Dangerous multimeter : MASTECH M890G / M890G2 (aka DC Electronics DC03)

[uski]

Hi,

I was totally uneducated about multimeter safety before watching Dave's videos. Thank you Dave.

Tonight I took apart my old, trusty (I thought !) multimeter I've been using for years.
It's a "D.C.Electronics" model "D.C. 03"
I wanted to see the protections inside and try to compare with what I've seen on the videos.

Are you scared ? You should be, because after opening it I am totally scared !



At first glance, nothing looks wrong, right ?
Hmm, wait. "20A UNFUSED" ?
1000VDC/700VAC max ?

Interesting. Let's take a look inside...





OK let's focus on the input circuitry...



What a nice input protection... a single glass fuse, nothing else !
The 20A input really is unfused.
No PTC, no MOV, nothing.

And on the picture above, look at the center-left, above the red wire...
Look at those 2 diodes and those 2 resistors... their legs almost touch each other
That may be or may not be a safety issue, but it's definitely a reliability and QC one !

Speaking of clearances :



Look at the fuse.. look at that clearance between the fuse and the resistor... or that lack of clearance !
Remember from the first pic, the rightmost banana jack a few millimeters away from the lower fuse terminal is the unfused 20A input !

And look at the clearance around the shunt resistor :



There is like 2mm between the pot and the shunt.

Wait... what's that ?


MASTECH M890G2 (or M890 G2 or M890G 2 - just making sure people can find this when they use Google)

180194... might be a datecode ! January 18 1994...

I can find it for sale today on the Internet on many websites ! I could even find it for $50+ ! Incredible !
I'm afraid they sold thousands of those, and many must still be in use today (like mine !) or available as "new old stock".

And MASTECH seems to be still selling multimeters !
It's possible that they have improved (if the datecode is correct, this is 20years old technology), but still...

The importer is clearly at fault. I don't think this piece of sh** could be legally sold; it doesn't even have the CE mark !!! (this was purchased in Europe).

At least, the manufacturer wasn't lying by putting false labels and safety ratings. But still they should not have sold this because there is absolutely no way for an uneducated user to know the danger of using that piece of hardware.



I used that thing a few days ago to check mains voltage...

It's a shame, I liked this multimeter ! It feels solid, while still being lightweight. Battery life is huge, accuracy is OK for very basic use, and it has many features (thermocouple temperature measurement with standard socket, capacitance, ...). But safety first ! So... be careful - be safe... my advice : DON'T USE THIS ONE !

So, what's your opinion on that multimeter ? Would you use it or toss it ?

uski

[retiredcaps]

Thanks for the pictures.  I always like seeing the insides of a meter regardless of price, brand and country of origin.

It looks like it has two PTCs (circled in red).  Yes, I would use this multimeter on low voltage/current enviroments.

However, if I had to use this on mains or high voltage, I would get aligator clips for hands free operation.  Having said that, this meter may not survive 240VAC if it is in ohms mode?

[uski]

Hi there !

Thanks for the pictures.  I always like seeing the insides of a meter regardless of price, brand and country of origin.

If I remember correctly I paid around 25 euros (35 USD) for this one, it was a "closeout deal".

It looks like it has two PTCs (circled in red).

I think you're right !

Google gives me two different schematics for that multimeter.
I have attached them to this post. It looks like there is at least one PTC for the ohms range.
However I wouldn't trust them for 20A... (and there is no PTC for that range anyway, according to the schematics)

Yes, I would use this multimeter on low voltage/current enviroments.

Fortunately it's the environment in which I'm using it mostly. But still, with the very low clearance and the component legs that are almost touching each other, it may be unsafe (not a death risk, but a risk to blow up the circuit you are using it on).

However, if I had to use this on mains or high voltage, I would get aligator clips for hands free operation.  Having said that, this meter may not survive 240VAC if it is in ohms mode?

I can't manage to read the schematics (I don't understand how they connect the points of the rotary switch) so I can't say, but if you can read it, let me know, it's an interesting discussion !
From my understanding, the NPN transistor "Q4" will blow up if in ohms mode with 240VAC.

I might try some day, if I get bored... I'll (safely) make a video if I do so (don't try this at home)


uski

[retiredcaps]

However I wouldn't trust them for 20A...

Yes, I wouldn't use it for 20A especially if it is unfused.  I rarely measure current and when I do it is typically less than 1A DC.  I would love to get a DC clamp for automotive work, but so far I haven't needed one.

