Tenma 72-2050: Review and Teardown. Well - Kind of.

[schwarz-brot]

There we go, i will try my first documented teardown ever right here. This will be in parts as I am not allowed to attach too many too big pictures. Therefore this is going to be part one whicht starts off with the outer appereance and general overview.

Part I: Tenma 72-2050

Direct links to the other parts:
Part 2: The Insides - mechanical construction
Part 3: The PCB

This DMM is out of production. Funny enough farnell still lists it for some countries (46$ in australia...). There you can get the .pdf with the technical data which I also attached as .jpg for quick and easy viewing:


Pic 1: Documentation

See for yourself, I won't go into it. After all the specs aren't too good. So what's the deal with this meter?


Pic 2: Overview standing with rubber enclosure


Pic 3: Overview front with rubber enclosure


Pic 4: Overview side with rubber enclosure


Pic 5: Detail inside rubber enclosure: dampening fins and joint of stand

First thing that comes to your eye is the very massive black rubber enclosure in which the meter sits (Pictures 1 to 4). It is a nice fit. No way to fall out accidentally. The rubber is soft and has some dampening fins inside on the top and bottom (See Picture 5). Plus, there are large protruded edges on the front top and bottom which protect the frontpanel quite well (see Picture 4).
Integrated in the rubber enclosure is a massive stand (you can vaguely discern the huge joint in picture 5) that works reasonably well. Using this one can easily turn the switch without the meter falling over. The rubber also stops it from moving around while in use. I pretty much like that design. Also the stand clips into little notches in the rubber while not used and cannot move around wildly then.
Also integrated is a velcro adjustable hand strap which I did not use too often but sometimes comes in handy. No worries about that.




Pic 6: Overview front


Pic 7: Overview back

Now let us take off the enclosure and see the rest of the beauty (Pictures 6 and 7). First thing to realize is the very sturdy case. One can try like dave does in his reviews: It is not going to warp when twisted by hand. The material looks and feels like it could take some hard hits without breaking. My Meter is in regular use since I got it back in 2006. The print does not show any little mark of wear which I kind of like.


Pic 8: Detail Display and soft buttons

In Picture 8 you see a close-up of the display (still got it's protection foil...). The display lies very deep inside the case and is covered by a strong plastic window. There is no way to apply any pressure to it, not even if you try it. In contrast to all that stability the softbuttons feel a bit worn out nowadays. The modeswitch will sometimes stick down. This was not the case when this meter was new. This is not a big deal yet but may become one over time. Finally kind of a negative point! While we are at it, there are some more negative points about the display: Because it is burried so deep inside the case one cannot read the bargraph from a wide angle seen from the bottom direction. This is hardly ever a problem, as I feel the bargraph is just kind of a handy feature on such a basic meter. Plus one will only have this issue when the meter lies around and you try to read it from a far place. If it stands up there will be no problem. Also the rest of the display can easily be read from nearly any angle. If only there is enough light! One won't be able to use this meter in dark spots, as there is no backlight. I don't understand why they spared on this, because this meter just wants to be taken out in the field and be used under any hard condition one can imagine (while not explicit made for that). After all - no problem with proper lighting.


Pic 9: Detail rubber feet

Enough ranting - lets move on to some good points. Dave would for sure like the very soft rubber feet (see picture 9). The meter can hardly be moved around the bench even without its rubber enclosure. These little pieces of rubber simply work!
Next let us take a look at the large rotary switch. Its handle is not too large but large enough. One might turn it with working gloves. It goes with a nice click between the positions. Hard enough to not be turned accidentally while working but easily enough to not move the beauty around while switching. There is no way to accidentally stick in a middle position, although you can force that if you try very sensitive. I like that it has two off positions. If one uses the right one it is possible to toggle through the ampere-ranges in safe direction. I like that but miss the possibility to turn it all the way around from one of position to the next. So one still has to think of turning it to the right position prior to amp-measurements. There is no beep when switching between the positions. It only beeps once after being turned on when it is ready to use. Also it beeps all the time when you have your test leads connected to the amp connectors and try to use anything else but the amp ranges. There even is this safetybeep when one tries to use the 10A range while connected to the µA/mA jack. I like that.
The continuity tester is not the fastest. It won't beep when the leads just click together but it is at least ok. I guess Dave would not like that.
What he for sure would like: Amp measuring always starts in the DC-Range which then can be toggled to AC.

