Trying to fix dead dishwasher. AEG F35020VI

[maturecheese]

Hello all,

first time poster here. I thought I would try to fix my dishwasher as a project - seems such a shame to throw it away and I might just learn a thing or two with a practical challenge. I realise though that I can't go any further without some assistance. Let me start by saying I am not great with electronics but not a total beginner, I do some arduino projects in my spare time, so I have learnt a little and have some basic equipment to help me!

The machine wont turn on, so I got out the pcb and noticed a blown power resistor, 100R 3W 5%. I replaced it with the exact spec and plugged in and boom, the same resistor blew in dramatic fashion.

I cant see any other obvious damage, except for the fact that two contacts on the bottom of the board of the relay module (situated next to the resistor) have gotten real hot. The burnt contacts are the COM and the NO. I used my multimeter to check that there is no continuity between COM and NO at 0V. There is between COM and NC. Then I put a 9V battery across the coil of the relay it switches fine and there is continuity between COM/NO, not COM/NC, so the relay appears ok?

There were two capacitors on the board 4.7uF 400V in those empty circles on the board. I removed them so I could test them (I have a budget component tester from ebay). I thought maybe they could have shorted. Unfortunately one of them got damaged when I took it out but the other tested ok. I have a replacement on order. Looking at what happened to the resistor I would expect some visual sign if either has shorted but the one I damaged looked fine?

Also, the green resistor next to the blown resistor measures 2.1 ohms on my multimeter which seem weird. Dont the bar colours indicate 100 Ohms? I havent removed it from the circuit but without the capacitor that is removed one end is not connected to anything as far as I can see

In red on the underside of pcb i have circled the blown grey resistor, blue is the COM and NO points on relay that must have got because there looks to be burning. Green is the green resistor.

Could it be that there is nothing wrong with the pcb at all, but that when I plug it in it is immediately switching the relay on and whatever device that it is turning on is itself shorting - it's really obstructed at the bottom of the machine and havent yet figured to where the wire is going. Or maybe its just the pcb, but I am not at the level where I can understand what it is doing.

Anyway, extremely grateful for any advice/pointers....
Thanks

 

[(*steve*)]

As you've found out, resistors don't burn out all by themselves. What you've done is similar to replacing a fuse only to have it blow again.

What you'll need to look for is the cause of the problem. It may be a failed semiconductor, a short, or some other failed component.

The first trick is to find it, and for us to help you, you will need good photographs. In our resource section we have a resource describing how to take good photographs. I recommend you take a look at that resource.

 

[Bluejets]

Also, the green resistor next to the blown resistor measures 2.1 ohms on my multimeter which seem weird.

It's a choke( inductor) ...marking on the board says "L3". One cannot read a value for that with ohms scale.

1st photo, where does the white plug connect to?
Assume it is incoming power and switching to a motor or whatever.

Right hand corner could be a power supply setup for converting mains level to on-board low voltages D5, L3, YDR6, U2, your blown resistor and the grey x2 cap by the look of it.

 

[maturecheese]

Ok, I have done my best to take some improved photos. With it being daylight there is much better lighting. Please try to zoom in on them as they appear small when I open them but there's pretty good resolution on them I think.

It's a choke( inductor) ...marking on the board says "L3". One cannot read a value for that with ohms scale.

I see, thankyou!

1st photo, where does the white plug connect to?
Assume it is incoming power and switching to a motor or whatever.

Yes, uploaded a photo with plug attached, there is mains live, neutral and the grey wire is going down to a motor underneath.

Right hand corner could be a power supply setup for converting mains level to on-board low voltages D5, L3, YDR6, U2, your blown resistor and the grey x2 cap by the look of it.

Interesting. That makes sense, thanks. You think one, or more of these components is the source of the problem? ...including the blown fuse-like resistor. Its not likely to be the motor or something external as that would blow the plug fuse, not the power converter for the microcontroller, right?