Extech EX330 digital multimeter failure

[ileacristian]

Hello!

I'm a super newbie electronics enthusiast.
About a year ago I decided to buy an Extech EX330 digital multimeter after watching a bunch of Dave's videos where he said some nice things about this multimeter (not to blame him or anything  ).

At the beginning it seemed to have SOME weird readings. A restart would usually fix them. Then I took a long break from hobby electronics.
When I tested the multimeter again it acted very weird. The display would show totally weird numbers & things would appear and dissapear sort of randomly. I thought the batteries were dead and replaced them but it behaved the same way. The only thing that works reliably is the non-contact voltage detector.

Can anyone recommend any troubleshooting steps? Is this thing repairable? 

Thanks!

[rsjsouza]

I would open your meter and post some photographs here. There is a chance that either dirt/grime/grease may be causing all sorts of strange readings. There is also a possibility the contacts of the LCD with the base PCB may be failing.

After Dave's review of the Extech, several other users of this forum mentioned severe quality problems with units purchased new from this brand. Unfortunately the old review is more visible than threads here in the eevblog.

https://www.eevblog.com/forum/testgear/a-tale-of-two-meters/
https://www.eevblog.com/forum/testgear/extech-ex430-teardown/
https://www.eevblog.com/forum/testgear/extech-ex420/
https://www.eevblog.com/forum/testgear/extech-build-quality/

[ileacristian]

Sorry for the long delay. I attached some pictures of the multimeter.

I used a microscope to see better some parts that looked weird.

Here is the back of the multimeter:

The front witout the LCD:

Here is the  LCD that ... just fell off. I think it makes it's contacts through that rubber band at the top, but I wasn't able to make it work again.
It's such a weird design. You can't move the lcd to align the contacts because you need to put the case on in order to turn the damn thing on (because the dial needs to be rotated to turn it on) - and I didn't have time to see what contacts need to be  shorted to turn it on without the dial.

I saw that the main chip was looking a bit messy so here are some close-ups.
One of the pins seems to have a broken contact? The bottom pins seem to be shifted very ugly. And there is a lot of gunk to be  honest.

Top pins:

Right (top)  pins - where the broken? one seems to be

Right (bottom) pins

Bottom Pins - very shifted

Here are some caps that look very ugly. They are next to the chip.

And the brand of the caps. Chang Yang chinese brand I think:

So what do you guys think?

[rsjsouza]

Thanks for sending the photographs. It seems the main IC was subjected to a rework or it was manually soldered at the factory - there is a lot of flux residue and black crud around its pins. I strongly recommend you to clean the entire board with a good amount of isopropyl alcohol - just be careful with the buzzer.

But, before you do that, replace all the electrolytic capacitors. They look toasted.

Good luck with the repair.

[ileacristian]

Ok, so I cleaned the board, I even went with the soldering iron to clean up some of the IC mess (I don't have a hot air station yet).
I replaced all the caps as you suggested. It still behaves erratically.
Here's a video:


Any ideas on what should I do next?

[mzacharias]

First I would confirm the input impedance of the meter is around 10M Ohms, if good this suggests the input jacks and input circuit at least is OK. You can check using another meter.

I would then clean the function switch and associated circuit board contacts. Other than that - if nothing is visible bad, and given that you have already reworked the solder connections - there may not be any more you can do.

[rsjsouza]

First I would confirm the input impedance of the meter is around 10M Ohms, if good this suggests the input jacks and input circuit at least is OK. You can check using another meter.

I would do that as well, but using a different wire or probe that you are currently using - just to be sure the cables of the current probe are not the culprits. Also, check if the jack contacts are ok and not have cold solder joints. One additional detail: when you insert the probe on the jacks, check if they are snuggly fit or loose - this may be the difference between a stable and an unstable reading.

If none of this works, you would need to debug the input circuit to see if there are any faulty protection transistors, cold solder joints, intermittent contacts on the rotary switch, etc.