UNI-T UT71E failure - Page 1
Forum section: electronics / testgear

author: SerjP
author_rank: Contributor
timestamp: March 12, 2021, 09:09:26 am
content: Hi,see what happened to my UT71E after measuring the AC current of ~399mA on mA range.After 5-10 minutes of measurement the load was suddenly turned off. I'm realized it was a400mA500mA fuse which was blown. The meter was quite warm on the left side.Imagine how surprised I was when I opened the meter and found that the fuse survived. I think I was not doing anything wrong according to the documentation, measuring amperage near the end of the allowed rangeIt funny, I just bought the fluke recently as a more reliable tool, but I did not plan to destroy my UT71EReferred threaduni-t-ut71d-review(forum does not allow me to post there, so created new thread).Thank you @Wytnucls for UT71E diagram (mentioned thread 1st page), so I can find that failed element.Why it happens, what do you think guys?I was measuring the consumption of incandescent lamp connected to the mains. So I concerned what other components to check as high voltage may be applied into wrong path due to failed component.As far as I can google it the 5342b is a 6.8v Zener diode having max rating as of 700mA and 5W, but I'm not sure about the exact specs.

author: Wytnucls
author_rank: Super Contributor
timestamp: March 12, 2021, 12:18:59 pm
content: The 500mA fuse will only blow after 1 hour, if the current is close to 750mA.It is better to measure on the A scale, unless you need the mA resolution, especially if measuring for a long time, when close to the end of the scale.Hopefully, only the zener diode was affected.That applies to all multimeters.The zener diode on my 71D is SMD type

author: SerjP
author_rank: Contributor
timestamp: March 12, 2021, 01:03:57 pm
content: Quote from: Wytnucls on March 12, 2021, 12:18:59 pmI was calibrating my Chinese power meter, so high resolution was important to me. On AC 10A range both fluke and uni-t has zones of uncertainty like from 15% to 100% of range. Here is cite from uni-t manual from AC Current part of specs: "True rms are valid from 10% of range to 100% of range". From fluke: "AC conversions for Model 87 are ac coupled, true rms responding, and valid from 3 % to 100 % of range, except 400 mA range (5 % to 100 % of range) and 10 A range (15 % to 100 % or range)".So measuring 400mA is a 4% out of 10A range. I was trying to avoid that additional errors.Thanks for the fuses info!

author: SerjP
author_rank: Contributor
timestamp: March 12, 2021, 01:25:19 pm
content: I just figured out what could probably go wrong with my setup below:~ ------------ ~I was using two meters in a row to double check values. But later I turned off the uni-t and go on with fluke only. I mean turned the uni-t switch to OFF position, while did not disconnect the leads.I was sure it should not be dangerous for the meter in Current measurement mode to turn off the meter while leaving the load connected.But now looking into schematic I can see that in OFF position the measurement current does not go through sense resistive network any more. But instead current goes through that zenner diode. Hmmm...Not sure If I read the scheme right though. But if it is that probably can be the reason for the temperature raise. 6.8v zenner multiply by 0.4A gives 2.72W of power dissipation here.Please check the comparison with fluke schematic. From first glance it looks the same, but there is critical difference here. Check the polarity of zenner comparing to rectifier.

author: Fungus
author_rank: Super Contributor
timestamp: March 12, 2021, 01:30:44 pm
content: Have you tested the diode? Are you sure it's not just desoldered?Measuring current near the limits of a fusewillheat it up. It's not recommended for long periods. Use the 10A range instead,The other option is to get a 0.1Ohm resistor and put that in series with the power then measure the voltage across the resistor.https://www.aliexpress.com/wholesale?SearchText=0.1+ohm+1%25+Wirewound+Aluminum+Resistor(or 1Ohm, or 0.01Ohm, depending on your application)

author: SerjP
author_rank: Contributor
timestamp: March 12, 2021, 01:46:26 pm
content: Quote from: Fungus on March 12, 2021, 01:30:44 pmYes, checked Zenner and it looks good: forward bias is 0.65v and reverse bias is OL. Will later connect power supply to check actual reverse bias.So it just desoldered.Indirect current measurement via voltage drop over resistor is possible, but it'll introduce one more error. I don't see a good reasons for making my measurements more complicated and hunting that additional error while tool allows me to perform direct measurements (I know, I know we anyway measure voltage drop on resistor inside the meter, but it is hi-spec resistor and calibrated meter).I already seen that Daves video on fuses. Was shocked by the temperature and voltage drop, lol. That actually why the first thing come into my mind was - ooops, my fuse is blown or even drop out from the board due to hi temp.

