# Trying to repair some clamp meters

Discussion in 'Repairing Electronics' started by BGAmodz, Sep 25, 2013.

1. ### KeepItSimpleStupidWell-Known MemberMost Helpful Member

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I was looking at the datasheet last night. If it goes back to the COM terminal, then it's likely the input is differential. So, those 0.4 VDC values are probably right. High value probably occurs when it's overloaded. Basically a drop across the 10 M ohm resistor.

For now, no more measurements in autorange. And lest look at 6 VAC and Zero VAC for an input. Except for Zero VAC short the meter input terminals. Lets now call them 0V (open) and 0V (Short), )

Measuring a 10 M resistor is going to depend on your meter. A meter with a 10 M input Z in parallel with another 10 M resistor results in 5 M ohms. You have to know your meter. Might be a combination of capacitance in your meter and your meters input Z. 10 M || 10 M is 5 M

Drop the Mega Ohms. It's megohms.

A resistor marked 1001 may be a precision resistor of 1000 ohms. You said you had other meters. Compare the resistance of that part in the other meters.

So, looks like we have been barking up the wrong tree a bit. Sorry about that.

===

So three paths:
a) Compare the 1001 resistor value with another meter

b) If the meter is reading Overload all of the time with no input: That answer was never posted. Try replacing/paralleling the 1001 resistor with a 1K with NO VOLTAGE applied and see if the meter reads zero.

c) More measurements

Post a link to a manual.

Let's look at pin CC (VAC, VDC) and Vin (VAC, VDC) relative to IC COM for the next series of measurements, and Vout (DCV) with 0V (s), 0 V (opn) and 6 VAC applied to the meter. Ignore autorange.

Don't do any measurements on any scale that would cause the meter to be in overload. Always start from the highest range first.

I have NO IDEA if we will be successful or not.

Last edited: Oct 1, 2013
2. ### BGAmodzMember

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Thanks KIS , am really appreciating your help and your time .

I will do the tests and get back to you .

3. ### BGAmodzMember

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Hey mate , i don't know if this is good news or bad news but , i just tried to unsolder that 1001 resistor , and it just broke , i guess it was already fragile enough to break up easily .

Any ideas ???

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5. ### Nigel GoodwinSuper ModeratorMost Helpful Member

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SM components are really fragile, and easily broken - they are also glued to the board as well as soldered.

6. ### BGAmodzMember

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Yes but the component unsoldered was a a resistance of 1001 ( 1 kilohm ) , i wasn't reading that on my meter before i remove it , and i noticed that this resistance was already broken when i applied heat on its pads , one half just moved easily , haven't experienced the same thing with previous ones they were hard to remove .

7. ### BGAmodzMember

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Hi again , KIS

I changed that resistor by a new one but i couldn find it in SMD so i just chose a cylindrical one ( don't think its a issue ) .

Now i notice some changes ; So always on autorange , i see the reading on screeen going from 0.5xx to 0.4xxx as soon as i connect the brobes to the 24 V AC voltage , i guess the resistor is doing its job right now , but still no measurement .

8. ### KeepItSimpleStupidWell-Known MemberMost Helpful Member

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I said forget about autorange. If I understand correctly, the meter reads something (Zero?) rather than overload with nothing connected or shorted.

Did you have another meter with the same resistor in in that you could measure? That resistor's power disipation is crucial to maintaining the meter's protection since it is in series with COM of the meter. It's value is probably critical too, but not at this stage. Your probably not used to the 1001 markings?

More concerned about non-auto range at the moment.

The input of the RMS chip might not rise like it did before.

So we have a hint that the AC range was subject to some major stress. My lines of thinking are, that the CC input is buffered by an OP amp like the application notes suggest. A DC voltage at the CC input might suggest an op amp failure if there is one. With a schematic, it;s like the blind leading the blind.

So, let's find out what's happening at the CC and Vin inputs, both AC and DC wise.

