you to it!

Take it easy!