Can A Resistor Fail Like This From Decades Of Normal Use?

[The Doktor]

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.

[amyk]

That's a resistive dropper. Yes, they do get hot. As long as the resistors measure OK, they should remain working.

[Fryguy]

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 .

[Swake]

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.

[mikerj]

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.

[coppercone2]

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-it

7. POOR EMISSION — Rejection due to insufficient electron emission from the cathode will show a sharp drop in the output power and moding during operation. Poor emission is due to residual gas in the tube, improper cathode temperature, etc.

An increase in the residual gas contaminates the cathode and quickly deteriorates emission quality. Excessive cathode temperature also contributes to poor emission from excessive evaporation. Cathode temperature that is too low, on the other hand, not only produces insufficient electron emissions but also represents temporary degradation of the thoriated-tungsten filament.

The filament or heater voltage must be maintained within the specifications. This care is especially critical because the cause of emission rejection is typically deceptive (e.g., poor contact, increased resistance of filament circuit). Additionally, variation of the supply voltage leads to excessive anode current, which shows the same phenomenon as emission failure. It is advisable to minimize any potential changes in the supply voltage greater than 2.5%.

8. DROP IN OUTPUT POWER — The cause of this rejection is not always related to the drop in the emission. Sometimes the cause is secondary electrons ejected from the cathode by back bombardment, which also contributes to the emission during operation.

The major reason for the drop in the output power is in the decrease of efficiency. This is usually due to deposits from the cathode or other parts creating increased surface loss in the cavities. Another potential cause is a disturbance of the interaction space, which results from the easily-deformed form and asymmetry of the thoriated-tungsten filament. The higher the temperature, the more evaporation, deformation, and asymmetry of the cathode are observed. It is imperative to take care to monitor and avoid this.

9. MODING — Moding occurs when the MW electric field inside of the tube can no longer maintain the oscillation in-mode. This is often due to the disturbance by the reflected waves from the load that causes an unusual oscillation. Insufficient electron emission from the cathode also discontinues the in-mode oscillation, which gives rise to moding. In order to avoid this, the operator of the microwave is advised to check the operation of the circulator. Additionally, the manufacturer’s advice should be sought regarding the load impedance, reflection from load, load variations, etc.

[factory]

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 . . .   

Probably built 100x better than any crap made these days, if it's lasted this long it must either have decent paintwork or be made from stainless steel.
Just beware of flammable plastic interiors used in the in some old microwaves.

David

[BILLPOD]

40  years is a good long time for ANY appliance to be operating.  I'm just curious as to who the manufacturer is/was.