Can a resistor in an old circuit show signs of damage by overheating?

Thread Starter

MarkAB

Joined Feb 27, 2016
80
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?

KW715256.jpg
 

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takao21203

Joined Apr 28, 2012
3,696
my guess its a tr less psu.

Replace with USB adapter some are easy to remove from shell.
There are such modules sold without shell.
Even LED drivers do as the voltage wont rise infinitely you could patch the current as well.

If your circuit needs more than 5v either patch the feedback loop or use dc dc converter booster.

They use resistors because at 10,000 units its a considerable saving and failure rate is a bit higher probably as well, odors, you name it.
 

Alec_t

Joined Sep 17, 2013
10,592
I'd guess those fat resistors are droppers in a transformer-less supply. Check that diodes D2 and D3 haven't gone short-circuit.
 

wayneh

Joined Sep 9, 2010
16,124
D3 looks odd in the photo, like it's translucent or reflecting something. Maybe burnt or maybe a camera anomaly. Anyway, check it.
 

Thread Starter

MarkAB

Joined Feb 27, 2016
80
D3 looks odd in the photo, like it's translucent or reflecting something. Maybe burnt or maybe a camera anomaly. Anyway, check it.
Sorry but that is not the actual PCB but a picture I found online of an identical one.

Will check Diodes anyway and post picture of the actual one.

Thanks for the help.
 

WBahn

Joined Mar 31, 2012
24,970
The fact that you had PCB traces burning out in the first place means something is wrong. That something may or may not even be on this board -- you could have a short somewhere in this "robot". That may well have caused damage to this board beyond the burned trace such that, even if you fix the original problem, this board may now cause problems that might damage the robot in other ways. Best bet would be to check out the robot itself and then replace this board with a new one, if possible. Troubleshooting and repairing this board may not be practical.
 

Thread Starter

MarkAB

Joined Feb 27, 2016
80
Hi guys just pulled out all the diodes and resistors and I am starting to think as WBahn said the problem could be elsewhere !
The only thing is that there is nothing else but a motor and all this PCB does is handle some safety switches and spin a motor at different speed in one direction and the other.
It is just a food mixer.
Why could these resistors be heating up?

kenwood km260.jpg
 

Alec_t

Joined Sep 17, 2013
10,592
All four of those big resistors look burnt and both the relays look to have suffered too.
Is that glue, or spewed innards, on top of the big electrolytic cap below the resistors?
 

Thread Starter

MarkAB

Joined Feb 27, 2016
80
Removed and checked the resistors and they measured fine 2.14k,(both in series) well within the 5% parameter. and yes that is glue from the resistors. why do they glue them?
The yellow one is a Carli MPX40 capacitor and I think its good, did a check on it but with a ohms meter. will pull out the relay but think its fine too.
Motor looks good too.
 

SLK001

Joined Nov 29, 2011
1,541
Why could these resistors be heating up?
Does the motor drag when running? Or, does it spin freely when spun by hand?

Do you run your "robot" for long periods? How long does it take for the resistors to start smelling?

Where is the picture of the back side of the board?
 

Thread Starter

MarkAB

Joined Feb 27, 2016
80
Motor does not drag and yes it spins freely.
I used to run it for 10 min as instruction manual states but now after 1 minute it starts to smell.

front PCB.jpg

And the rear

Rear PCB.jpg
 

SLK001

Joined Nov 29, 2011
1,541
No that is just glue form the resistors support ):

Thanx anyways
No, I mean check it in the area where the board is burned (when it was mounted). The capacitor is probably across the lines and may have broken down, which would have created a jet of plasma that would have shot out of a hole in the shell (or the bottom) and damaged the board. If you look at the actual damage, it starts under the capacitor.
 

Thread Starter

MarkAB

Joined Feb 27, 2016
80
No, I mean check it in the area where the board is burned (when it was mounted). The capacitor is probably across the lines and may have broken down, which would have created a jet of plasma that would have shot out of a hole in the shell (or the bottom) and damaged the board. If you look at the actual damage, it starts under the capacitor.
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.
 
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