Author Topic: Harbor freight Cen-tech 90899 small teardown  (Read 37951 times)

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Offline rsjsouza

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Harbor freight Cen-tech 90899 small teardown
« on: December 05, 2012, 03:37:37 pm »
I just found lost somewhere a brand-new Cen-tech 90899 DMM and decided to open it up and see if anything interesting popped up. Well, I decided to take some pictures that evidenced the unfortunately but expected terrible assembly and design.

Interesting: I just saw PedroDaGr8's review of another Cen-tech precious piece of equipment...

Sorry for the low quality of the pictures; I was testing a new portable camera (also found in the same somewhere place as the DMM)...

Frontal view. Typical Mastech el-cheapo DMM, but with an ON/OFF switch. It says CAT II but does not have any indication of fused/unfused inputs.


Back cover. Several paper label warnings slapped to the back, probably to appease any lawyers...


Back cover, inside view. The spring is not only loose from the PCB, but the aluminum stick is peeling. Putting the cover back without shorting everything will be interesting...


Full inside view. Typical board for cheap DMMs, with average soldering and the unfortunately way too narrow clearances...


Input circuitry. Also typical narrow clearances, but the novelty is the slanted fuse...


Rest of the board. Also typical of cheap DMMs, although I haven't seen them that small yet


10 ohm resistor


Comparing with a Fluke 179


7k5 ohm resistor


Comparing with a Fluke 179


Voltage test (12V sealed battery)


Ampères test (12V sealed battery with 10 ohm resistor across)


Miliampères test (12V sealed battery with 7k5 ohm resistor across)


In summary: do never put this DMM across the mains leads... It may be a shocking or explosive experience... It is fine for small electronics circuits, provided the loose spring is not shorting anything inside... :)







« Last Edit: August 03, 2016, 07:01:55 am by rsjsouza »
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Offline AlphZeta

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Re: Harbor freight Cen-tech 90899 small teardown
« Reply #1 on: December 06, 2012, 01:50:09 am »
I bought one of these last year from Harbor freight when it was on sale for $5. It seems that this meter is just a rebranded DT-830 as everything looks identical.

But hey the price, it is OK if you just want to use it to check your batteries and trouble shoot some connectivity issues.
 

Offline rsjsouza

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Re: Harbor freight Cen-tech 90899 small teardown
« Reply #2 on: December 06, 2012, 02:00:08 am »
AlphZeta, that is exactly the work I will put this DMM to do... My biggest surprise is the handy ON/OFF button, which helps when doing subsequent measurements (it also saves battery lifetime and the crank selector switch).

I'll just have to shut my eyes when opening the case to replace the battery, so I won't be hit with a flying spring... :)
Vbe - vídeo blog eletrônico http://videos.vbeletronico.com

Oh, the "whys" of the datasheets... The information is there not to be an axiomatic truth, but instead each speck of data must be slowly inhaled while carefully performing a deep search inside oneself to find the true metaphysical sense...
 

Offline AlphZeta

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Re: Harbor freight Cen-tech 90899 small teardown
« Reply #3 on: December 06, 2012, 02:04:02 am »
Quote
In summary: do never put this DMM across the mains leads... It may be a shocking or explosive experience...

And I forgot to add, these meters have been around for at least 15 years if not more, and they are surprisingly resilient to rough treatment. I recall back in 2000, a friend of mine accidentally stuck the leads in the mains to measure voltage but forgot that the range was set to mA. I heard a big bang and heard a loud curse. To our surprise, besides a blown fuse and somewhat melted tips of the test leads, the meter actually survived!!! And best of all, everything still works after we replaced the fuse.

Now, that may be the exception so don't try this at home. :-DD
 

Offline rsjsouza

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Re: Harbor freight Cen-tech 90899 small teardown
« Reply #4 on: December 06, 2012, 02:12:24 am »
That is very interesting. Before I bought the Fluke 179, I had (and still have) an almost identical DMM (Steren 602-110) that was put to high voltages for quite some time and it worked well (check it here in action). However, I can tell the Steren is better built and has better clearances across the input terminals...

