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Trying to repair some clamp meters

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BGAmodz

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Hi everyone .

So some days ago i broke one clamp meter due to a high voltage measurement that my device could not handle .

Now after getting a new one , the responsible of the products warranty sold me 4 clamp meters with a nice price , those ones are similar to the other but they are bigger in size and handle very high current ( 2500 A ) , but they are not working .

So i just decided to try my luck and see if i can get at least 2 of them fixed up and sell them to buy an ESR meter , i have 4 clamps ; 3 of them boot but some functions don't work .
1 measures everything but AC voltage ( iso tech )
2 cant measure anything at all except the buzzer
- the last one don't start at all .

After analyzing the one with AC voltage dysfunction , i checked all the caps , transistors , resistors , diodes / zener diodes , and they all seem working fine , no shorts or anything abnormal .
Now things get serious in checking 6 ICs that i suspect at least one of them is responsible for the AC voltage measurement problem .

Here is a list :
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
This one is the main core processor it is labeled NJU 9210 : 3.3/4 digit sigle chip digital multimeter LSI , and am suspecting is the one responsible for the Clamp problem .
A picture of the actual clamps model i got and i hope i get some opinions from you guys .

Cheers
**broken link removed****broken link removed**
 
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Without one working, I'd start with looking at In and out on the AD736 with an Ac signal present. https://www.analog.com/en/special-linear-functions/rms-to-dc-converters/ad736/products/product.html relative to COM of that IC.

What is the recommended AC signal i should apply to that AD736 ? and i guess i should apply it to the COM and IN pins ???

For the one that doesn't start, check the power supply first. I think you need +- 3 V.
I connected the power supply to it ' 9v ' battery , and still nothing , i will focus on this one later .
 
Don't apply to the AD736. Use the normal meter inputs. You should see an AC signal at the input of the IC and a DC signal at the output. The voltage should be grossly attenuated when it hits the IC.

I would suggest 24 VAC or less to the NORMAL METER AC input, just for safety sake.
 
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Don't apply to the AD736. Use the normal meter inputs. You should see an AC signal at the input of the IC and a DC signal at the output. The voltage should be grossly attenuated when it hits the IC.

I would suggest 24 VAC or less to the NORMAL METER AC input, just for safety sake.
Thanks for that , so this IC is responsible for the V-AC measurements ??
I will try to apply this tings and see if i get a signal
 
The AD736 is just a place to start without a schematic Probably because of the TRMS capability, the attenuator has to be designed differently. The AD736 takes an AC signal and converts it into an RMS DC value, so it's a good place to start when only the AC measurement doesn't work.

Unless the OP amps are acting as comparators, looking at the difference between the inverting and non-inverting inputs would be something else I would look at. Check to see if the outputs are at a rail. The difference should be less than a few mV.

We don't know what the analog switch does, but you could look create a table of the logic inputs and measuring mode, to see if their is correlation. It MIGHT route the converted AC signal to the DVM and the other signals. The 4066 is a relatively fragile device, if the supply rails are exceeded. Then you can compare the inputs and outputs using the truth table definitions. If the state changes are dynamic, then it becomes more difficult.

Nothing is certain. You can only hope to find something wrong. Once you have one working, it can be come the basis to fix the others.

One heard part is finding the "reference". The power pin for the AD736 looks like a good choice. you might try to figure out if this is connected to meter COMMON with an ohmmeter.
 
The AD736 is just a place to start without a schematic Probably because of the TRMS capability, the attenuator has to be designed differently. The AD736 takes an AC signal and converts it into an RMS DC value, so it's a good place to start when only the AC measurement doesn't work.


One heard part is finding the "reference". The power pin for the AD736 looks like a good choice. you might try to figure out if this is connected to meter COMMON with an ohmmeter.
Thats my vision too , this IC converts AC signals to DC so they can be converted to digital data , so maybe this one is faulty .

And indeed the meter's common is directly connected to the pin number 8 of the AD736 :

AD736.gif
 
The AD736 is just a place to start without a schematic Probably because of the TRMS capability, the attenuator has to be designed differently. The AD736 takes an AC signal and converts it into an RMS DC value, so it's a good place to start when only the AC measurement doesn't work.

Unless the OP amps are acting as comparators, looking at the difference between the inverting and non-inverting inputs would be something else I would look at. Check to see if the outputs are at a rail. The difference should be less than a few mV.

We don't know what the analog switch does, but you could look create a table of the logic inputs and measuring mode, to see if their is correlation. It MIGHT route the converted AC signal to the DVM and the other signals. The 4066 is a relatively fragile device, if the supply rails are exceeded. Then you can compare the inputs and outputs using the truth table definitions. If the state changes are dynamic, then it becomes more difficult.

Nothing is certain. You can only hope to find something wrong. Once you have one working, it can be come the basis to fix the others.

One heard part is finding the "reference". The power pin for the AD736 looks like a good choice. you might try to figure out if this is connected to meter COMMON with an ohmmeter.
Ok i set the meter to V AC and put under test a 24 V AC , now what are the pins i shood check in the AD736 IC ???
 
Hi again and thanks ,
I am trying to power up the meter but it seems the battery is getting low , but before i loose power i have had an AC signal of 5 VAC between COMM and IN pins , i will try to find out the output voltage after i get the meter back on the road .
 
What would be the point? - you've already been told what to do, just do it - 'playing' with a simulator isn't going to help you in any way.
Well to see how it works out of circuit , i cant get a clear idea on the IC when i simulate it ..
 
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There really isn't any point of simulation. We are trying to answer two questions:
1) is an AC voltage getting to the IC?
If it is getting to the input, then is it converting it to DC
If it's not getting to the input, then who knows where to look.

The block diagram of the IC is really all you need. It only has one needed capacitor and one optional one.
 
OK , i checked the IC input its reading is 5 Volts AC , now for the output should i check the voltage between the pin 6 and COMM ???
 
Hi again am back with results of the tests , i have checked 3 voltages 24 VAC ; 220 VAC and 6 VAC , this is what i get on the AD736 input and output :
24 V-AC measurement : Input : 4.3 V-AC ; V+ :4.3 V-DC
6 V-AC measurement : Input : 1.7 V-ac ; V+: 5.3 V.dc
220 V-AC measurement : Input : 6 V-ac ; V+: 5.3 V.dc
 
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