Circuit board charring  - Page 1
Forum section: electronics / repair

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: September 20, 2016, 10:07:05 am
content: Hi, does a chard circuit board under resistor mean the circuit is about to malfunction? I have a 10 year old working microwave oven I was cleaning and I noticed a charring under a few resistors in the power supply section. I did a visual inspection of the the capacitors and they all looked normal. Does this mean something is going out of spec and about to render the useless?Obviously something is causing these resistors to overheat enough to burn the board?Any thoughts?nico

author: tec5c
author_rank: Frequent Contributor
timestamp: September 20, 2016, 10:19:47 am
content: Some charring on the PCB for high power resistors in a 10 year old bit of equipment is kind of expected/normal.It would really depend on how bad the charring was as to whether or not there is any underlying issues.

author: amyk
author_rank: Super Contributor
timestamp: September 20, 2016, 10:30:16 am
content: Those are probably part of the resistive dropper for powering the controls, and they do runveryhot, although they do usually tend to last a long time. If you are knowledgeable and so inclined, replacing them with a capacitive dropper might be an improvement.The main thing to be aware of is if the carbonisation bridges the terminals, it will short out the resistor and burn the board.

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: September 20, 2016, 11:51:20 am
content: I think a capacitive dropper would limit the current and the display might not work. The controller for the mico contains a few relays too.

author: BMack
author_rank: Regular Contributor
timestamp: September 21, 2016, 03:23:41 am
content: It depends, the traces are the main thing to look at. Post a couple pictures.Some of the hospitality TVs I work on are really bad, especially in smoking environments...but they work fine. Light colored boards discolor rather easily also, so that should be considered.

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: September 21, 2016, 11:39:56 am
content: I would like to show some photos. Its back together now. If I need to fix it or clean it out I will.By the way the reason I took it apart in the first place may interest some.I had a terrible cockroach problem which I couldnt eliminate in my kitchen. The two appliances that could harbor a nest were the micro and the fridge. I took the micro apart and it was crawling with them may be 500 plus big live adults.The fridge is next. Just fumigating the with sprays and powder was useless. Doing this teardown did the trick.Gives new meaning to the term "bugs" in electronic equipment.

author: BMack
author_rank: Regular Contributor
timestamp: September 22, 2016, 05:33:15 am
content: If you can get to the area you can spray WD-40 it will flush them out and kill them.The thing I'd worry about is the roach crap on the inside. I've worked on some electronics that had roaches in it and they do a number to electronics. lizards do a number on them too. I worked on this not too long ago.https://www.reddit.com/r/techsupportgore/comments/4u1672/fried_to_a_crisp/?ref=share&ref_source=link

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: September 22, 2016, 07:55:59 am
content: This is kind of a disgusting but needs to be aired.The lizards actually may be eating the cockroaches. Cockroaches shit all over the inside of appliances causing a musky smell in the kitchen. Your wife will insist you fix it and may even leave you if you don't do so. If you call a pest control expert he most probably woun't take your microwave oven apart to get the nest. There is no way this will go away unless you throw a perfectly good working micowave oven in the dumpster or take it apart to clean the inside around the microcontroller panel. It will become your responsibility to fix this or move to another house.This is a real horror show.nico