Harbor freight Cen-tech 90899 small teardown

[mrflibble]

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.

That is indeed the hack I mean. And thanks for pointing that out. I guess this is a good example of why sleep is a good plan. I totally misread that yesterday. I mean, it's still a cute hack but less useful than I thought it was. Doh!  Guess it's back to my original plan of decoding the lcd outputs then. At least that piggyback hack did have some nice inspiration on decoding the rotary encoder.

[d3javu]

I have one made by pro's kit. It is my first ever multimeter which i use during my university years. 

[Jidis]

At the risk of being pelted with rocks, I had this opened and figured someone might want to see it. It should be the currently selling "no backlight" version. It's a model 69096. Board looks similar to the OP's, but not exactly.

I'm a fan of these things. I've got one of the black ones on the way here now (DT830D) with square wave and continuity functions. Need to get that yellow one next. There's also a DT830-CLOCK that's black with a yellow label. It appears to have reset and adjust buttons, but I'm guessing they might just be for the clock part. If anybody spots that on eBay or something, let me know.

Take Care

[retiredcaps]

I had this opened and figured someone might want to see it.

I'm always interested in seeing what the insides look like regardless of price or manufacturer.

Your Greenlee DM-55 is way better built compared to this.   

[Jidis]

Your Greenlee DM-55 is way better built compared to this.   

Yeah, I noticed that too. 

These pictures seem to show stuff my eyes have trouble seeing. Do you know if those Intersil,etc. chips make it very far up the multimeter ladder, or do they only show up in the ultra cheap stuff?

Take Care

[retiredcaps]

These pictures seem to show stuff my eyes have trouble seeing.

Yes, I have noticed that too.

Do you know if those Intersil,etc. chips make it very far up the multimeter ladder, or do they only show up in the ultra cheap stuff?

If you want to spend about 1.5 hour finding out how the 7106 came about read and listen to

https://www.eevblog.com/forum/testgear/old-fluke-multimeters/

http://www.theamphour.com/180-an-interview-with-dave-taylor-multi-talented-meter-maker/

[Jidis]

Much Thanks!

I got glued to that Fluke thread and couldn't get off of it.

Re: The HF meter- I notice there are 2 non-backlit versions on their site right now: My 69096, and a 98025. Neither of which is the OP's 90899. Specs look the same, but they added a "Frequency" spec of 45-150 Hz to the 98025. I also noticed last night that the one I cracked open (69096) is a rich darker red vs. the bright red plastic backlit model here. Reminds me of that Merlin game from back in the 70's.

I can see me years from now like you guys with the Fluke 806x collections, posting pictures of my army of DT830 clones. 

[Stonent]

Also the HF versions all have the battery tester where the others usually don't.

[Jidis]

Also the HF versions all have the battery tester where the others usually don't.

Yeah, it looks like most of them with that same enclosure have twenty positions for the switch, so I guess they have to make trade-offs. I do remember at least one of the yellow ones having the battery test down in the lower half of the dial somewhere. I also remember some others where certain positions actually shared a couple functions. Not sure how they handle that.

The reason I asked about those chips was that seeing all these different versions got me wondering if the same chip is capable of a much higher level of accuracy in a different circuit, and how much variance there might be between different revs of that PCB or different models of this clone. The one I checked here was surprisingly close to the values of the reference board I bought, but it would still be interesting to know if there's one specific brand/model which is consistently more accurate than the others.

Take Care 

[SeanB]

7106 in it's various varieties is a very good design, and is very accurate and linear, depending only on short term oscillator stability and 2 low leakage capacitors and 2 high stability resistors for operation, along with a good reference. The internal one is pretty good, so it is mostly the one used.

You get a 1999 count meter that will work from -1999 to +1999 with nearly perfect linearity and almost no offset, provided the supply is noise free and the layout is as recommended in the app notes ( and still good even without that to some extent), and the clock is a multiple of the local mains frequency. You normally get it set for 2.5 readings a second, which makes it reject both 50 and 60Hz noise. Only caveats is that you need to remember it is slow to come out of overload, and that the internal reference is limited in common mode range and the current it can supply. Also do not leave it in test mode for a long period as this damages the display.

[dannyf]

A big 830 fan here. I have always been amazed by what those little buggers can do.

[Jidis]

Thanks for the info SeanB!

I see there's another thread on it here. That guy's got mine, and I notice mine is missing a cap about an inch below the trim pot. Don't know if that's something they decided to stop using or if someone was just in a hurry when they did mine. My board isn't even silkscreened.

