UNI-T UT81C multimeter error not displayed - Page 1
Forum section: electronics / repair

author: dungntvn
author_rank: Newbie
timestamp: March 26, 2025, 06:17:05 am
content: Hello my friends.I have a UNI-T UT81C multimeter that currently has a power failure and the screen does not turn on. I have checked the device and it currently has an IC error. Can you tell me where I can buy this brand's microprocessor IC?

author: tunk
author_rank: Super Contributor
timestamp: March 26, 2025, 11:11:44 am
content: You may be more likely to get answers if you post a photo.

author: ondraN
author_rank: Regular Contributor
timestamp: March 26, 2025, 11:28:00 am
content: You might buy a CPU, but the problem will be with the firmware. It is impossible to read it from the CPU even if it is fine (locked) and it cannot be obtained anywhere.

author: tunk
author_rank: Super Contributor
timestamp: March 26, 2025, 11:57:07 am
content: The firmware could be installed in an eeprom.There's also an old thread here:https://www.eevblog.com/forum/testgear/uni-t-ut81c-scopemeter-teardownreview/

author: Zenith
author_rank: Frequent Contributor
timestamp: March 26, 2025, 12:08:20 pm
content: I am not familiar with this item at all, but what makes you believe it's a faulty microprocessor, rather than something else, such as a bad connection or short caused by debris? I recently fixed a problem with another UNI-T meter caused by a leaky electrolytic capacitor. It prevented it from powering up properly. I had a circuit diagram to work with. Is there any maintenance documentation for the 81C? Without it you are very much proceeding by guesswork.

author: dungntvn
author_rank: Newbie
timestamp: March 26, 2025, 02:36:30 pm
content: Hi Tunk!I have attached the file but for some reason it doesn't go with the title of the post I posted.It just shows a black line when I turn it off and on again

author: dungntvn
author_rank: Newbie
timestamp: March 26, 2025, 02:40:45 pm
content: hi ondraNI contacted UNI-T directly but they didn't provide me with the programmed chip, it's too bad for me that they refused to service the very product I bought from them

author: dungntvn
author_rank: Newbie
timestamp: March 26, 2025, 02:44:49 pm
content: hi zenithi have checked the power supply for the processor, all have full input power but the processor cannot output clock, this brand is used a lot in my country because of its cheap price but it is terrible that the company ignores the warranty for their own products in my country, if they are broken, they are thrown away

author: tunk
author_rank: Super Contributor
timestamp: March 26, 2025, 02:51:35 pm
content: I was thinking about a photo of the microcontroller.

author: dungntvn
author_rank: Newbie
timestamp: March 26, 2025, 02:54:23 pm
content: UT81C microcontroller photo here tunk

author: tunk
author_rank: Super Contributor
timestamp: March 26, 2025, 03:43:07 pm
content: The chip marked NXP LPC2138 could be the microcontroller:https://www.nxp.com/docs/en/data-sheet/LPC2131_32_34_36_38.pdfLooks like it has built in flash memory, so if it's dead, youcould be out of luck. Look and see if there's any eepromsnear it. But as Zenith suggested, it could be something else.

author: Zenith
author_rank: Frequent Contributor
timestamp: March 26, 2025, 04:39:31 pm
content: Quote from: dungntvn on March 26, 2025, 02:44:49 pmIf it has failed under warranty I would have thought you would have a claim against whoever you bought it from.Unfortunately most of these low-end, mass produced products are not intended to be repaired. There's very likely no technical documentation for them which has been published. If you can't even find a schematic, it makes fault finding very difficult.If they fail, it's worth opening them up and having a look. Faulty connections, bad switches and other elementary faults cause most problems. I've found that ICs rarely fail. There's only so much effort it's worth investing.QuoteI assume this is the usual arrangement where the processor runs a crystal oscillator or resonator - it can also be an RC oscillator. Sometimes a crystal oscillator module is used. I've never come across that feature of a processor failing. I'd check the soldering, the tracks and the crystal or resonator. You could also try injecting a signal from another source. As for changing the processor as a guess, it depends on how cheaply you can find the processor, and how confident you are you can remove the old one and solder in the new one. If it has onboard ROM of some sort, it may need a programmed processor only available within UNI-T.

author: dungntvn
author_rank: Newbie
timestamp: March 27, 2025, 02:41:30 am
content: hi tunki checked the product very carefully there is no eeprom on both boards, this line of flash memory is integrated into the processor

author: dungntvn
author_rank: Newbie
timestamp: March 27, 2025, 02:49:53 am
content: hi zenithi reported to the seller and uni-t and got the answer that there is no warranty station in my country and they can't do anything elsei have removed the entire machine inside and have checked the external oscillator crystals are still working the power supply to the microprocessor is still enough voltage uni-t's DMM line does not have an isolator between the microprocessor chip and the measurement processing chip like fluke lines all have so the chip is very easy to die when the input signal receiving pins are punctured by external agents

author: Teledog
author_rank: Regular Contributor
timestamp: March 27, 2025, 02:50:18 am
content: I've had problems with a "dead" Uni-T meter display.Turned out it was the LCD/zebra strip connection.Took it out, cleaned the glass/strip with IPA , reassembled & all was well.G'luck!

author: dungntvn
author_rank: Newbie
timestamp: March 27, 2025, 02:54:00 am
content: hi teledogthis line has an external communication port if mine has a broken LCD connection then I connect the external communication port of UT81C to the laptop it should display but in fact it reports an error that cannot be connected and says to check again

author: darkspr1te
author_rank: Frequent Contributor
timestamp: March 27, 2025, 05:28:56 am
content: If this model has the same issues my 81B I would return it under warranty. My unit killed the CPU 3 times in a row same issue each time.The device's "universal" DMM ic keeps blowing, normally between the DMM ic and the cpu that does all the display.i found the FNRI dmm scope to be way better.darkspr1te

author: dungntvn
author_rank: Newbie
timestamp: March 27, 2025, 05:44:15 am
content: This line of non-isolated microcontrollers has an old circuit design that is very easy to dieMy machine is the same line as the 81B, only the frequency bandwidth is widerHow can you return it under warranty? I sent it for warranty and paid a fee, but the company did not accept it and replied that it was an old model and they did not accept it anymore.

author: Zenith
author_rank: Frequent Contributor
timestamp: March 27, 2025, 01:29:26 pm
content: A warranty operates over a period, usually a year, possibly longer, starting from when you bought the item. If it fails in that period the seller is under an obligation to replace or repair it. It's a good idea to keep the receipts for things you buy. If it fails after the warranty period, that's the buyer's problem.It sounds as if you sent it for repair and the seller, or UNI-T, refused to do anything with it because it was too old. Most of the better makers will repair items that are outside the warranty period, at a price. It usually only applies to more expensive items. They may do that for items they no longer sell, up to a few years after they stopped selling them.It would be a good idea to check by email, whether whoever you want to repair an item, is willing to repair it, before sending it off and parting with money. If it's an old item they are no longer prepared to accept for repair, they should know that from the start and be able to tell you.