What do we have here? A meter with a problem.
Let’s start with the high level stuff. I’m not sure who is the ODM for this meter. It doesn’t seem all bad. The case is somewhat cheap feeling and not overly solid. Like an original Fluke 87 you have to pull the back cover to change the battery. The switch doesn’t feel too bad. The display is very easy to read with very good contrast. Fluke needs to figure out how to have this level of contrast. Their old meters were much better than the new stuff.
Anyway, I’m not a fan of the lots of small buttons. None of them are dual purpose and some like the Pwr Reset button seem to have very little purpose (waking the meter from sleep). I have a theory why that might be. The meter’s main IC is a JRC (Japan Radio Co) NJU9210. You know, the old NJU9210… well like me you probably didn’t know. Anyway, I looked the thing up and found some of the specs. Nowhere in the specs does it talk about engine RPM or even temperature measurements. All of those functions appear to have been scabbed onto the basic meter chip. This can be seen in the number of trim pots for the temp and automotive specific functions. For example there is a separate trim pot for Fahrenheit and Celsius. I first noted this when I realized the bottom row of buttons do not beep when pushed, unlike the top row. This suggests the bottom row of functions are actually controlled and even control display segments outside of the main IC. I’m not sure how many other meters are designed like this. It definitely seems klugey but perhaps that was the best that was available at the time… or was cheap.
Anyway, the case/boot is soft and likely will protect the meter nicely. The fit is somewhat lose but I guess OK. The thermocouple input is separate from the normal inputs. Again I think this is part of kluging functions onto a basic meter IC. One last thing before we talk about the guts. The meter doesn’t show OL when it’s over range. Instead it shows 4000 with a flashing 4 and in some modes beeps each time there is a flash. Not the most enjoyable of things.
Upon opening it is clear the magic was released from a few parts and a trace on the back! The burn marks are on the other side of the PCB from the current shunting safey diodes Dave has talked about. We can also see a trace from the mA fuse holder has been… well fused. No real voltage slots or walls in the case. Looks like a cheap meter on the inside. Let’s pull the PCB.
Hmmm glass fuses and a burnt current sense resistor. We can clearly see the burned diodes. Funny, that big fuse doesn’t look like it fits very well. The small fuse looks like it’s got a really thick wire in it….
Well the big fuse is a 20A as the design requests but it’s not the original size. It doesn’t properly fit the fuse holder and obviously someone replaced the original fuse and jury rigged the holder. The smaller fuse seems to be physically the right size but it was a 9A (!) fuse. No wonder this thing failed! The traces and diodes were clearly never meant to take 9 amps and neither was the 1 ohm sense resistor. It died as well.
So that explains the reason why the meter couldn’t measure in the mA range. The rest of the ranges work fine as they were free of burnt traces!
I don’t think anyone will be surprised that the rest of the meter is nothing special. It has limited input protection. It at least appears to have tried to use good fuses. There are quite a few trim pots to adjust the add on functions. The wiper switch operates on both PCBs.
Anyway, it was an interesting tear down. It certainly reinforces the point that you shouldn’t use the wrong size fuse. Had this meter contained the correct fuse size I doubt it would have been damaged at all.
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