The general rule is to bisect a circuit, but since we don't know what we are dealing with the RMS chip and it's datasheet is the best place to start and you have to keep the meter out of overload.

The RMS chip can read DC+AC, but I doubt very much the meter supports that mode. Again, no link to the manual.

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10. ### KeepItSimpleStupidWell-Known MemberMost Helpful Member

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Resistor: Don't change it. Just verify that it is a 1K in the other meter and stay away from any ranges that would cause an overload with an input voltage.

11. ### BGAmodzMember

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OK i will proceed then

12. ### BGAmodzMember

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Ok here is the results in the highest range :

Open

Vin : 0.5 v AC ; 0.4 v DC
Vout : 4.3 v AC ; 2.25 v DC
Cc : 3 v AC ; 1.64 v DC

Short

Vin : 0.5 v AC ; 0.4 v DC
Vout : 4 v AC ; 1.9 v DC
Cc : 3 v AC ; 1.64 v DC

6 Vac

Vin : 0.5 v AC ; 0.4 v DC
Vout : 3 v AC ; 1.5 v DC
Cc : 3 v AC ; 1.6 v DC

Am trying to figure out the relation between the IC and an OpAmp IC just beneath it . 27L2C

13. ### KeepItSimpleStupidWell-Known MemberMost Helpful Member

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best idea. Follow CC. Other thought is the connection is Open and your picking up AC nearby. Readings don't make a lot of sense.

14. ### BGAmodzMember

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Hi , i am drawing allthe connections of that IC and i have a path from an operational amplificator .

Do you have some tips on how to check that OpAmp
??

15. ### KeepItSimpleStupidWell-Known MemberMost Helpful Member

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The general tip is that the difference of the inverting and non inverting inputs should be a few mV of each other (ideally zero) if it's used as an amplifier.

In any event, you have to look at the inverting, non-inverting inputs and the output relative to power common. Probably with a zero and some input. AC and DC values may be important.

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17. ### KeepItSimpleStupidWell-Known MemberMost Helpful Member

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Totally confusing, If the OP amps output whent to the CAP at CC, I could make a lot more sense.

If you can verify the OP amp connections.

And let's pretend that we don;t know where it goes/comes from.

Verify common. I think it would be the the DVM chip and I think it would be connected to COM on the meter.

Use the 200 V scale. Apply 6 VAC, and 0 V via a short to the inputs.

Look at the AC and DC voltages at inverting/non-inverting and output of the OP amp.

For fun and giggles, you could look at the above for ALL of the OP amps,

I'm grasping at straws at this point,

18. ### BGAmodzMember

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My guess is that NJU 9210 microcontroller is faulty , but the problem is that there is no supplier who sells it , its a very rare IC .

Whats a DVM chip ??

here is the ICs list from my clamp meter :
NJM022B : dual low power operational amplifier
TS27L2C : Precision very low power cmos dual amplifiers
OP07 : Ultralow offset voltage operational amplifier
AD736 : Low power , true RMS to DC converter
AIC1578 : High efficiency step down DC/DC converter
HEF4066B : quadruple bilateral switches
NJU 9210 : 3.3/4 digit sigle chip digital multimeter LSI

19. ### KeepItSimpleStupidWell-Known MemberMost Helpful Member

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NJU 9210; Digital Multimeter - or DVM Digital Voltmeter. DMM is digital MultiMeter. DMM is proper term.

The NJU9210: http://www.alibaba.com/product-gs/586183052/_IC_NJU9210.html

Quantity 1.

Quickly poke around (DC and AC voltages) at the inverting/non-inverting and outputs of the OP amps with 6 VAC applied on the highest AC range.

You might also probe the 4066 IN/OUT (AC and DC) and other ins just DC.

20. ### BGAmodzMember

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Thanks for the link .
I will order this IC and see if the unit works on AC voltage test , if it doesn't work time to check the others one by one .

21. ### BGAmodzMember

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Hi again am back on topic , please is there a way to make a variable ac input from 0 v to 5 v AC using trimmer ??