You may skip the comments and direktly jump to:
Part 2: The Insides - mechanical construction
Part 3: The PCB

[schwarz-brot]

Pic 10: Overview test leads with focus on connectors


Pic 11: Detail test leads with rating, wiring and tip

Last point for this post is going to be the test leads (Pictures 10 and 11). They are well constructed in my opinion while not the best I have ever worked with. Starting with the tip (see Picture 11) one will find it sharp enough to easily pierce through any oxide but not too sharp. Therefore it sometimes is hard to push them to a specific point, especially if that point is very small. The handle shows the CAT III / 600V /10A rating and the wire is also properly rated (both in Picture 11, sorry could not get the writing on the wiring right). So these are a perfect fit for the meter. The handles are none of these modern ones with rubber. But they prevent the fingers slipping down into the circuit with a large ring. That is a well working solution, so I am ok with that. The writing on the wires by the way is: "RU AWM 1015 E199279 VW-1 105°C 600V 22AWG LEO HUI    CSA TYPE TEW LL115797 FTI 105°C 600V 22AWG" This is for the black one. I am not sure about what is international safety markings and what is just the type of the wire. Well, figure out for yourself what information you need.
The wires are soft enough to not get in the way while measuring what I quite like. The connectors are safety types with protecting caps on them (see Picture 10). The length is about 1m (not measured).

What else is to say:
You cannot reach the battery and fuses easily, the meter has to be unscrewed. Also I miss a place to put the testleads. There is a holder missing in my oppinion. Also there are no bonus-functions: No temperature, no capacity, frequency, etc. Also the accuracy is pretty bad. This is not for the most exact measurements! Instead this is for field use, I guess. And it is perfectly build to withstand pretty much anything one could mechanically do to it. In fact mine fell pretty often in the seven years I own it. The largest drop was about 3 meters from a ladder. It survived withoud any problem. I hope you do not mind I did not clean it up before taking my fotos. This one was always carried around in a toolbox.

Next time I will take it apart and see what's inside - stay tuned! Any questions or remarks? Do you wish some detail pictures of something particular? Just tell me. Please remember, this is the first time I ever did this.

[Lightages]

Wow! You have had that meter for all this time and you still have that ugly piece of shipping plastic on the display? Take it off!

[PedroDaGr8]

The details for the wire used in the test leads checks out kind of:
http://data.ul.com/link/stylepage.aspx?style=1015

On the cable, the RU denotes underwriters laboratories. The e199279 is the manufacturer file number at ul, 1015 is the model. It's listed as 700vdc 600vac. Though it's 30awg not 22awg. Maximum voltage rating is 2500v

Sent from my EVO using Tapatalk

[retiredcaps]

Nice review.  Thanks for taking the time to share and please follow up with pictures of the insides.

[schwarz-brot]

Yeah, I still have that foil on the display! Guess it protected the display quite well while in the toolbox.

I will do a follow up the next days, but please be patient. I am sure you realized english is not my native language. Therefore it takes lots of time to write this up. Many technical terms are just missing in my active vocabulary. But you will get my points. If not - just ask.

What about the pictures? Are they too big? Is there an easier way to link them in? I attached them, then uploaded my post and linked them after that manually in place. I feel there has to be a direct way to do so. I don't want to use imagehosters since the pictures get lost after some time.

[Lightages]

Yes the images are just right.

[Monkeh]

Though it's 30awg not 22awg.

What makes you say that?

[Tepe]

Yeah, I still have that foil on the display! Guess it protected the display quite well while in the toolbox.

So in the hope of keeping the lens pristine you have kept it covered with plastic foil that makes it look anything but that. What's the benefit? Surely not the resale value?  O0

[schwarz-brot]

You come pretty close!

As I said I got this meter from my employer together with a hole bunch of other brandnew quality tools and a toolchest. This was destined for my apprenticeship and we were told we have to give it back in good and working condition. This is why I decided to let this foil on there - back in the days.

I somehow managed to keep all of my tools and some more    Since then I never thought of taking off that silly little foil. Why would I care about it? It does not matter to me. But I think it's kind of funny to see anyone rant about it. If this meter will someday blast I can take at last away that foil and take a look at a beautiful display in all it's virginity.





Maybe I'll just take it off next time.

[schwarz-brot]

Alright, there we go. This is

Part 2: The Insides - mechanical construction


Pic 12: Overview of opened meter


Pic 13: Detail Screws

When it is time to exchange the battery or a fuse one will find there is only one way to reach these items: One has to unscrew the case and open it up. There are three screws (picture 13) that  go directly into the plastic - no metal inserts. Not the nicest construction. But when the case is opened you realize there are no wires getting in the way and the fuses can easily be reached. I am ok with that. Also the top half of the case slides all the way down into the bottom case. The whole outer walls are doubled up that way. In case of a blast I would trust this construction.