As you said, "past performance does not guarantee future results"... :)
Vbe - vídeo blog eletrônico http://videos.vbeletronico.com

Oh, the "whys" of the datasheets... The information is there not to be an axiomatic truth, but instead each speck of data must be slowly inhaled while carefully performing a deep search inside oneself to find the true metaphysical sense...
 

Offline IanB

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Re: Harbor freight Cen-tech 90899 small teardown
« Reply #5 on: December 06, 2012, 02:25:47 am »
Those little Cen-Tech meters can be adjusted to improve their accuracy using the trim pot inside.

Unfortunately they have a major failing: if you leave them unused for a while the contacts oxidize on the rotary selector switch and they give faulty readings. It's fixable by rotating the switch vigorously a few times to clean the contacts, but if you are not paying attention you will get a completely bad reading.
I'm not an EE--what am I doing here?
 

Offline robrenz

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Re: Harbor freight Cen-tech 90899 small teardown
« Reply #6 on: December 06, 2012, 02:43:33 am »
Also only 1meg input impedance  :o

Offline rsjsouza

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Re: Harbor freight Cen-tech 90899 small teardown
« Reply #7 on: December 06, 2012, 05:28:19 am »
if you leave them unused for a while the contacts oxidize on the rotary selector switch and they give faulty readings.
That's exactly what I was getting when I unpacked this DMM... Since I found it sitting in a box for "don't know how long" and even the low-bat indicator was on right from the start, that is probably the reason.

Also only 1meg input impedance  :o
Bummer... Oh well, that is what you get when you "buy" a free DMM...  :D
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Oh, the "whys" of the datasheets... The information is there not to be an axiomatic truth, but instead each speck of data must be slowly inhaled while carefully performing a deep search inside oneself to find the true metaphysical sense...
 

Offline T4P

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Re: Harbor freight Cen-tech 90899 small teardown
« Reply #8 on: December 06, 2012, 07:01:45 pm »
In my head they are better and cheaper versions of those crappy panel meters you can find for about the same price  :-+
That said, they are good for being a panel meter voltage and current wise  :-DD
 

Online amyk

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Re: Harbor freight Cen-tech 90899 small teardown
« Reply #9 on: December 07, 2012, 06:49:37 pm »
An "830" has sort of become a Chinese generic name for a cheap DMM (just like a 936, 937, etc. is not only a Hakko iron anymore, and an "858" is a hot air rework station.)

The separate power switch is odd, but maybe they added that with the savings from having the input PCB be cut out of the main one (notice the matching indentation on one side).
 

Offline (In)Sanity

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Re: Harbor freight Cen-tech 90899 small teardown
« Reply #10 on: January 13, 2013, 06:31:30 pm »
I appear to have a tendency to bring up some old posts.  Not exactly what I was setting out to do. 

I just purchased one of these from Harbor Freight for $2.00 on sale.   I had one goal in mind when I did so,  see how accurate a $2 meter could be.   Shockingly using my 3457A as a reference this little meter was an average of 0.25% on DC and about 0.5% average on AC.   The ohms ranges came in around 0.35% average and the ma ranges were again around the 0.25% average.  The only range that was out far was the 10 amp current range scoring in at 3% which is just in spec based on the paperwork. 

So I adjusted the trim pot to improve the DC accuracy and then using my 3457A as a reference I was actually able to get the 10 amp current range within 0.25% (or better).  How?,  by scraping a razor blade along the shunt wire until the reading came up to my reference meter.   Removing a tiny bit of material each time. 

So if your high current reading is to low one option is to shave the shunt.   Sounds a bit weird now that I write it out.   If it's to high you could push it down a little bit in the solder connection to in a sense shorten it or just cut it remove a tiny tiny amount and solder it back together again..shave as needed.  Has anyone else shaved the shunt before ?

