UT210E Show wrong Resistance Readings (OHMS) - Page 1
Forum section: electronics / testgear

author: sawmavarte
author_rank: Newbie
timestamp: July 01, 2024, 03:38:53 pm
content: 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.

author: Paul T
author_rank: Regular Contributor
timestamp: August 29, 2024, 03:17:09 am
content: 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-installRemove the clamping transistors SS8050 (Y1), check function, re-installRemove the input 5pF capacitors, check function, re-installCheck 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

author: DavidAlfa
author_rank: Super Contributor
timestamp: August 29, 2024, 04:11:10 am
content: 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!

author: Gyro
author_rank: Super Contributor
timestamp: August 29, 2024, 08:40:49 am
content: Quote from: DavidAlfa on August 29, 2024, 04:11:10 amThat's the beauty of no-clean flux.Edit: Actually it might also apply to incompletely removed water soluble flux, which will naturally be hygroscopic.

author: Paul T
author_rank: Regular Contributor
timestamp: August 29, 2024, 10:18:01 am
content: 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?

author: DavidAlfa
author_rank: Super Contributor
timestamp: August 29, 2024, 02:50:39 pm
content: I think I actually caused the issue when cleaning it in first place, as it was in pretty isolated area, just a 0603 resistor far away from the analog parts, it wouldn't have affected anything.Probably the alcohol diluted it, making it runny and getting under capacitors, resistors, ics... then it dried and the moisture did the rest over time, I recall seeing the readings changing when breathing on the pcb.Then I started poking with my fingers until I found the sensitive zones, flooded with alcohol and started brushing hysterically, probably the pcb is thinner in that area nowPerhabs a barely conductive path between an analog input and VDD, just microamps... but it was enough. No corrosion whatsoever, it's been fine for months now.Is it happening in all resistance scales?I rememeber about 15 years ago, I had a very cheap DMM.Before measuring resistance I had to compulsively rotate the selector back and forth to clean the contacts, it was so cheap that they weren't gold plated, just bare copper, so they oxidized very easily.

author: Paul T
author_rank: Regular Contributor
timestamp: August 29, 2024, 04:57:17 pm
content: Quote from: DavidAlfa on August 29, 2024, 02:50:39 pmThis device has a selector wheel to engage the resistance, capacitance, diode, continuity. From there, you select the function you want using a press button, cycling through the modes. There is no range button, so all you can do is set it to resistance and let it auto-range.The selctor wheel has gold plated pads and the typical dual-V-spring forrmed from a rectangle. There isn't anything flaky going on here.Based on your recipe for success, I'm going to get more aggressive, perhaps using the finest bristle toothbrush and a longer time in the IPA tank. Hopefully there isn't ionic contamination trapped under the soldermask that I can't get to.

author: Paul T
author_rank: Regular Contributor
timestamp: August 29, 2024, 09:22:31 pm
content: Well that extra bit of alcohol scrubbing seems to have done the trick! I turn on the multimeter and the R mode displays "OL" once again.Woo-hoo!My Uni-T Unit isn't a goner! Thanks for the helpful suggestions and encouragement.