Problem with chinese multimeter ? - Page 1
Forum section: electronics / general

author: Aldobrandi
author_rank: Regular Contributor
timestamp: April 06, 2012, 09:13:23 pm
content: A big hello to the fantastic eevblog community ! This is my first post but I've been lurking for a little bit and already have learn so much from all of you in just a few weeks.Alright let's get down to the matter at hand, shall we ?I am relatively new to electronics so I apologize in advance if this has a really obvious answer.Being away from home for a year I decided to put together an inexpensive but hopefully decent learning workshop (to clarify I can't spend too much money on this because I'll have to leave most of it behind me when I come home in a few months, and being a grad student living in Paris I have a very limited budget). My first move was to buy a multimeter, and being frustrated that most of those recommended by Dave (within the $50 range) were simply unavailable to purchase in France without a prohibitive shipping fee, I decided, and this was probably a huge mistake, to buy a 60 euro off-brand one from a local electronics store : I present to you the (not so) glorious MASTECH MAS-345 :http://gsmserver.com/shop/equipment/measuring_equipment/multimeter_mastech/digital_multimeter_mastech_mas345.phpThe specs looked OK, from a complete beginner's perspective, and I had even found a couple of rather enthusiastic reviews on Amazon of the one sold under the brand "Sinometer". Being an avid viewer of Dave's videos it didn't take long for me to adopt his "Take it apart" moto. So here I came to crack open my brand new yellow rubbery brick. It all looked OK (once again this is coming from someone who had barely any idea of what he was looking at) except for one strange looking little resistor on the front panel : one of its leg had simply snaped, as I hope you can clearly see in this picture :Here is my actual question: I have done some rough testing, every feature seems to work alright and measurements fall within the various margins of error. From the little you can see, do you think this is even a problem? How could it affect my prolonged use of this meter? Is it worth replacing the resistor (I have already successfully replaced components in devices before)? I am planning to get a decent meter once I get back to the States (in the $100 range) but hopefully this one will hold on until then.Thanks a lot for your advice and sorry for this longwinded post.TLDR: bought cheap chinese meter, it has a broken resistor, what to do now ?

author: Rerouter
author_rank: Super Contributor
timestamp: April 06, 2012, 09:23:41 pm
content: From the look of it, it seems like an optionional resistor, some station on the factory fitted it for the wrong model and it was snipped,at a guess to confirm myself, would the big resistor above it happen to have a net name 46? with no A?

author: Aldobrandi
author_rank: Regular Contributor
timestamp: April 06, 2012, 09:26:39 pm
content: Thanks a lot for the quick reply ! I would have to open it again to find out, which I'll do in a few minutes. I'll keep you posted.

author: Aldobrandi
author_rank: Regular Contributor
timestamp: April 06, 2012, 09:32:06 pm
content: You are correct, the big resistor's net name is 46 indeed, 46A having been left out! So I guess it's nothing to worry about then, thanks a lot !

author: MBY
author_rank: Regular Contributor
timestamp: April 06, 2012, 09:37:36 pm
content: I have a old Velleman DVM345DI. I think its the same meter. I took my DMM apart and got a view exactly like yours. But no broken resistor lead.If I follow the PCB both these resistors are in parallel, measuring 100 ohms. It is the 4 mA current shunt, I think. Decoding the color rings, the R45 is 100 ohms and R45A is 5600 ohm.It is really strange to parallel a 100 ohm with a 5.6 kOhm, but I can make sense of it I think: Tolerances in other components and the design calls for two near calibration ranges. Those units than cannot meet specs with (100^-1 + 5k6^-1)^-1 ~ 98.25 ohm get the 5k6 R45a cutted, to rise the resistance a few ohms.Thats my guess: All units are manufactured with both resistors in place and the designer knows that this is slightly too low resistance for _some_ units, but is correct for most. During trimming some units get their R45A cutted too meet specs.So, there is nothing to worry about. The worst case scenario is that your 4 mA range is outside specs, especially if my theory is wrong and the cut is by mistake.Hope it helps.Edit: I have no R46A either. Probably the same story there. R46 is the shunt (I think) for higher currents, the 40-400 mA range. (The 20A shunt is the big, long copper wire stretching from the COM jack to the 20A jack.)This meter has been around for many years and has many rebrandings. Its not exactly a high end but as you say a cheap Chinese meter, but its okay. As your first meter it's not bad at all. I love the backlight - its a hard switch to a LED. You can even turn on the backlight when the meter is hard off! No unnecessary SW-control but a simple solution!But I rarely use my meter, its a "backup".Edit: Some grammar and spelling.

author: Aldobrandi
author_rank: Regular Contributor
timestamp: April 06, 2012, 09:43:37 pm
content: This makes sense, thanks a lot for your detailed reply, I am learning everyday and hopefully next time I open a device I won't freak out at such a sightCheers.

author: Unknown
author_rank: Unknown
timestamp: April 06, 2012, 10:22:37 pm
content: My guess is that it's used for some kind of trimming. Solder in all resistors, measure some value, and cut one of the resistors based on the measured value. It's more stable and cheaper than a real trimmer, and may be sufficient if the value is stable across the life of the meter.

author: ModemHead
author_rank: Frequent Contributor
timestamp: April 06, 2012, 11:00:52 pm
content: I've gotten two of those meters free from Circuit Specialists in the past. I gave one away but I still have one. I looked inside and no leads are cut, but it is also missing R46A. And there are obvious signs of lousy hand soldering around those resistors. Also flux residue, and solder splashes. I will now return it to it's place on the back of a shelf, gathering dust...

author: Bored@Work
author_rank: Super Contributor
timestamp: April 06, 2012, 11:03:46 pm
content: The legendary late Bob Pease described such a technique in an app noteA New Production Technique for Trimming Voltage Regulators

author: Aldobrandi
author_rank: Regular Contributor
timestamp: April 08, 2012, 01:10:58 pm
content: Thanks a lot BoredAtWork (so am I), ModemHead and alm for your replies. I am so glad I took this thing apart and posted pics here because I have learned quite a bit about how meters are made. A big thanks to the community !