Fault finding - Page 1
Forum section: electronics / beginners

author: rthorntn
author_rank: Frequent Contributor
timestamp: June 20, 2013, 02:57:23 am
content: G'day All,I have a DECT phone that worked for a while and now it won't power on (there were some batteries in it that had leaked on to the contacts), I opened it up today and this is what I found (warning 4MB file):https://dl.dropboxusercontent.com/u/2436853/20130620_124855.jpgI see nothing obvious, can anyone point me in the right direction on how to troubleshoot it, I have a digital oscilloscope and a multimeter?Thanks.Richard

author: c4757p
author_rank: Super Contributor
timestamp: June 20, 2013, 03:04:05 am
content: I see what looks suspiciously like a cut trace at the top of the battery contacts, but it's hard to tell with the photo. I'd check there.OT, but I wish cameras had a feature to decrease the image size automatically when the focus isn't good enough to justify the resolution.

author: rthorntn
author_rank: Frequent Contributor
timestamp: June 20, 2013, 03:41:52 am
content: Thanks c4757p, apologies on the photo size is from a SGS4 stupid MP count, cut trace, what am I looking for there, is it the contacts to the left or right of the picture, is the cut on the PCB?Anybody any idea what the little components are that look like they have a green bottle on them, to see it on the image there are four of them one on each side of the MCU, are they re-settable fuses?CheersRichard

author: MacAttak
author_rank: Unknown
timestamp: June 20, 2013, 03:42:54 am
content: It's clearly upside-down, and therefore all of the electrons have fallen outSorry, I've watched one too many EEVBlog YT vids.

author: c4757p
author_rank: Super Contributor
timestamp: June 20, 2013, 03:45:48 am
content: Left of the picture, on the PCB directly at the upper right corner of the contact assembly.

author: MacAttak
author_rank: Unknown
timestamp: June 20, 2013, 03:49:59 am
content: Quote from: rthorntn on June 20, 2013, 03:41:52 amHe is talking about the scratch on the green PCB you can see in center of this part of the board. Notice how the scratch appears to cross a faint lightly-colored trace that seems to lead from that tiny brown capacitor to something else (maybe the battery contact clip itself).

author: c4757p
author_rank: Super Contributor
timestamp: June 20, 2013, 03:52:06 am
content: Thanks - my computer's messed up and I didn't have a familiar image editor handy.

author: rthorntn
author_rank: Frequent Contributor
timestamp: June 20, 2013, 03:55:30 am
content: Thanks, it was battery acid gunk, a bit of IPA appears to have taken it off, do you think I should cable my bench psu to the contacts to get steady power?

author: Yago
author_rank: Frequent Contributor
timestamp: June 20, 2013, 11:22:28 am
content: Quote from: c4757p on June 20, 2013, 03:04:05 amSimpler to just achieve focus before pushing the button !On topic , yes bell out that track to start .

author: rthorntn
author_rank: Frequent Contributor
timestamp: June 20, 2013, 11:44:24 am
content: Thanks Yago, what does bell out mean?

author: fuffkin
author_rank: Contributor
timestamp: June 20, 2013, 12:01:34 pm
content: I would definitely power it from your bench PSU and set a lowish (reasonable) current limit on it say a 100mA. Apart from anything it'll tell you if anything if drawing power - just because it doesn't seem to power on, doesn't mean it isn't powered.If it's not taking power try and trace the voltage around from where it comes in to where it stops. If there are IC's, see if you can look them up and find out which are the power pins. If not guess. Looks for any breaks etc or dodgy components.I would think based on the fact that the batteries corroded you are looking for something physical, either corroded track or something. Try and poke around a bit and see if anything you do alters the current taken from the psu (no, don't just short the battery!).If it is taking power when you first powered it on, but is in current limit. Power off and physically inspect for shorts. If yo can't find anything, steel yourself and crank up the current limit. At some point, if there is a fault something will start to get hot. Use the tried and tested "finger" probe. The hot component is likely your problem.All a bit rough and ready, but should be a start!

author: dfmischler
author_rank: Frequent Contributor
timestamp: June 20, 2013, 12:03:47 pm
content: Quote from: rthorntn on June 20, 2013, 11:44:24 amHe means "ring out", or to perform a continuity test.

author: KarlMonster
author_rank: Contributor
timestamp: June 21, 2013, 01:40:10 am
content: It won't power on with the leaky old batteries, or it won't power on with fresh clean replacement batteries?I agree with Fuffkin - there are a host of 'not-on' conditions that are not 'off'.If this image is post-battery-gunk-clean-up, then I would try to get a better look under that left side battery holder. I can see bits of component solder peeking out from under there, and Gremlins love to hide in those little places.(Personally, I'd do something stupid like use a Water-Pik to get pressurized water under there, and then rinse with IPA.)