I also noticed the yellow one with the through hole parts in post #3 over there has what looks to be a 42? pin version of the chip rather than a 44, unless there's a couple which weren't dragged out to those pads.

George

PS- I got the "fall apart" test leads everyone talks about with this non-backlit one. They're different but still cheap looking on the other, so apparently it sells with a couple different types.

[Jidis]

Here's the bright red one (92020) with the backlight. The board in this looks very similar to the through hole yellow version that guy at BadCaps had. Some of the through hole parts are even in the same spots as mine, but I didn't want to pull the board out now to see if there are holes under them. This also has the 42 pads surrounding the processor (vs. 44).

Funny, but I ran into the same deal on that other one that someone joked about here with the "pre-cracked" threads. Almost all of them were cracked on the darker red one. The cracks don't seem to go all the way down (yet), so there's still something to grab. They're mostly where the threaded posts neck down to a smaller size at the top. That said, maybe the bright red one has softer plastic and is a better choice for longevity. Also, the owner of the yellow one mentioned the PCB being wedged in and not wanting to pry it out for risk of breaking it. My non-backlit had that too. It looks like they didn't cut the 'U' shaped notches in the board wide enough on mine to fit around the threaded posts. I Dremeled mine out before reassembly, but had also noticed a spot in the case where it looked like someone had dropped the soldering iron and didn't pick it up for a few seconds (there was even solder still smeared into the plastic). When I was putting it back together, I noticed that the melted dip in the plastic lands right on the post where the board is too snug. I think they got a batch of boards where the notch was too tight, so they squashed the iron into the case to make the post smaller. 

Good to know our three dollars goes to such innovative thinkers. 

[marshallh]

I've now had 2 of these types of meters blow their brains out when turned on. (I.e.  the glop top explodes and shatters itself).
They work great for low level DC work.

[Jidis]

Here's the board for the black DT-830D. Unfortunately, it's a DOA. In some modes, a character or two will show up on the display, but in many, it stays blank and there's no sign of life from the added continuity beeper, so I doubt it's just the screen. Soldering is somewhat messy as expected.

Interestingly, the board looks quite different from my others or any I saw online. There's a 358 opamp added, *no* DC trim pot, and the IC (chip on board thing) appears to be installed at an angle, with only 40 pads drawn from it (11-11-10-8). It's also got nice looking "hatched" patterns filling the empty spaces (grounded, I guess). The little daughterboard for the jacks is at a different height and is using three connecting wires as standoffs. Soldering for the wires looks like crap on the mainboard end. There's barely enough to fill the holes and the main V/mA one looks like it may already have broken free. Otherwise, it looks like a sturdier means of holding them. They sit lower than the mainboard, so that the back of the enclosure acts as a stop now when you press on them.

This is the model with the square wave output and continuity features. The diode test appears to share the setting for 2000 ohms. It's also got a battery compartment with spring clips in a unique "reverse polarity" configuration. There's also another unstuffed spot on here by the center hole. As with the other, I have no way to know if there was supposed to be a part there or if it was optional (no labels either). I'm guessing those three pads in the upper right with the 100 ohm resistor are where the trim pot went.

BTW, these pictures are sized and aligned so that if they're in the same location, you should be able to bounce between them with the pads and holes lining up (top layer is mirrored).

Take Care

PS@marshallh- Do you mean they blew up with nothing connected?

[retiredcaps]

Here's the board for the black DT-830D. Unfortunately, it's a DOA.

The first ever multimeter I bought was one of those DT-830 from ebay.  Not surprisingly, it arrived DOA.  If you flex the pcb around the COB, you can see more lcd segments, but nothing makes sense on any rotary dial setting.

I still have it in hopes of fixing it one day for educational purposes.

[Jidis]

If you flex the pcb around the COB, you can see more lcd segments, but nothing makes sense on any rotary dial setting.

Funny, I did the same thing with this one after noticing that it's got the same cracked threads issue as the dark red one. I thought maybe the LCD pad wasn't making contact with the board, but it turned out some weird area over on the edge of the board was the best place to flex. It still didn't bring it back to life or put anything worthwhile on the screen. Also, the original reverse polarity battery setup shows more life than the correct way. With the battery in backwards, you can get decimal points and an occasional "HV" or something.

Are those displays some sort of standard pinning? All I'm familiar with are those HD44780 ones, but I'm guessing those Intersil chips were setup to use something common.

