Harbor freight Cen-tech 90899 small teardown

[SeanB]

Followed by a thud and an ambulance perhaps.........

[(In)Sanity]

Followed by a thud and an ambulance perhaps.........

LOL,  no kidding. 

Let me list out the pro's and con's of this device. 

Pros:

• Good accuracy for the price. This is great to keep you from second guessing the meter and using it longer.
• LCD has good contrast.  This comes in handy when it's fallen down near your trasmission in your car and you have to use a flashlight to see the reading.
• Range selector switch hasn't fallen off yet.  This will save you the expense of a pair of locking pliars
• Plastic case screw holes come pre-split and cracked so you don't have to.
• The HFE test connector appears to have no noticeable contact resistance to help prevent you from wasting your time trying to use it.
• The test leads don't actually contain wire so the risk of electrical shock is minimized
• A convenient fuse has been provided which is built in to the test leads to protect the user from overloading the 10 amp current range.   This also doubles as a camp fire starter in the event your stranded and all you have is a car battery and your meter.
• The gracefully supplied battery has extra short life to prevent you from using this meter longer then needed.
• If you agree not to change the already short life battery you may receive a discount (or lack of an increase) in your life insurance policy rates.
• In the event your meter should expire (clearly do to your own incompetence) then the plastic shell can be melted down to make decorative candles for your home. 
• A large bright yellow on/off switch has been provided with a limited number of life cycles to further aide in reducing the time using this meter.

Cons:

I really can't think of any.

Jeff
 

[Bored@Work]

• The HFE test connector appears to have no noticeable contact resistance to help prevent you from wasting your time trying to use it.

But one contact is directly connected to one of the probe sockets. So if you have some transistor stuck in it, and then probe mains voltage you can easily do a haptic voltage check in parallel.

[Stonent]

I couldn't resist buying another one of these since they were $2.00 today. I purposely bought the "other" meter that has the red switch so we could compare the insides and see which may be better.

Here's the one with the "UL830B" from the teardown above.



And here's the one I bought. I'll call it "K2404":



1. Different pots.  Mine had about a 2 to 3 mm piece of bare tinned wire sticking out of the middle leg of the pot.  I snapped it off.
2. Mine has the hole on the left for the backlight button. There's another meter that looks like this one that has a backlight button. So I'm guessing mine is the same brand/source as that.
3. I've got a big ground area.
4. I've got a soldered fuse. 
5. Under the battery on mine is a + and - in the case showing the battery should be flipped the other way so the wires don't lay across the inputs and I'm assuming wires tucked away on the other side.  Sadly they did not install the battery correctly as you can see and caused a potential dangerous situation.
6. My test leads have 1000V melted in to them that looks like it was done with a cattle branding iron with gloopy plastic around it.
7. My capacitor appears physically bigger. It was marked ":N." and "154"
8. It looks like both boards were designed with the input jacks on the side and then were just snapped out and moved to the other side.

Anything else anyone notices to be different aside from board layout?

[Stonent]

Also the battery tester function... According to the insert it puts a 370mW load across the battery and checks the voltage drop.

[IanB]

Here's a picture of a 90899 from three or four years back:



I think the build quality might have been slightly better then?

1. There is a grounding spring on the PCB connecting to a shield inside the back cover
2. The 10 A shunt looks tidier
3. The DC trim pot is possibly of better quality?
4. I see a PTC marked below R4 and C2--is there one in the later model?

[(In)Sanity]

The one I got yesterday and have been talking about actually had the fuse holder soldered in at an angle.  Instead of fixing it they just rammed the fuse in and forced the blades way apart. I guess it technically worked..so they just went with it.   I was able to correct the issue in just a few seconds by just hitting it with the iron.   It's a $2 meter.........

Jeff

[Stonent]

Oh yeah, my meter did not have any foil inside the case.

[Stonent]

http://habometer.en.alibaba.com/product/558303742-213524963/Digital_multimeter_DT_830B.html



Looks like you can buy them in bulk for $2 to $2.50 (Yellow)

http://habometer.en.alibaba.com/product/558561950-213524963/Digital_multimeter_DT_830B.html?tracelog=cgsotherproduct1

Or 2.50 to 3.00 each for a red one that looks identical to the Harbor Freight one except for the brand being different.

[(In)Sanity]

Ohh and by the way I did measure 1000v DC with this thing and 500 VAC and it survived.   I guess I could try 750 VAC,  I don't however have a current controlled source for that high of voltage AC so I'm a bit worried    The 1000V DC was current limited to around 100ma.  Still very dangerous. 

Jeff

[rsjsouza]

5. Under the battery on mine is a + and - in the case showing the battery should be flipped the other way so the wires don't lay across the inputs and I'm assuming wires tucked away on the other side.  Sadly they did not install the battery correctly as you can see and caused a potential dangerous situation.

In both mine and IanB's two holes for the battery wires can be seen, but in yours I think someone goofed up on the drilling and broke the PCB in the corner close to the transistor testing socket. maybe that is why they put the battery in any position.

8. It looks like both boards were designed with the input jacks on the side and then were just snapped out and moved to the other side.

I haven't noticed that. Quite interesting.

I think the build quality might have been slightly better then?
4. I see a PTC marked below R4 and C2--is there one in the later model?

Wow, yours look much more acceptable than the newer ones. If you notice, the PCBs areas are being reduced at every assembly iteration, as well as the number of passive components... Incredible!
BTW, no PTCs in mine...