Ohh and as a side note for anyone playing with one of these toys...don't use the supplied leads for anything much over 3 amps.   They will light up like a fuse at 10 amps.   5 amps they just get kinda hot.   Then again yours may be less crappy then the ones I got.   Just use caution.

Jeff
 

Offline retiredcaps

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Re: Harbor freight Cen-tech 90899 small teardown
« Reply #11 on: January 13, 2013, 07:16:23 pm »
Has anyone else shaved the shunt before ?
:-DD.  Best line of the year so far.

Well, it looks like they nick and shave $250 USD multimeters.

http://www.eevblog.com/forum/reviews/teardown-extech-ex570-my-first-but-also-last-extech/
 

Offline saturation

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Re: Harbor freight Cen-tech 90899 small teardown
« Reply #12 on: January 13, 2013, 11:55:22 pm »
See this too.

http://www.eevblog.com/forum/chat/kelvin-50le-multimeter-review/msg170027/#msg170027

I'd interested to see how the accuracy changes with ambient conditions.  What would happen if you used that Centech outdoors in winter, or summer?  Leave in a refrigerator for 1 hour to get in down to 40oF and recheck any of its ranges.
Best Wishes,

 Saturation
 

Offline (In)Sanity

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Re: Harbor freight Cen-tech 90899 small teardown
« Reply #13 on: January 14, 2013, 04:22:11 am »
See this too.

http://www.eevblog.com/forum/chat/kelvin-50le-multimeter-review/msg170027/#msg170027

I'd interested to see how the accuracy changes with ambient conditions.  What would happen if you used that Centech outdoors in winter, or summer?  Leave in a refrigerator for 1 hour to get in down to 40oF and recheck any of its ranges.

I would love to do something like that.   I've actually been playing around with a peltier system which I could build in to a mini environmental chamber,   or I could just set the freezer to -25C and toss the meter in for a while.   Hmm,   I think I'll try the easy part first and just heat it a bit.   Hot air rework on 100C pointed in it's general direction should help warm it up.   Not very accurate,  but might show a trend.   
 

Offline Stonent

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Re: Harbor freight Cen-tech 90899 small teardown
« Reply #14 on: January 14, 2013, 05:11:44 am »
Those little Cen-Tech meters can be adjusted to improve their accuracy using the trim pot inside.

Unfortunately they have a major failing: if you leave them unused for a while the contacts oxidize on the rotary selector switch and they give faulty readings. It's fixable by rotating the switch vigorously a few times to clean the contacts, but if you are not paying attention you will get a completely bad reading.

I have one of these I bought to troubleshoot a problem as my meter had gotten lost. There were two very similar looking models at the same price... I think I have the other. The accuracy seemed way off on everything I tested.  I played with the trim pot but it seemed to do nothing at all.

Holding the leads together seems to just cause it to wander around from 2 to 6 ohms randomly.
The larger the government, the smaller the citizen.
 

Offline (In)Sanity

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Re: Harbor freight Cen-tech 90899 small teardown
« Reply #15 on: January 14, 2013, 05:24:07 am »
Those little Cen-Tech meters can be adjusted to improve their accuracy using the trim pot inside.

Unfortunately they have a major failing: if you leave them unused for a while the contacts oxidize on the rotary selector switch and they give faulty readings. It's fixable by rotating the switch vigorously a few times to clean the contacts, but if you are not paying attention you will get a completely bad reading.

I have one of these I bought to troubleshoot a problem as my meter had gotten lost. There were two very similar looking models at the same price... I think I have the other. The accuracy seemed way off on everything I tested.  I played with the trim pot but it seemed to do nothing at all.

Holding the leads together seems to just cause it to wander around from 2 to 6 ohms randomly.

Sounds like you just had a defective device.   They are built to such low quality it just amazes me how accurate they really are.  I was expecting them to be out by many percent,  not tenth's of a percent.   

