EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: makoulis on January 31, 2011, 11:21:57 am
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a friend of mine take my multimeter and try to masure mains AC at ohms range :o
and on all ranges the display shows like the picture below... How I can fix it??
(http://img502.imageshack.us/img502/489/2011013147817.jpg) (http://img502.imageshack.us/i/2011013147817.jpg/)
Uploaded with ImageShack.us (http://imageshack.us)
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The display looks to be showing overrange, so its possible that some component has failed to a short and is pulling the input high.
You need to draw out the circuit for the input path and follow the voltage of a 1.5V battery (for example) on the volts range as it enters the meter and passes through all the electronics before reaching the main IC. Hopefully you will find one of the components along the path that is dead and can be replaced, if its the main IC thats blown then it's not worth fixing.
You could start by checking components for any that have failed to a short-circuit state, as this can easy be checked with the component still in circuit.
Diodes can also be checked in circuit for their forward voltage drop using a DMM on the diode range, it's quite possible that a failed diode is to blame.
However, you're going to need a working multimeter to fix it, so if you only had the one meter then it's a good opportunity to get a second one. Everyone should have atleast two meters anyway.
If you could post a pic of the whole top side and bottom side of the meter we might be able to suggest a few components to check.
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Of course, the first step would be a visual inspection looking for any obvious damage like scorch marks, blown fuses (unlikely on ohms range, mind you), exploded components, lifted tracks... I see the device had been opened, so it might already have been done, so in that case, start electrically checking components as mentioned above.
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I need to see more pictures about the other ranges .
So to predict ( estimate ) if the damage could be just in some parts in the way to the master IC,
or if the main IC are damaged for good.
Also the PCB on the ranges selector , it must be checked too.
( Geia sou patrida ) :)
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here are the pictures.....
http://img203.imageshack.us/i/p1280035.jpg/ (http://img203.imageshack.us/i/p1280035.jpg/)
http://img717.imageshack.us/i/p1280036.jpg/ (http://img717.imageshack.us/i/p1280036.jpg/)
http://img843.imageshack.us/i/p1280037.jpg/ (http://img843.imageshack.us/i/p1280037.jpg/)
http://img145.imageshack.us/i/p1280038.jpg/ (http://img145.imageshack.us/i/p1280038.jpg/)
http://img209.imageshack.us/i/p1280039.jpg/ (http://img209.imageshack.us/i/p1280039.jpg/)
http://img585.imageshack.us/i/p1280040.jpg/ (http://img585.imageshack.us/i/p1280040.jpg/)
http://img218.imageshack.us/i/p1280041.jpg/ (http://img218.imageshack.us/i/p1280041.jpg/)
http://img87.imageshack.us/i/p1280043.jpg/ (http://img87.imageshack.us/i/p1280043.jpg/)
http://img141.imageshack.us/i/p1280044.jpg/ (http://img141.imageshack.us/i/p1280044.jpg/)
http://img844.imageshack.us/i/p1280046.jpg/ (http://img844.imageshack.us/i/p1280046.jpg/)
http://img829.imageshack.us/i/p1280047b.jpg/ (http://img829.imageshack.us/i/p1280047b.jpg/)
http://img585.imageshack.us/i/p1280049.jpg/ (http://img585.imageshack.us/i/p1280049.jpg/)
http://img222.imageshack.us/i/p1280051.jpg/ (http://img222.imageshack.us/i/p1280051.jpg/)
http://img822.imageshack.us/i/p1280052z.jpg/ (http://img822.imageshack.us/i/p1280052z.jpg/)
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Sadly the ADC chip is often the victim in such circumstances. If the Ohm's range protection was inadequate, a significant voltage will find it's way to the sensitive ADC input and kill the chip. From your photo's I see a hybrid thick film chip with 'ADC' written on it....
http://img829.imageshack.us/i/p1280047b.jpg/ (http://img829.imageshack.us/i/p1280047b.jpg/)
That could be the ADC stage and sadly it also appears to be a METEX custom part.
As others have advised, I would first trace the circuit from the Ohms and DC volts terminal(s) through to the ADC, and create a schematic with which to consider potential failure points and where to stimulate the ADC in order to test it's input on say the DC Volts ranges.
It would be worth checking the function of the microprocessor by changing ranges and making sure the display shows the correct units....i.e Volts = mV or V, Current = mA or A etc. If that is not working correctly, the microprocessor is not behaving normally.