UT71E Dead or something else!

[Mustapha.Othman]

Hello there,

Am a beginner in electronics so cheer me up   

Anyway my issue is my multimeter uni-t 71e is dead yesterday, i was doing an experiment on it trying to power it on a rechargeable battery with 3.8v and boosting it with a module for an 9v, what happen is the boost module have fail and gave an 37v for the ut71e after that it just run but giving 0.00000 on the voltage result or any other measurement, So right now if anyone can help me to troubleshooting where do i get start or what should i check first

screenshot:
https://jmp.sh/dz9WGog


https://jmp.sh/vYf2rOw


Thanks in advance

[krolu]

First of all try to find where do traces go from one of the input terminal, the best would be the most right terminal V/Ohms. It should go somewhere to the A/D converter. Apply some voltage on the input and check if you have some voltage on the input for the A/D converter. If there is none than maybe there is some part that is on the way that blow up. If it's there than its probably the ADC.
That's the start I think.

[coromonadalix]

Thread here with schematics, since Voltcraft vc940 is a copy

https://www.eevblog.com/forum/repair/adjustment-for-uni-t-ut71e/

[JFJ]

... the boost module have fail and gave an 37v for the ut71e after that it just run but giving 0.00000 on the voltage result or any other measurement...

From your pictures, the display and main selector switch appear to be behaving correctly. That suggests that the display controller IC (U9 M430FE425) and the I/O processor (U2 M430F149) survived the supply voltage being raised to 37V. The DMM IC (U1 ES51966F), however, may not have survived (it uses separate input pins for different types of measurement, but now all measurement inputs appear to read zero).

A good way to start your troubleshooting would be to check the supply voltage to U1. It should be 5.5V and can be read between V+ (pins 62 & 63) and V- (pins 57 & 58) on U1. If you're lucky, the pre-regulator transistor (Q12) or the voltage regulator IC (U7 7201U55) that supplies the 5.5V, might have died and protected U1.

[Mustapha.Othman]

First of all try to find where do traces go from one of the input terminal, the best would be the most right terminal V/Ohms. It should go somewhere to the A/D converter. Apply some voltage on the input and check if you have some voltage on the input for the A/D converter. If there is none than maybe there is some part that is on the way that blow up. If it's there than its probably the ADC.
That's the start I think.

Hey @krolu Thanks for this, I have done what u suggestion i inject an 9v to the voltage input i traced down the tracks to the ADC terminal pin #29 "VR1" it's giving 0.716v from 9v input

I also checked the V+,V- pin 62,63+57,58 it's giving 5.2v

screenshot:
https://jmp.sh/2XTLQF1

Now my Q is how do i verify the U1 is working normally i mean the measurement!

[Mustapha.Othman]

From your pictures, the display and main selector switch appear to be behaving correctly. That suggests that the display controller IC (U9 M430FE425) and the I/O processor (U2 M430F149) survived the supply voltage being raised to 37V. The DMM IC (U1 ES51966F), however, may not have survived (it uses separate input pins for different types of measurement, but now all measurement inputs appear to read zero).

A good way to start your troubleshooting would be to check the supply voltage to U1. It should be 5.5V and can be read between V+ (pins 62 & 63) and V- (pins 57 & 58) on U1. If you're lucky, the pre-regulator transistor (Q12) or the voltage regulator IC (U7 7201U55) that supplies the 5.5V, might have died and protected U1.

Hey @JFJ, indeed all the measure recall function is working and display normal but not the measurement inputs they all display as a zero's even the diode mode what it should be showing O.L is showing zero's as well so my brain is going to U1!

i guess the U1 is having his juice! with 5.2v in the pin's u mention, but i didn't measure The transistors or the regulators yet, if they are working normally too what should be the next step !

if someone can help me to figure out if the U1 is measurement output is normal or it's functional working with How-to to heads up i'll be more appreciate   

[JFJ]

... i inject an 9v to the voltage input i traced down the tracks to the ADC terminal pin #29 "VR1" it's giving 0.716v from 9v input...

Something must be sinking a lot of current to cause most of the applied 9V to be dropped across the short chain of low value input resistors. The input impedance of U1's VR1 should be around 10MΩ, but I can't see anything else on the schematic that could be the source of the short circuit.

For me, the easiest way of testing if U1 is the culprit, would be to remove it (using something like ChipQuick). If a repeated test, with U1 removed, found 9V present at U1's VR1 pad, then you would know for certain that U1 is defective.

[Mustapha.Othman]

Something must be sinking a lot of current to cause most of the applied 9V to be dropped across the short chain of low value input resistors. The input impedance of U1's VR1 should be around 10MΩ, but I can't see anything else on the schematic that could be the source of the short circuit.

Yes indeed, and to be insure i measure the resist from the input to the pin VR1 is giving 10M\$\Omega\$

For me, the easiest way of testing if U1 is the culprit, would be to remove it (using something like ChipQuick). If a repeated test, with U1 removed, found 9V present at U1's VR1 pad, then you would know for certain that U1 is defective.

Buddy we have a winner  i removed the U1 chip i inject again 9v to the input i measure it out and i found a 4.72v on the VR1 pin

i guess i'll go with a new chip for U1 ES51966P

https://www.ebay.com/itm/1PCS-New-Manufacturer-CYRUSTEK-MPN-ES51966-ES51966P-Encapsulation-QFP/312195598167?hash=item48b04ec757:g:2csAAOSwQrJbUuQD

Please let me know if i should check anything else! i already confirmed the regulator and the resistor's which on the power for those U's are working normally and giving the what voltage should be

[JFJ]

... i removed the U1 chip i inject again 9v to the input i measure it out and i found a 4.72v on the VR1 pin ...

That voltage is what you would expect, when you take into account the loading effect of the meter used for the measurement. I forgot about that, in my previous post, when I said "If a repeated test, with U1 removed, found 9V present at U1's VR1 pad...". Also, I don't know why I prefixed "input resistors" with "low value" (I must have been half asleep).

Please let me know if i should check anything else!

I cannot think of anything else to check before a new ES51966 is fitted. If the new chip fixes your UT71E, then the meter will, of course, need a full re-calibration.