Jeff
 

Offline SeanB

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Re: Harbor freight Cen-tech 90899 small teardown
« Reply #16 on: January 14, 2013, 05:32:26 am »
Testament to just how robust dual slope integrating meters are, along with how good the 7106 design is, even many years later.
 

Offline (In)Sanity

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Re: Harbor freight Cen-tech 90899 small teardown
« Reply #17 on: January 14, 2013, 05:44:09 am »
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
 

Offline Stonent

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Re: Harbor freight Cen-tech 90899 small teardown
« Reply #18 on: January 14, 2013, 05:47:47 am »
FYI: These meters are on through the end of today (Sunday 1/13/13) for $2.00

As always they have 2 different model numbers that look alike.

http://www.harborfreight.com/7-function-multimeter-98025.html

http://www.harborfreight.com/7-function-digital-multimeter-69096.html

I'm tempted to either buy the same one, and put the old one in the box (had it for a few months) and return it or buy the second one that I don't have and keep it.
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Offline (In)Sanity

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Re: Harbor freight Cen-tech 90899 small teardown
« Reply #19 on: January 14, 2013, 05:57:58 am »
FYI: These meters are on through the end of today (Sunday 1/13/13) for $2.00

As always they have 2 different model numbers that look alike.

http://www.harborfreight.com/7-function-multimeter-98025.html

http://www.harborfreight.com/7-function-digital-multimeter-69096.html

I'm tempted to either buy the same one, and put the old one in the box (had it for a few months) and return it or buy the second one that I don't have and keep it.

The one I have is the 69096 model.  But honestly..what's the difference?   The board is identical,  the case is identical,  the leads are identical.   Just go buy one for $2,  why bother to return anything.  I should have purchased 2 or 3 of these for $2.   They are somewhat unsafe meters,  but they appear to work pretty well.   It's hard to find just a panel meter from China for $2 let alone a complete meter that works.

Jeff
 

Offline Stonent

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Re: Harbor freight Cen-tech 90899 small teardown
« Reply #20 on: January 14, 2013, 06:21:58 am »
True but the wandering all over the place makes me a bit irritated.  I took it to work for basic troubleshooting and have it on my desk.  That and since I paid more than $2 for it...
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Offline Stonent

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Re: Harbor freight Cen-tech 90899 small teardown
« Reply #21 on: January 14, 2013, 06:29:56 am »
I bought one of these last year from Harbor freight when it was on sale for $5. It seems that this meter is just a rebranded DT-830 as everything looks identical.

It does seem to have a rather strong resemblence to the Uni-T DT830b





There also appears to be another DT-830b that is unbranded that looks even closer.



The only thing I can see different in these meters is they took the off switch and moved it to the knob. And removed the battery tester.

I do like in the grey one how they actually put lines around hFE and encircled the transistor tester to indicate that's what hFE means.

The best variant has to be the "FUKE" DT-830b.



« Last Edit: January 14, 2013, 06:54:48 am by Stonent »
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Offline (In)Sanity

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Re: Harbor freight Cen-tech 90899 small teardown
« Reply #22 on: January 14, 2013, 07:01:24 am »

The best variant has to be the "FUKE" DT-830b.

 :-DD  OK,  that's just hysterical. 
 

Offline Stonent

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Re: Harbor freight Cen-tech 90899 small teardown
« Reply #23 on: January 14, 2013, 07:05:12 am »

The best variant has to be the "FUKE" DT-830b.

 :-DD  OK,  that's just hysterical.

A little white pen could fix the word "FUKE" into something more descriptive.
« Last Edit: January 14, 2013, 07:07:43 am by Stonent »
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Offline (In)Sanity

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Re: Harbor freight Cen-tech 90899 small teardown
« Reply #24 on: January 14, 2013, 07:42:03 am »

A little white pen could fix the word "FUKE" into something more descriptive.

Perhaps they are just prepping you for what you might ultimately say while using their product.
 


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