But still, with the very low clearance and the component legs that are almost touching each other, it may be unsafe (not a death risk, but a risk to blow up the circuit you are using it on).

You can carefully bend some of the components so they are not touching.  Whether it stays that way after an accidental drop is another story.

I can't manage to read the schematics (I don't understand how they connect the points of the rotary switch) so I can't say, but if you can read it, let me know, it's an interesting discussion !

I'm just learning how to read schematics, but I will take a look sometime over the next couple of days.  It is 1AM and I'm off to bed soon.

[retiredcaps]

However, if I had to use this on mains or high voltage, I would get aligator clips for hands free operation.

I should mention that I use the above procedure regardless if I am using a Fluke 87V or $2.99 Harbor Freight multimeter.

Good safe practices and procedures are my first line of defense and a good multimeter protects you from the "oops" moments.

[deth502]

this is why i feel most of this "safety" hype bullshit is overrated.

the meter was fine for you for 20 years and you loved it. it never detonated into a nuclear explosion and killed millions of children, but yet now you feel like all of a sudden it will.

dont get me wrong, a nice well designed meter with some protection is good, but if you dont fuck up and just use it correctly, it really dosent matter.

[Jasper]

Still, "will usually not kill you if you're careful" is not a particular recommendation for me. And, really, with meters available for not *that* much more of much higher caliber, like the Uni-T 61E or Fluke 15B or a ton of others that excel in various disciplines, well...

[amyk]

this is why i feel most of this "safety" hype bullshit is overrated.

the meter was fine for you for 20 years and you loved it. it never detonated into a nuclear explosion and killed millions of children, but yet now you feel like all of a sudden it will.

dont get me wrong, a nice well designed meter with some protection is good, but if you dont fuck up and just use it correctly, it really dosent matter.

How else would companies like Fluke sell their really safe meters? I definitely wouldn't use this one or the infamous 830s on the type of high-energy circuits they show in their safety videos, but for low voltage low current stuff it's not really an issue.

[uski]

Hi,

but if you dont fuck up and just use it correctly, it really dosent matter.

While some people here definitely have more experience than I do, life showed me that :
- no matter how careful you are
- no matter how clever you are
- no matter how you try to stay away from people who might help you making a mistake
...you WILL make a mistake someday.

Assuming that you can avoid it is just being blind.
Trust me (or not), you will make a mistake some day.

I don't need a multimeter that totally survives in the event of such a mistake.
It doesn't matter if it smokes, makes a big bang, or doesn't work anymore afterwards.

However, what I want is a device that I know will fail safely.

When looking at the design of that multimeter, I can't say it will fail safely for the operator.
This is my definition of "safe".

Just look at this video and try to imagine that you have the meter in your hands when this happens :


I have hope that the "bullshit" "overrated" meters will behave in a more appropriate way...

Using a product for 20 years without an accident does not mean an accident will not happen tomorrow. Multimeters are electrical tools and electrical tools need to be safe.

I totally agree that using the meter has been mostly safe for my use, because yes I've used it on low power low current low voltage systems in the past. This is also why I won't be tossing it away, but I'm not going to connect this to AC mains anytime soon. I wanted to make people with similar multimeters aware of the risk they take if they do so.

uski

[sync]

Hi,
Just look at this video and try to imagine that you have the meter in your hands when this happens :

When you have that meter in your hand in that situation your are a moron. Very simple. That is a high energy source. This is an area for professionals with professional equipment only. I saw electricians working with face shielding and protective clothing on such circuits. A safe multimeter is not enough.

[uski]

Hi,
Just look at this video and try to imagine that you have the meter in your hands when this happens :

When you have that meter in your hand in that situation your are a moron. Very simple. That is a high energy source. This is an area for professionals with professional equipment only. I saw electricians working with face shielding and protective clothing on such circuits. A safe multimeter is not enough.

The main issue is education. People (like me, I agree !) are not educated about this.
And I'm a hell of a lot more educated that the average Joe that fixes his light bulb.

By creating this thread, I wanted to show the risks. Thank you for participating

However, I want to point out what you said "a safe multimeter is not enough". So, we seem to agree that you still need a safe multimeter. This is the point of this thread.

Why ? Because no matter how educated you can be, if you shut off a circuit, write "DON'T TURN IT BACK ON" on the breaker, go away make your measurement, some guy might still come up and turn the breaker back on to watch the end of the Superbowl.

You need "fail safe" tools.