I have a Steren 602-010 (Mastech M830B, circa-2002) but it is completely different (all discretes):

[Stonent]

OK team I just got back with my Feng Shui master and we now have a change of plans... I want you design a multimeter and I want you to move the components as far as possible away from the center as possible.  Don't ask. Just do!

So I'm guessing the 830B model is more of a front face specification rather than an inside specification.

Just make it look like "this" and put whatever the hell you want inside.

[amyk]

I almost thought that link said "Hobometer"...

The selector switch is in the centre, that's why all the other components end up at the edges.

[rsjsouza]

So I'm guessing the 830B model is more of a front face specification rather than an inside specification.

Just make it look like "this" and put whatever the hell you want inside.

That is pretty much it. I was always told that the price of the plastic mold for the case is tremendously expensive, therefore they need to reduce the price on the internals to get the most out of the same mold.

DMMs are the cheapest ones, but I've seen this "design recycling" in other classes of instrumentation equipment as well... Not that this is a bad thing, but definitely the quality becomes a factor of manufacturing date, not model or revision.

All that, I still find incredible how they can still make money by selling these at US$2.00...

[saturation]

Did you hot air the PCB or the chassis?  If its the PCB that's impressive.  90F would simulate early summer, as in leaving the DMM in a car or such, and the fridge effect would be like working with it say, in your car, during the fall or 'warm' points of winter.

I just did a quick for fun test on this little guy.   It had been sitting all night in my basement lab at around 68F,  I used my hot air rework to slowly bring the thing up to around 90F and it only increased the least significant digit by 1.  That's a 0.1% increase.  Still not bad for a $2 meter.   I may do a more comprehensive test in the future instead of the quick and dirty test.

Jeff

[SeanB]

I must buy one tomorrow at the house of tat, Osmans clocks and watches. R42 each, in a blister pack. Check it out, tear it apart then freeze it to -13.4c in the freezer and check again. Single point check on 10V only, possibly a resistor as well.

[(In)Sanity]

Did you hot air the PCB or the chassis?  If its the PCB that's impressive.  90F would simulate early summer, as in leaving the DMM in a car or such, and the fridge effect would be like working with it say, in your car, during the fall or 'warm' points of winter.

I just did a quick for fun test on this little guy.   It had been sitting all night in my basement lab at around 68F,  I used my hot air rework to slowly bring the thing up to around 90F and it only increased the least significant digit by 1.  That's a 0.1% increase.  Still not bad for a $2 meter.   I may do a more comprehensive test in the future instead of the quick and dirty test.

Jeff

I applied hot air from a distance for some time to slowly warm the plastic case and also the inside indirectly.   It needs to be an even heat for a true test.

Jeff

[T4P]

i have several ... heat packs, those that you use during winter as a leftover from my japan trip
Would make a even heater for a tiny meter like this 
Or a peltier hot side bolted down to a steel plate covering the entire meter

[saturation]

Yes, but cold works as well, after all you're just looking for an  ambient different from standard, so why worry about heat? Just throw in into the refrigerator for an hour and see what happens.  You'll get a lot of condensation when you remove it, and that's another test right there.  1 hr, and that's all it takes.

I applied hot air from a distance for some time to slowly warm the plastic case and also the inside indirectly.   It needs to be an even heat for a true test.

Jeff

[(In)Sanity]

My little test was just quick and dirty,  hardly scientific.   I'd be interested to know just how much it drifts under a more controlled test.   Temperature coefficients and even thermal junctions are a subject of interest to me lately.   

Jeff

[(In)Sanity]

Ok,  in the interests of science...umm.. Yah something like that I've done some more thermal testing. 

Temperature as measured at the bottom of the 10 amp current jack with a Type K thermocouple and socked for at least half an hour at each temperature, here is what I've got.

Voltage reference was a fresh 1.5V AA battery. 

@ 102F (38.89C)  the meter reads 1.569 - 1.570 with flutter. 

@ 56F (13.33C) the meter reads 1.572 without much flutter.

So based on that test and temperature range it drifted 0.191%.  That's 0.0042% per deg/F or 0.0246% per deg/C or 246 ppm/C which if you think about it sounds about right for the type of components used.

Heating and cooling source provided by a 100W peltier with a feedback loop to the thermocouple.   It was just a quick toss together but appeared stable.   The meter was placed on top of the heating and cooling source and allowed to soak and stabilize.   Not exactly an environmental chamber,  but a test none the less.   I was having a hard time with cooling below the noted temperature with my current setup.  It's a $2 meter,  pretty damn stable for a $2 meter.

[mrflibble]

I noticed that in some of the meters IC1 is 44 pins, and in others it's 42 pin. Any idea what IC1 is? Any datasheet?

[amyk]

ICL7106 or equivalent.

[mrflibble]

ICL7106 or equivalent.

It is the "or equivalent" I am worried about. Case in point, I have the datasheet for the TelCom TC7106 in front of me and that is available in 40 pin dip (which is what is in my super old el cheapo DMM) and 44 pin PQFP/PLCC. And neither of these TelCom TC7106's has an RS-232 output as reported for the Cen-Tech. I'm interested in using the RS-232 port so if you or anyone else knows more specifics about that or what exact chippies are in the DMM's in this thread that would be real handy.

[sync]

I don't think the cen-tech has RS232. The hack (http://www.instructables.com/id/DMM-Piggyback-add-RS-232/) taps an analog signal and uses the microcontroller ADC.