How to test network for leaks without desoldering everything - Page 1
Forum section: electronics / repair

author: ignislion
author_rank: Contributor
timestamp: November 28, 2022, 01:23:03 pm
content: I am trying to repair S27A950D monitor.Most of the power ics were blown( photo as good as i always do =) so i replased all mosfets ldos and power controllers, just in case.At the first run the board signaled to me that something is wrong by spiting one of mosfets with fire.I was lucky to get service manual for this thing and next i found that 3.3v line is leaking 2A to ground. After desoldering ic202 and ic207 multimeter shows resistance from 3.3 to ground is just couple of Ohms. I have tried to sprincle the board with alcohol and look for hotspots while sourcing power, but nothing.I am still newby at electronics repair, so for me it look like i need to desolder and test every bit of passive components from 3.3 network and test it. Or there is some better trick

author: ignislion
author_rank: Contributor
timestamp: November 28, 2022, 01:27:33 pm
content: Some more photos of board with destroyed components

author: RoGeorge
author_rank: Super Contributor
timestamp: November 28, 2022, 01:44:03 pm
content: Sometimes there are solder bridges along a PCB trace (by design), or zero ohms SMD resistors that can be desoldered, meant to isolate the power supply from its loading circuits. Sometimes there can be fusibles or ferrite bead that can be removed to isolate the power supply stages.If not, might be easier to just cut the power trace right after the voltage stabiliser circuitry then to desolder parts. Cut away the load, repair the voltage sources, then connect back the traces with their load, one by one, using solder bridges over small (milimeters long) pieces of copper wire.

author: ignislion
author_rank: Contributor
timestamp: November 28, 2022, 01:49:39 pm
content: Currently i have desoldered two ICs on both ends of the 3.3v bus. So it is isolated from both power and load and still leaks to ground. I imagine that some of passive components are broken but before i have time to check it i am trying to find best way to do so.

author: RoGeorge
author_rank: Super Contributor
timestamp: November 28, 2022, 01:58:32 pm
content: Measuring ohms between V+ and GND with a DMM is not relevant (unless it's a 0 ohms short circuit). The indication depends a lot by the type of DMM. A few ohms can mean it's all OK or it can mean it's defective, you'll never know for sure, unless you compare it with a working board. That is why the need to debug it stage by stage, and connect the load only after the power supplies are in good working.Might be cheaper to search for a replacement board.

author: ignislion
author_rank: Contributor
timestamp: November 28, 2022, 02:15:43 pm
content: Id like to agree if i am not leave 3.3v net without semiconductors at all, like on imageBefore i have isolated 3.3 bus the ic202(mosfet) exploded due to overcurrent. And after isolating attaching 3.3v to conductor eats 2A of current, so there isclearly something shorting.Finding new board is out of my expertiese. So if things are bad i am just going to return all the stuff to the owner.

author: ignislion
author_rank: Contributor
timestamp: November 28, 2022, 06:28:40 pm
content: So it was controller =( Board to the trashI have found other parts of 3.3 bus, coupled it one by one, gave some current, and only controller left, and alcohol shows it is the hottest part