Do you mean the Carli MPX40 capacitor? it looks good, sealed and I have tried to check it with a normal digital meter but not to sure how to check it without a proper capacitor meter.

Yes, that's the cap. Look for areas of charring on its bottom. Post a high resolution closeup of the bottom of the cap. Your damage isn't the type that you would normally see from an overheated resistor.

 

Yes, that's the cap. Look for areas of charring on its bottom. Post a high resolution closeup of the bottom of the cap. Your damage isn't the type that you would normally see from an overheated resistor.

It has a little charring on the corner but that is just from the resistor, I think that glue must have burned it.

 

If it's not the capacitor, then one of your resistors really blew its guts out - literally. Take your ohm meter and measure each resistor's value. Also measure the resistance of your cap. Post those values (a total of 5 values). Also post the marked value of the four resistors.

 

If it's not the capacitor, then one of your resistors really blew its guts out - literally. Take your ohm meter and measure each resistor's value. Also measure the resistance of your cap. Post those values (a total of 5 values). Also post the marked value of the four resistors.

Resistors measure 1k each, all four measure 4.29k in series. how do you measure resistance in a cap ?

Resistors are brown, brown, red, gold = 1.1k 5%

 

I suggested that you measure the resistance of the cap, because if it blew up, it could now be shorted.

Here is your photo of the top PCB:


The failed component shot out a jet of superheated gas, burning the board in a characteristic "V" pattern of this type of failure. The apex of the "V" usually points to the failed part - in this case, the cap. This pattern of damage is NOT what you would see if your resistors heated up too much. In that case, you would see brown circles around BOTH of the resistor holes. This browning is not enough to burn through traces. Your failure not only burned thru the trace, it almost burned thru the entire board.

I'm going to go out on a limb and suggest that if you replaced the capacitor, your robot should work once again (but don't quote me on that!!).

 

I have just repaired a burned out trace on the main PCB of a kitchen robot leading to a pair of serial connected resistors.
The robot in now working but the resistors are overheating to a point that they smell. I do not think this is normal and I wonder if I should replace them or look at what is causing this to overheat.
Can a resistor's value decrease as it deteriorates?

View attachment 101444

Possibly the same sort of thing as the startup resistors in a SMPSU, in normal running the control chip is fed by an extra winding on the chopper transformer - but it needs some juice to get started.

Generally resistors of that type aren't rated for rectified mains voltage, so you almost always find 2 in series. Startup resistors in SMPSUs are a common weak spot, one of the resistors fails open circuit.

If the repaired item seems to be working OK, I'd suspect that the manufacturer is a cheapskate who used the cheapest resistors they could get away with.

For safetys sake, I'd replace those resistors with correctly rated parts. There are various component supply houses that specialise in TVs, monitor etc spares. They should carry stocks of startup resistors. Probably a good idea to upgrade to 2W resistors, and make sure that each can withstand rectified mains voltage on its own.

 

Possibly the same sort of thing as the startup resistors in a SMPSU, in normal running the control chip is fed by an extra winding on the chopper transformer - but it needs some juice to get started.

Generally resistors of that type aren't rated for rectified mains voltage, so you almost always find 2 in series. Startup resistors in SMPSUs are a common weak spot, one of the resistors fails open circuit.

If the repaired item seems to be working OK, I'd suspect that the manufacturer is a cheapskate who used the cheapest resistors they could get away with.

For safetys sake, I'd replace those resistors with correctly rated parts. There are various component supply houses that specialise in TVs, monitor etc spares. They should carry stocks of startup resistors. Probably a good idea to upgrade to 2W resistors, and make sure that each can withstand rectified mains voltage on its own.

Yes this sounds logical as it does work OK just it overheats and they could have used cheap resistors so I like the idea to upgrade but sorry I am a bit of an armature here, what are "start up resistors" and "hold rectified mains voltage"

Could you suggest the parts to upgrade to please?
is this a 47k or 0.47k ???



any other upgrades worthwhile?