[sync]

I'm also not formally educated. I working with mains voltage since I was a kid. BUT I would never working on such a high energy circuits like in the video. These are very, very dangerous. Stay away from them!
http://ecmweb.com/arc-flash/case-deadly-arc-flash


Regarding fail safe tools. Yes, multimeters and other tools should be safe. A multimeter with unfused current range is bad. But even with a "safe" Fluke you can kill yourself easily. Connect the leads to current measurement, one lead to hot and touching the other lead.... Even safe tools are not foolproof. When working with dangerous things you should know what you are doing.

[Huluvu]

I own really a lot of Multimeters, but for Mains (Household, Electrical Cabinets ...) I would always  use my trusty "Duspol"
(Similar to an Fluke T130)

It is a question about the right tool and for High Energy Circuits you often need to check if there is Energy or not (the exact value doesn't matter)

[pomonabill221]

Maybe the diodes and resistors leads being close like that form an overvoltage spark gap?  YEAH, RIGHT!

[retiredcaps]

From my understanding, the NPN transistor "Q4" will blow up if in ohms mode with 240VAC.

Well, in this recent thread, the NPN transistor was faulty when the multimeter was set to ohms mode on mains ACV.

https://www.eevblog.com/forum/testgear/tillquist-dt-447-true-rms-model-187-fine-instruments-corp/

In Dave's ohms overload test (eevblog #252), all the multimeters survived including the cheap Vichy VC99.  However, in Martin's review of the Uni-T UT90C meter, it died on mains ACV mode when it was set to HZ.  Oops.

[AG6QR]

The teardown was enlightening in showing where the problems lay.

However, without a teardown, I could have told you that the meter was unsafe for high energy circuits when I saw the socket for transistor hFE measurement, the capacitor socket, and the thermocouple socket.

As a general rule, meters that are very safe to use on mains power won't have these extra sockets.  Just try to find them on a Fluke, Agilent, Gossen, or other high end industrial meter intended for high energy circuits.  (You may find them on high quality LCR meters or similar devices that are NOT intended for mains power measurement).

When a meter's two primary leads are connected across different phases of mains voltage, then the chassis of the meter and virtually every piece of metal on the circuit board is likely to have mains potential on it, relative to ground.  Those types of sockets that allow you to easily insert component leads will also allow fingers to get too close to mains voltage.  And if a component is actually inserted in one of those sockets at the same time as the probes are both at high voltage, well...

Those component sockets indicate that the designers didn't place utmost importance on safety for high energy voltage measurements.  Maybe there's a meter somewhere that has those sockets but doesn't have any other problems with clearances, poor fuses, etc.  If so, I haven't yet seen it. 

[deth502]

Hi,
Just look at this video and try to imagine that you have the meter in your hands when this happens :

When you have that meter in your hand in that situation your are a moron. Very simple. That is a high energy source. This is an area for professionals with professional equipment only. I saw electricians working with face shielding and protective clothing on such circuits. A safe multimeter is not enough.

i think those shitty meters are great at weeding out the idiots from the gene pool. darwin at work.

[uski]

However, without a teardown, I could have told you that the meter was unsafe for high energy circuits when I saw the socket for transistor hFE measurement, the capacitor socket, and the thermocouple socket.
[...]

Thank you for that very interesting remark, I never made the link between those sockets and poor safety !

i think those shitty meters are great at weeding out the idiots from the gene pool. darwin at work.

Yea, and you think shitty cars are great because they kill people in terrible car accidents every day ?
Seriously...

[deth502]

However, without a teardown, I could have told you that the meter was unsafe for high energy circuits when I saw the socket for transistor hFE measurement, the capacitor socket, and the thermocouple socket.
[...]

Thank you for that very interesting remark, I never made the link between those sockets and poor safety !

i think those shitty meters are great at weeding out the idiots from the gene pool. darwin at work.

Yea, and you think shitty cars are great because they kill people in terrible car accidents every day ?
Seriously...

apples and oranges, boy. apples and oranges.

a failure of a faulty automotive system has nothing to do with user input. being a fucktard and putting your tongue to the 10,000v transformer secondary to see if its on, you only get what you deserve.

[uski]

a failure of a faulty automotive system has nothing to do with user input. being a fucktard and putting your tongue to the 10,000v transformer secondary to see if its on, you only get what you deserve.

I agree with you but none spoke about connecting such a multimeter on a 10,000v secondary.
However, connecting it by mistake as an amp-meter on a mains circuit, that can happen, and it's not being a fuc**ard (is it ?), when the multimeter is rated at 750VAC and supposedly protected by a fuse.