I notice you saw this guys meter. It's weird that they use the same model numbers for a whole slew of different versions. His is also a "D", but it's nothing like mine.

Regarding some of those cases, I don't know why the heck they would keep using something which has proven to fail before it even reaches the customer. You'd think with all the changes in PCBs, they'd have redesigned or reenforced the threads somehow.

Take Care

George

PS- Sadly, I wasted about an hour tonight cleaning up and resoldering everything on that black one only to see it do the exact same thing afterward.

[Stonent]

I've had a couple of these where the zebra strip is the cause of the problems.  Commonly if the screws stripped out and there's not enough pressure being held on the display the numbers will go all wonky.

[Jidis]

Hi Stonent,

I suspected that as well, but I seem to get less activity from it when pressing near the strip than in other areas. Strange thing is, I can get random LCD segments to display like Retiredcaps mentions, but more so by simply touching certain areas of the PCB, rather than pressing. Like his though, it never really makes any sense, and if I short the probes in continuity mode, I can go around the board pressing and touching everything and it never makes a peep.

Still wondering about that LCD though. Is there some sort of standard for those things? Also been wondering why, with the few external components you typically see in these cheap meters, the DIY stuff you see using those ICs always seems so rudimentary. I didn't see one with anywhere near the range of functions these meters have.

George

[Stonent]

Hi Stonent,

I suspected that as well, but I seem to get less activity from it when pressing near the strip than in other areas. Strange thing is, I can get random LCD segments to display like Retiredcaps mentions, but more so by simply touching certain areas of the PCB, rather than pressing. Like his though, it never really makes any sense, and if I short the probes in continuity mode, I can go around the board pressing and touching everything and it never makes a peep.

Still wondering about that LCD though. Is there some sort of standard for those things? Also been wondering why, with the few external components you typically see in these cheap meters, the DIY stuff you see using those ICs always seems so rudimentary. I didn't see one with anywhere near the range of functions these meters have.

George

Well many meters use the ICL7106 chip or a clone living in the blob.  It has everything you need including LCD driver, with just a few passive parts on the outside.



There's like 5 caps and 5 resistors in total. 

There's also the 7107 which is the LED version.

[dannyf]

A good application with those meters is to repurpose them as dedicated volt or amp or ohm meters: take out the pcb and selectively solder the pcb traces to select the range / measurement type. This thing can be easily powered from a 12v source and you have a very accurate volt / amp or ohm meter.

or a frequency meter or capacitance meter (not so accurate).

[Jidis]

Well many meters use the ICL7106 chip or a clone living in the blob.  It has everything you need including LCD driver, with just a few passive parts on the outside.

Thanks Stonent. Yeah, I saw some of the example circuits for that chip, they're just never really specific about what lines are what. I also noticed that the meter's PCB has a much higher number of contacts for that zebra strip thing (something like 30?).

A good application with those meters is to repurpose them as dedicated volt or amp or ohm meters.

Only problem with that is that there seems to be a whole slew of those little mini panel meters already out there which sort of do the same thing, with the exception of the resistance measurements, but since it doesn't auto-range there, that would probably require a mess of switches or something.

Take Care

[CatalinaWOW]

Since the price of these things ranges from free to a couple of bucks, their best application is to solve the "Its never where I need it" problem.  I have a couple of dozen.  Just like the free flashlights from the same source.  They end up in each of my cars, in the kitchen drawer, by the computer, in the shop and so on. Don't worry about fuzes, batteries or anything.  I haven't had much problem fuzing the leads, but do break them regularly.  In any case, just like DOA, the solution is to fill the landfill and break out another one.

For the collectors out there, the most fun one I have seen got a packaging error.  Had a long list of functions that it supposedly could do, including capacitance, frequency, temperature etc.  Not bad for a free meter, but it was the same little unit.  I think I kept the packaging somewhere, but not with the meter inside, because I needed a meter NOW!

[Jidis]

For the collectors out there, the most fun one I have seen got a packaging error.

How about this one? Ran into it the other day and had forgotten I had it. I think it may be a miniature predecessor to those red things. It takes six of those AG13 type batteries and cost three bucks or something (probably less than the batteries). Unfortunately, it's also dead. 

[CatalinaWOW]

The little meter that "could".

See in the claims on the package that it has gained the ability to measure AC current, frequency, capacitance, humidity, sound level and luminosity.  Maybe that is what hfe translates into in Chinese.  Strange that temperature and inductance weren't on the list of things it can do.