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UT210E Show wrong Resistance Readings (OHMS)

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sawmavarte:
My UT210E Clamp meter is not showing correct readings for resistance mode as the value show keeps on flactuation with random vlaues. I wonder is some components could have gone bad and cause this issue. I have attached some images and schematic for this perticular model for reference. Pls help im a beginner.

Paul T:
Mine has recently developed a similar problem.  By default, the unit reads between 1 and 15 Megohms on resistance and nothing on the input jacks.  When it's on the low side, capacitance values are measured higher than they typically do.  The stray resistance never completely goes away.

If I measure a higher value resistor, it comes in low.  I did the calculations, it's as though the errant resistance is in parallel with the resistor I connected.

It's like something is going short, might be inside the board or the CPU. I've tried...
Remove the MOV's, check function, re-install
Remove the clamping transistors SS8050 (Y1), check function, re-install
Remove the input 5pF capacitors, check function, re-install
Check the large 1 nF grounding capacitor,
Check the range selector tracks and springs,
Clean between all CPU leads with a needle,
Inspect the entire PCB and all solder joints with a microscope,
Apply high voltage to the input jacks and use a thermal imaging camera to detect heat (none found)
Checked the isolation resistance of the input jack holder,
Removed and cleaned the entire board in ultrasonic IPA and blown with high pressure air.
This last step I did because the resistance shunting the input seems to be sensitive to humidity.

Still looking for a fix. Meanwhile, I bought a WinAPEX 268B as a stand-in in case any other functions start to go, especially the all-important DC Amperes.

Like you, looking for some help from the experts here.  I'm usually good at fixing things, but I'm at my wits-end here.  I'm glad I found someone else who might be dealing with the same thing   :-BROKE

DavidAlfa:
Had this in mine,  the only thing that worked was to give it a deep clean with IPA and brushing.
My fault, as I had modded the extremely loud beeping circuit, adding a large resistor to the driving transistor so it would't drive so strongly.
Apparently some flux leftovers caused a weird/phantom resistance measurement issues two years later.

Even if you did nothing to it, a deep cleaning won't hurt!

Gyro:

--- Quote from: DavidAlfa on August 29, 2024, 04:11:10 am ---...
Apparently some flux leftovers caused a weird/phantom resistance measurement issues two years later.

Even if you did nothing to it, a deep cleaning won't hurt!

--- End quote ---

That's the beauty of no-clean flux.  ::)

Edit: Actually it might also apply to incompletely removed water soluble flux, which will naturally be hygroscopic.

Paul T:
So if I put the unit on the output of the dehumidifier, the "leak" resistance slowly goes up to a higher megohm value and stays that way for a little while.  The time it takes to change and other experiments I have done tell me it's not surface ionic contamination.

@DavidAlfa , are you sure the problem came from your resistor change? It's a large distance from the input path and I assume you would have cleaned up your flux sufficiently. ;)  For me, aggressive cleaning did not help.

I suspect this is a PCB or component defect/weakness that takes some time to show up.  Hard to believe it would be CAF because it happened after sitting, not after use with high voltage.

What components will be affected by increases in humidity? 

MLCC? 
Tantalum capacitor?
SMD Resistors?
The CPU itself?

I also had the thought that one of the reference resistors (on front and back of the board) is wandering from it's original value.

Currently when I turn the device to R, the resistance is 1.3 megohms and settles to just under 1.1 megohms in what appears to be a capacitor charging curve.

Perhaps we should compare DMM manufacture dates?

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