 

Yes this sounds logical as it does work OK just it overheats and they could have used cheap resistors so I like the idea to upgrade but sorry I am a bit of an armature here, what are "start up resistors" and "hold rectified mains voltage"

Could you suggest the parts to upgrade to please?
View attachment 102126 is this a 47k or 0.47k ???

View attachment 102130

any other upgrades worthwhile?

The first picture appears to be a 0.47uF capacitor, the k is most likely the tolerance code and 275VAC is correctly rated for 230V mains - that could be a filter capacitor, or a capacitive reactance dropper which would violate the rules of this forum.

 

capacitive reactance dropper which would violate the rules of this forum

what why when I am even more confused now.

arrrrgggggggg

helppppppppp

The resistors measure 15 x 5 mm that suggest they already are 2w should I go up to 3w?
Can I fit one 4k 10w resistor instead of four 1k resistors in series?

 

what why when I am even more confused now.

arrrrgggggggg

helppppppppp

The resistors measure 15 x 5 mm that suggest they already are 2w should I go up to 3w?
Can I fit one 4k resistor instead of four 1k resistors in series?

If that capacitor is used for its capacitive reactance to drop mains voltage - its basically a transformerless power supply, and not allowed on this forum.

If its a filter capacitor to suppress mains spikes; that may put a different complexion on the matter.

I'm not sure why they insist on this rule - a microwave oven has a transformer, the 2kV 1/2A secondary is easily as dangerous as raw mains and can jump a lot further!

 

Ok guys,
I changed the capacitors and it seems to be working ok, would have liked to change the resistors but could not fit 3w ones and could not find 2.5w,maybe next time.
Resistors still get quite warm but the test will be when my wife makes a cake with it. Will keep you posted and thank you all very much for your great help.
Every day I learn a little more.

 

Ok guys,
I changed the capacitors and it seems to be working ok, would have liked to change the resistors but could not fit 3w ones and could not find 2.5w,maybe next time.
Resistors still get quite warm but the test will be when my wife makes a cake with it. Will keep you posted and thank you all very much for your great help.
Every day I learn a little more.

Vitreous resistors pack higher wattage into a smaller space - but probably not available in such a high resistance.

That type can also fail low resistance if they get *VERY* hot.

 

Ok so the kitchen robot has passed the test and as they say the proof is in the pudding, or in this case the cake:


Worked well and did not overheat so it must have been the capacitors that made the resistors overheat. Still don't know why thou?

Need to learn a bit more on electronics I guess?
Thanks for your help guys.

 

Good work and thanks for the brownies.

 

Worked well and did not overheat so it must have been the capacitors that made the resistors overheat. Still don't know why thou?

The capacitor failed. Typical dielectric breakdown failure, which resulted in a high temperature jet of gas exiting the device. If you have access to a microscope, you will probably find a hole in the blackened area.

 

The capacitor failed. Typical dielectric breakdown failure, which resulted in a high temperature jet of gas exiting the device. If you have access to a microscope, you will probably find a hole in the blackened area.

The scorched bit is right on a corner - about as far as you can get from the roll of foil inside. If it was going to vent, it would have done so at a weaker point.

Occasionally I've seen them with swollen cases, but mostly the high frequency pulse types for flyback tuning in PC monitors. Once or twice I've seen them split the plastic case they're potted in.

 

The scorched bit is right on a corner - about as far as you can get from the roll of foil inside. If it was going to vent, it would have done so at a weaker point.

I have no idea where this particular cap's weak point is. It is in a corner, but this corner is also outside the ground zero for the blow-out (look at #25). Without better forensics, all our guesses are just that.

I would have looked at the cap under a microscope to see what I could see.

 

I have no idea where this particular cap's weak point is. It is in a corner, but this corner is also outside the ground zero for the blow-out (look at #25). Without better forensics, all our guesses are just that.

I would have looked at the cap under a microscope to see what I could see.

You don't need a microscope to see that the epoxy potting resin is much thicker at the corners.

 

You don't need a microscope to see that the epoxy potting resin is much thicker at the corners.

Thicker doesn't mean stronger.

 

Thicker doesn't mean stronger.

Depends what kind of thick..........................