author: Wytnucls
author_rank: Super Contributor
timestamp: March 12, 2021, 02:28:18 pm
content: Quote from: SerjP on March 12, 2021, 01:03:57 pmThat shortcoming is mentioned in my review.The meter should have had a 4A range also, like more expensive multimeters.

author: rsjsouza
author_rank: Super Contributor
timestamp: March 12, 2021, 02:49:18 pm
content: Quote from: SerjP on March 12, 2021, 01:46:26 pmI wonder if the zener may have warmed enough to melt the solder on its terminals...Looking at the circuit and the measurement mode, when the sense resistor is opened, a much higher impedance path through the zener will increase the voltage across its terminals. Since you are measuring mains, the zener will drop down its own Zener voltage (6,2V) and, depending on the load, will have a significant current flowing through it.By the way, just like you I also learned from a cheaper meter (Richmeters RM102Pro / Aneng AN113) to not turn off the meter with the probes plugged. In this particular case, the V/Ω/mA input is shorted via its fuse when in off position.

author: SerjP
author_rank: Contributor
timestamp: March 12, 2021, 06:01:50 pm
content: Quote from: rsjsouza on March 12, 2021, 02:49:18 pmYep. It was warmed enough to drop out from the board. I even can see the light radial waves of deformation of the meter plastic case around that diode (diode located 2-3mm close to the case).2.5W of dissipated power looks not so much to warm up board to 250 degrees C. But we can see results from the photo. The load was a 60W or 100W bulb I think. Fluke shown me the 399-405mA. No way current was higher than that, otherwise 0.5A fuse should be blown.Quote from: rsjsouza on March 12, 2021, 02:49:18 pmThanks for confirming that!I checked that 5342b reverse bias with power supply and limiting resistor. And it is definitely a 6.6v Zenner which wasn't damaged. So meter can be fixed easily. Nice!Quote from: rsjsouza on March 12, 2021, 02:49:18 pmWow! That probably was a more loud experiment when your fuse was blown up. Any fireworks?

author: SerjP
author_rank: Contributor
timestamp: March 12, 2021, 06:24:05 pm
content: Quote from: SerjP on March 12, 2021, 06:01:50 pmWell, not so easy to remove the PCB. Need to desolder the small PCB on the top of the input jacks. Input contacts are melted into plasticI want to take out PCB not only to solder the Zenner back but also to check rotary switch and contacts for wear.

author: Fungus
author_rank: Super Contributor
timestamp: March 12, 2021, 06:27:01 pm
content: No need to remove the PCB, push a piece of component leg through the hole where the diode is supposed to go, solder it, attach the diode to the other end.

author: rsjsouza
author_rank: Super Contributor
timestamp: March 12, 2021, 09:59:14 pm
content: Quote from: SerjP on March 12, 2021, 06:01:50 pmYes, 2.5W on such small part is quite a lot of power.Quote from: SerjP on March 12, 2021, 06:01:50 pmNo, it was very uneventful as it was tied to a low voltage regulated 5A power supply - I only realized this days later when I put it in mA mode.

author: SerjP
author_rank: Contributor
timestamp: March 18, 2021, 11:04:00 am
content: Soldered Zener back in place. And meter came back to life.Also tried to desolder the small PCB to check the selector switch wear level. Unfortunately did not get success in it (see attachment)Actually found the same attempt and results on Youtube (link with time code):Finally was able to straighten the input jacks, but it took me couple of hours. So I decided not to look into the selector switch to avoid the risk of completely destroying the meter.DMM accuracy looks the same as before the issue with Zener. Happy with that.PS: forgot to mention that I found one more component failed due to mentioned problem with Zener. It was 'R105 1k' which is located within FREQ signal path. I suppose it limits the current to U1 ES51966F during frequency measurement. The resistor was open. Replaced it. Don ask how I found it. I'm just 'cherry picking' components on the board with another multimeter, measuring them to try to check their tolerance. And fortunately found broken resistor.