Pic 14: Overview bottom half of case

Picture 14 shows the bottom of the meters case which carries a standard 9V battery. The meter was originally delivered with one that was extra insulated with a plastic tube. When I had to exchange the battery the first time I found it hard to put a standard 9V (Varta Brand) in that tube. I got it in there but eventually pushed too hard, making it explode about an hour later. Nothing happened to the meter, but I decided to let go of that tube as it just helped to give the battery extra stability in the body. I found the battery sat securely in there because of a little piece of foam on one side providing some pressure. None of my collegues had such a problem, so I guess it was just a faulty plastictube in mine.
Else, there is not to much to see: A thin shielding foil that covers a large part of the back (spring connected) And some extruded parts on the bottom side, which support the connectors. Good detail there, I think.


Pic 15: Overview front of case with PCB

The upper half of the case carries the PCB and shows those large walls, going down deep into the bottom half. The PCB is held snuggly in place by some clips. I am ok with that.


Pic 16: Detail rotary switch and soft buttons


Pic 17: Detail soft buttons

One can get the PCB out easily if needed. Then the main rotary switch, the display and the softbuttons can be reached (Picture 16). The construction of the switch is of good quality. The contacts are spring types. Too bad, they look like bare copper.
The soft buttons (see picture 17) are double contacted and feel pretty stable. I kind of like them (yes, I remember one tends to stick down).
I did not try to take out the display. It is a strong clipped connection and I did not want to ruin it. Also I forgot to take a foto of its backside and connector, sorry folks! I find it kind of remarkable that there is no wired connection! The display sticks down onto a line of pads on the PCB to be connected. Since it never got an issue this seems to work quite well. Like it!

[schwarz-brot]

And what you all were waiting for:

Part 3: The PCB



Pic 18: Overview PCB top


Pic 19: Overview PCB bottom


The PCB reveals no magic nor anything special. It is a mixed construction of through hole parts and SMD components. The PCB is a standard green one with white silkscreen and a heavily structured surface. All pads are gold plated. Not very thick but at least! The soldering on the SMDs looks very regular and even, no problems there. Same goes for most through holes.
There is Writing in copper saying "4233-1307-A" on the top side and the same ending with "-B" on the bottom. Also on the bottom there is the number "0506" to be found. This is read in the same direction as the other text. Of course it may also be "9050" when turned upside down. Not sure about this (7-segment stile, both seems possible). I did not bother to take pictures of that.
The Silkscreen shows a "GTR" logo and "CD-1".
The PCB is clearly laid out to be used with different types of parts. The trimpot, for example, could be of various sizes, the pads are arranged in a way that allows several types to fit in, even through hole types. One can see this at various places.
Also, this PCB could be used for several models, I think. There are many spots which can take different parts. Not all of this is to make usable what could be found, I think. There is room for a much larger ceramic type, sand filled fuse (how do you call them?) instead of that simple one on the Amp-jack, for example. Also the pads for a third softbutton are cleared and gold plated. I am sure this is for a hold button, wich some other models show.



Pic 20: Detail input area top side


Pic 21: Detail input area top side from the other side


Pic 22: Detail input area bottom side


The input area looks pretty crowded and a bit chaotic (pictures 20 to 22). Also I would suspect things could go wild in case of a blast. There are no cutouts and there is no shielding. I am a bit dissapointed about that after the good start. The Fuse on the 10 Amp range is a proper 10A / 600VAC one of those blast protected types I don't know the english name for. The other Amp ranges are fused with a FF 0.5A / 250V rated type. It sure has a ceramic body but I am not sure if it is a proper one.
One can also see there some resistors underneath the large fuse. What the hell is going on there?! I would not want anything near that beast at all! Also I find that airgap between the hot end of the 10A fuse and the bare metal of the com jack a bit small (measured: 3mm). Not sure, what is allowed there.
Also, they used different types of input connectors. Why? The com port and the voltage jack are massive tube types while the amp jacks are split types. The overall construction of the jacks looks good to me. All the pressure that might be applied when plugging in a connector goes into the supports on the back of the case, not stressing the soldered connections too much. Additional posts on the upper half of the case support the hole structure (Refer to picture 15 in my previous post).
For the rest of it, please take a closer look to the pictures above, I did not dive any deeper into the magic of proper DMM construction.
In the top right of picture 20 one can see one of too logic-ICs. There is one NAND and one NOR type (4001B and 4011B).




Pic 23: Detail main chip area


Pic 24: Detail chip, powerconnector and oscillator

On the opposite side of the board one finds a 40pin samsung branded package. This seems to be a microcontroller, although I could not find any information about it. For the type please see picture 24. This picture also shows in Detail the spring type battery connectors wich I like. Also the glued down oscillator can be seen. As far as I can tell there is nothing special happening there.