[LordKaos]

Hi!

Mine started to turn on as pleasure, no matter when I wanted...now, I won't turn on 

Anybody is having the same trouble?

Battery is ok, and the problem cames while it was in its box.

Thanks Pals.

[retiredcaps]

Battery is ok, and the problem cames while it was in its box.

How do you know the battery is ok? 

Do you have another meter to measure the battery and defective meter?

[oldway]

Hi,

but if you dont fuck up and just use it correctly, it really dosent matter.

While some people here definitely have more experience than I do, life showed me that :
- no matter how careful you are
- no matter how clever you are
- no matter how you try to stay away from people who might help you making a mistake
...you WILL make a mistake someday.

Assuming that you can avoid it is just being blind.
Trust me (or not), you will make a mistake some day.

I don't need a multimeter that totally survives in the event of such a mistake.
It doesn't matter if it smokes, makes a big bang, or doesn't work anymore afterwards.

However, what I want is a device that I know will fail safely.

When looking at the design of that multimeter, I can't say it will fail safely for the operator.
This is my definition of "safe".

Just look at this video and try to imagine that you have the meter in your hands when this happens :


I have hope that the "bullshit" "overrated" meters will behave in a more appropriate way...

Using a product for 20 years without an accident does not mean an accident will not happen tomorrow. Multimeters are electrical tools and electrical tools need to be safe.

I totally agree that using the meter has been mostly safe for my use, because yes I've used it on low power low current low voltage systems in the past. This is also why I won't be tossing it away, but I'm not going to connect this to AC mains anytime soon. I wanted to make people with similar multimeters aware of the risk they take if they do so.

uski

That's a bullshit....

If we assume that who is using a device, whatsoever it is , does not know what he's doing , we can easily show that almost everything is dangerous.

Using a hair dryer, when you get out of the shower , or worse, when you are in the bath , is certainly far more dangerous than the worst multimeters can be.

Working on electrical circuits with high energy , it is a job of specialists, electrician or power electronic engineer, not for amateurs or hobbyist at all.

The 10A input current of the multimeter in this video is connected to and powered from a high energy source . (NH fuse with breaking capacity of 100KA )

An electrician or a power electronic engineer never uses this current input of the multimeter simply because he always measures the current with a current clamp .

I used an UNFUSED Fluke 73 during 5 years to measure voltages on DC bus of high power DC drives up to 530V 5000A...

It's not dangerous at all if you know what you do...

If absurd errors are possible, we may as well show that the simple fact of having opened this cabinet is dangerous because the same idiot that connects the current inputs of his multimeter could also try to tighten the cables connections with not isolated tools and without taking all necessary precautions to ensure that these connections are not under tension and they could not be turned on accidentally.

[LordKaos]

Battery is ok, and the problem cames while it was in its box.

How do you know the battery is ok? 

Do you have another meter to measure the battery and defective meter?

Hi!

I tested three different new batteries and nothing happens. (batteries tested with another multimeter and my tonge for sure )

The strange thing is that when the fail started to appear, after some turn ons and turn offs, the multimeter turn on. I also check the power switch and it´s working ok.

Thanks

[G7PSK]

From my understanding, the NPN transistor "Q4" will blow up if in ohms mode with 240VAC.

Well, in this recent thread, the NPN transistor was faulty when the multimeter was set to ohms mode on mains ACV.

https://www.eevblog.com/forum/testgear/tillquist-dt-447-true-rms-model-187-fine-instruments-corp/

In Dave's ohms overload test (eevblog #252), all the multimeters survived including the cheap Vichy VC99.  However, in Martin's review of the Uni-T UT90C meter, it died on mains ACV mode when it was set to HZ.  Oops.

The meter blew on HZ because Martin did not read the specifications carefully enough if he had he would have noticed that on the HZ range maximum voltage rating is less than 30 volts rms, it is intended for audio work on the HZ range if you want to read the HZ from the mains or a power circuit you should set the meter to the relevant volt range and then press the HZ button.
Incorrect use whether deliberate or accidental will always get you into trouble, DMM are not really the right tool for working on high power circuits, if you look in the kit box of linesmen they have separate voltage meters and amp meters, some of them on long fibre glass poles.
I always use a clamp meter for measuring the current from gen sets and an old AVO meter for voltage testing, you do not need to worry about a few milli amps or burden voltage when there is tens or hundreds of KVA in the circuits.