Uni-t UT301B IR Thermometer - Page 1
Forum section: electronics / repair

author: vagfou
author_rank: Newbie
timestamp: November 03, 2015, 07:28:11 am
content: Hi,I have the Uni-t UT301B IR Thermometer for about 4 years now. Until some time ago it worked fine.But one day I turn on the laser and the icon apears on the screen as always. But when I push the trigerto make the measurement the display flickers once, the laser turns on and off quickly and the icon for the lasergoes off. The thermometer works fine when taking measurements without the laser.So I decided to go in. Everything seems fine. I connect it to a voltage supply and start testing.At about 9.5 volts and up the laser works fine and there is no flickering.BUT after some testing while playing with the voltage from 4 to 10.5 volts the thermometer can't measureany more. It displays OL (over limit) every time.Can anybody help with this?PS. I am not an electronics engineer

author: retiredcaps
author_rank: Super Contributor
timestamp: November 03, 2015, 06:38:46 pm
content: Can you post some clear focused pictures of the pcb (both sides) so we have an idea?

author: vagfou
author_rank: Newbie
timestamp: November 04, 2015, 01:03:59 pm
content: Here are some fotos of the thermometer and the pcbs inside.

author: sdk.61
author_rank: Contributor
timestamp: November 05, 2015, 09:25:22 pm
content: Hi , what exactly is the breakdown I don't understand

author: Maxlor
author_rank: Frequent Contributor
timestamp: November 05, 2015, 11:17:41 pm
content: Dave loves to say that the power rails are the first thing you should measure, makes sense to me. I'm guessing that when the laser is switched on, VCC gets draged down to where the device stops working properly. Can you confirm that?

author: vagfou
author_rank: Newbie
timestamp: November 06, 2015, 10:06:11 am
content: Quote from: sdk.61 on November 05, 2015, 09:25:22 pmHi, the breakdown is that while trying to figure out the laser problem something went wrong and now it can not measure. It displays only OL (over limits)Quote from: Maxlor on November 05, 2015, 11:17:41 pmThat is what I thougt first, and hooked it up the voltage supply. But there is no any large voltage drop. Only a few mV when the laser and the screen is flickering. It doesn't drop less than 9V. Other than that, I have the problem of not measuring now......

author: Maxlor
author_rank: Frequent Contributor
timestamp: November 06, 2015, 10:34:49 am
content: Quote from: vagfou on November 06, 2015, 10:06:11 amThe electronics inside the device won't run directly off the 9V battery. U7 looks like an LDO to me, measure the voltage that it is outputting.

author: vagfou
author_rank: Newbie
timestamp: November 06, 2015, 11:07:11 am
content: Tested the grey ribbon cable by moving it and by taking a continuity test. No luck.Quote from: Maxlor on November 06, 2015, 10:34:49 amThe normal output of U7 is about 5.4V and when the laser flicker drops to about 4.1V (maybe even lower, my multimeter isn't that fast). So, maybe there is the problem with the laser? But what about the OL ?

author: vagfou
author_rank: Newbie
timestamp: November 06, 2015, 11:27:29 am
content: I just power the thermometer via usb and the laser works fine without the flickering. But the OL remains. Any suggestions?

author: Maxlor
author_rank: Frequent Contributor
timestamp: November 06, 2015, 01:32:54 pm
content: Quote from: vagfou on November 06, 2015, 11:07:11 amIt's a bit hard to make it out on the pictures, but it looks like that LDO might be powering the MSP430. If so, that's a problem, because that MSP430 is a 3.3V part. This might indicate that the LDO is broken, or maybe the capacitors next to it, which might result it in oscillating (you'll have to use a scope to be sure, not just a multimeter.)It's curious that there's this HOITEK chip there btw, which is a USB-serial converter also used in older revisions of the UT61 series. As far as I can see, they use USB as power source only, not for communication, if so, why put that chip there? Maybe because it has a built-in LDO, and they had so many in stock it was cheaper than using a real LDO?

author: vagfou
author_rank: Newbie
timestamp: November 12, 2015, 07:14:53 am
content: Many thanks Maxlor for your time. And thank you all guys for the responce.I'll try to figure out where exactly the problem is when I have the time.