Pic 25: Detail empty pads and trimpot

But here it begins to get funny. I did not dive in there, but I suspect those empty pads R19-1 to R22-1 to be in parallel to their counterparts R19 to R22 (only followed the traces for 1). while the distance of the pads would fit the used smd resistors exactly they decided to make extra room for through hole resistors. But those would have to be mounted standing up! I can see absolutely no reason for that. There is room enough to mount them flat on the board. Any ideas?





Pic 26: Detail pads and gold plating


Pic 27: Detail display connector

Finally, on to the backside. Picture 26 shows the nice gold plated pads for the rotary switch and the softbuttons. Also one can see there is absolutly no single part on the backside of the board. The large double contacted pads for the softbuttons are a nice touch. I for sure will investigate that third one in the middle. I am interested to see if I could get a hold funktion for free with the existing firmware. Picture 27 gives a detail  of the nice lined up pads for the display. As I said in the previous post: The display is only pressed on there (graphite pads, maybe? I cannot see any metal part there) and it works just fine.


So there is need for a conclusion... What can I say? The mechanical construction is pretty good for a simple meter. In fact - that is kind of overkill. There was paid much attention to detail which can also be seen on the inside. The PCB is also constructed well and designed with some thought. Just think of all those pads with several possibilities for the used part. What I don't understand is why they decided there is not so much thought needed in the input area. This just looks like an average everyday cheap meter to me while the rest looks and feels way better.

Again, feel free to request details, pictures or clarification. Comments are welcome, no matter if they are adressing the meter and teardown itself or my style of writing and showing it to you.

[PedroDaGr8]

Though it's 30awg not 22awg.

What makes you say that?

The UL listing says conductor 30 AWG


Those fuses are called HRC fuses (high rupture current)

[Monkeh]

Though it's 30awg not 22awg.

What makes you say that?

The UL listing says conductor 30 AWG

No, it says 30 AWG - 2000 kcmil. 2000 kcmil is 36mm in diameter. 1000mm².

[Lightages]

Well the construction inside looks a little weird with the layout but at least it has some input protection. Thanks the information.

[Robomeds]

The split inputs on the A and mA inputs look like they are setup for input warnings if you connect to the mA jack when in the V mode. 

[retiredcaps]

@schwarz-brot, thanks for a well written and photographed review.    

I appreciate the time and effort you put into it and understand that writing in English is very time consuming.  I hope you will do another in the future now that you have a template.

[tautech]

But here it begins to get funny. I did not dive in there, but I suspect those empty pads R19-1 to R22-1 to be in parallel to their counterparts R19 to R22 (only followed the traces for 1). while the distance of the pads would fit the used smd resistors exactly they decided to make extra room for through hole resistors. But those would have to be mounted standing up! I can see absolutely no reason for that. There is room enough to mount them flat on the board. Any ideas?

Could they be for Melf precision resistors if only 10% SMD's were available?

As others have said..... very nice job. 

[Tepe]

@schwarz-brot, thanks for a well written and photographed review.    

Yes, those photographs are GOOD.

[schwarz-brot]

Thank you all for your kind words! That feedback gives a lot of motivation.

I am glad you like what I did. Hopefully next time I have a tripod and decent lighting to do even better pictures. I am not too happy with most of these ones. But that is a different hobby.

[schwarz-brot]

What next folks? I have some vintage gear to tear down, if you like. An RFT G1001.500 Digital Bench multimeter (mid-eighties GDR-made!). This does not work well yet, so I have to take it apart anyways. Looks pretty crappy outside but is kind of interesting on the inside with all the Parts made in the GDR or wherever else in the former USSR.

I have a a philips PM 2422 A, too. Also a bench multimeter. Looking good on the outside, not tested yet. As far as I know this dates back to the seventies, maybe early eighties. Nixie-porn, baby! Did not open it up yet, so I don't know what to expect.

I have a Hameg HM303-4 Occilloscope. 30MHz, analog of course, but I am not sure if I want to open it, as I use it quite often and I do not want to ruin it. I would more prefer to open up my really old HM 312 scope, which I got me but never used, as I got the HM303 only a few weeks later. There is also a two-channel adapter for the HM312.

Are these units of any interest to you? Which one would you like to see opened? Too bad I don't have more test equipment to offer...
What I can offer is lots of vintage HiFi equipment, but I guess this is the wrong blog for that kind of stuff. Let me know, what you want! Or send me your unwanted stuff and I do a mailbag-teardown