Can A Resistor Fail Like This From Decades Of Normal Use? - Page 1
Forum section: electronics / repair

author: The Doktor
author_rank: Regular Contributor
timestamp: January 03, 2024, 09:01:55 pm
content: This is a control board from a microwave oven which I am attempting to repair, it has been in use for over 40 years continuous. If you look at the first picture, you can see a resistor directly in front of the big ceramic resistor which has much of its coating flaked off. The resistor appears to work just fine, it measures 10Ω, And is dissipating ~.65W. This is giving it an operating temperature of right about 90°C, And it is powered continuously. The picture of the bottom side of the board shows the same area, you can see how the board has been discolored by operating at high temperature for a very long time. This area is not related to the failure, and that resistor, the large ceramic one (dissipating about 1 W) are the only two hot ones, with one large carbon composition nearby getting somewhat warm. I can't find anything malfunctioning which is drawing excess current, I am wondering if the resistor could have the coating crumble just from 40 years of baking at 90°?The failure point was a triac output optocoupler Which I believe drives the contactor for the magnetron. It has three resistors associated with it seemed to be part of a filtering network Which are also cooked pretty nicely, but it looks like that happens fairly quickly, as the board is not at all discolored. That failure appears to because by something not on the board, the fellow who owns the oven will check that himself.

author: amyk
author_rank: Super Contributor
timestamp: January 03, 2024, 10:38:53 pm
content: That's a resistive dropper. Yes, they do get hot. As long as the resistors measure OK, they should remain working.

author: Fryguy
author_rank: Regular Contributor
timestamp: January 03, 2024, 11:02:52 pm
content: I agree , as long as they measure ok there should be no problem at all . And yes , they do lose their paint when they get cooked for a long time at high temperatures . BTW nice heatstream marks on the board .On the other hand - this microwave oven is 40 years old ?I'd not repair this thing - it has already earned it's money 10 times over and how long do you think this will continue to work if you can get it working again ? After 40 years parts will continue to fail .It is amazing the mains powered and HV parts did survive that long . . .And what is that part in the first picture left of the power resistor ? Looks like something that has been eaten twice . . .Just curious .

author: Swake
author_rank: Frequent Contributor
timestamp: January 07, 2024, 10:11:41 am
content: Overcooked (pun intended).Judging by the size it looks like a 1/2W resistor or maybe even smaller. Dissipating 0.65W is more than its rating. It might work but that is solely thanks to the over-engineered or under-rated resistor. 0.65W dissipation was not anticipated, they would have put in another ceramic resistor as they did with the neighbor. In other words you found either a design issue or there is something that causes that over-consumption.

author: mikerj
author_rank: Super Contributor
timestamp: January 07, 2024, 11:40:13 am
content: Note that the output power of magnetrons degrades with use over time, the oven is likely putting out much less power than it's original rating.

author: coppercone2
author_rank: Super Contributor
timestamp: January 07, 2024, 11:42:31 am
content: i wonder what epoxy you could dip it in to recoat it.maybe jb weld possibly with a drop of pigment, if you can dilute it with something.https://thermex-thermatron.com/blog/why-magnetrons-fail-how-to-address-itQuote

author: factory
author_rank: Super Contributor
timestamp: January 07, 2024, 12:58:31 pm
content: Quote from: Fryguy on January 03, 2024, 11:02:52 pmProbably built 100x better than any crap made these days, if it's lasted this long it must either have decent paintworkorbe made from stainless steel.Just beware of flammable plastic interiors used in the in some old microwaves.David

author: BILLPOD
author_rank: Frequent Contributor
timestamp: January 07, 2024, 02:48:37 pm
content: 40 years is a good long time for ANY appliance to be operating. I'm just curious as to who the manufacturer is/was.