Whirlpool gas oven ps/control board burnt resistor (Resolved door switch)

[Greezmky]

Hello all,

I am attempting to repair a control board for a gas oven. The initial fault occurred after a lightning storm, the digital read out and all oven controls ceased to work. Upon inspection I found a resistor on the line side to be open. I replaced it with a 1/4watt as that is all I had at the time. I applied power to the board while on the bench and it worked (display lit and buttons functioned), however when my dad plugged it all back in it "sparked, cracked and shut off". I then purchased .5watt resistors and we tried again. Again it burned out the resistor. I have not been present either time when he hooked it up and plugged in the range, but each time I have bench tested it and the display comes on and buttons work. I am including some pictures and further information below.

I started to trace out the circuit, curious why there is so much draw causing the 10ohm resistor to open. I believe the power circuit is a switch mode power supply. Sorry still kind of a noob.

One 470uH inductor in series to and before the resistor on the line side seems to have a small crack but ohms out at 1.1ohms or so.

A few questions:

1. If I traced the circuit out correctly and the diodes make up a full wave rectifier, then why is one diode larger/different from the other three?

2. What seems to be the most likely failed component that would allow it to function on the bench but burn the resistor in the range/oven?

3. Could the 470uH inductor be shorted? I read something about inductors in series on ac mains to reduce high inrush current on initial power up?

My circuit tracing efforts:
https://www.dropbox.com/s/w3pknf9tkyzake0/20190828_031823.jpg?dl=0

The two relevant pages from the service manual for the stove:
https://www.dropbox.com/s/ktzmewgkmi6qmp1/20190827_125236.jpg?dl=0
https://www.dropbox.com/s/klcv2fx7ar7aaio/20190827_125244.jpg?dl=0

Board top and bottom:
https://www.dropbox.com/s/3reeqtj99rx64b9/20190830_054356.jpg?dl=0
https://www.dropbox.com/s/ytjw1f8v2mamd4l/20190830_054443.jpg?dl=0

R71 is the resistor that keeps burning up

D49 is the odd ball diode 1N5406, unable to read others while on board

I tested dc voltage after rectifier and it read approximately 160vdc as that is close to what I had in a simulator on my phone

L3 is the somewhat questionable inductor

Y3, MOV tested OL out of circuit


I apologize for the long post and the dropbox links but I wanted to keep high res so others could zoom in, apparently you do not need a dropbox account to view.
Thank you in advance for any help and feel free to ask any questions. I have more info but this post is getting long.
Robert

[Rollin Hand]

I have had some similar experiences. Unless you have a lot of free time and nothing better to spend it on my guess is that chances are you will waste much time and not get far. From a practical point of view I would suggest finding a pro.

[fzabkar]

I agree that all 4 rectifier diodes should be identical. I would replace them all, just in case one of them is leaky.

The cracked inductor is a common occurrence in Philips CRT TVs. I have also seen a similar cracked inductor in the RGB outputs of an old VGA card. I would replace it, as it may eventually go open circuit.

Can you measure the current in the 10 ohm resistor while it is operating? You can infer this from the voltage drop.

As for engaging a "pro", s/he would only replace the PCB. Any amateur can do that.

[Greezmky]

I agree that all 4 rectifier diodes should be identical. I would replace them all, just in case one of them is leaky.

The cracked inductor is a common occurrence in Philips CRT TVs. I have also seen a similar cracked inductor in the RGB outputs of an old VGA card. I would replace it, as it may eventually go open circuit.

Can you measure the current in the 10 ohm resistor while it is operating? You can infer this from the voltage drop.

As for engaging a "pro", s/he would only replace the PCB. Any amateur can do that.

My plan is to attempt to check all items on the stove tomorrow then plug in the board with just line and neutral to start then start plugging in the others and see when it goes bad before hand I will measure vd over the resistor and see what I get. I totally agree with the pro statement and I am trying to learn so this is my path until I stop learning. Plus the board is $250 so until all the traces are lifted or it a puddle of goop I would like to keep trying.

Assuming my retracing is correct (which I am pretty sure it is as I have done it like 4 times) is there any viable reason for that one odd diode? Am I correct that this is a switch mode power supply?

[fzabkar]

Yes, it is a switchmode supply and no, there is no reason why one diode is larger than the others.

One common problem with these types of supplies is a dried out filter capacitor on the IC's Vdd pin. However, this usually results in a no-start condition.

https://www.st.com/resource/en/datasheet/viper12a-e.pdf

[Greezmky]

Thank you just wanted to make sure I was interpreting what I traced and read correctly. I noticed that the layout lines on the board for the second dc side is the same as the lines for the larger diode so perhaps they ran out that day at the factory  Anyway I traced it once again with the same conclusion. The smaller diodes are 1N4004 it looks like. Well I hope to have some more information tomorrow and I will post my results.

[Greezmky]

The door switch, for the light which tested good was the issue. It sparked when all other testing was exhausted and we tried it again. The difference was I was back there and could see it. Still not sure why the dc switch would cause this but it is working now with a jumper and I was able to turn the light on by removing the jumper thus simulating a switch. So mom has no door light for now but she can turn it on with the switch on the control board.

A little more detail is available here if needed in the future https://www.badcaps.net/forum/showthread.php?t=79141&page=2

Thank you everyone for your help!

[JFJ]

... mom has no door light for now but she can turn it on with the switch on the control board.

The door switch might have an additional function. If the oven has an extra (downward facing) burner at the top for grilling, then the oven door may need to remain open while the grill is in use - to prevent overheating (radiant grills do not, usually, have a thermostat). In that situation, a working door switch could act as an interlock to disable the grill when the door is closed.

[Greezmky]

... mom has no door light for now but she can turn it on with the switch on the control board.

The door switch might have an additional function. If the oven has an extra (downward facing) burner at the top for grilling, then the oven door may need to remain open while the grill is in use - to prevent overheating (radiant grills do not, usually, have a thermostat). In that situation, a working door switch could act as an interlock to disable the grill when the door is closed.

As far as the oven light is concerned it is closed all the time so that feature would be in-op, as far as I know this oven does not have that nor would she use it if it did. Thank you for the tip though.