Short-circuiting capacitor, what's its purpose?

Thread Starter

ChrisRep

Joined Mar 24, 2019
9
Hello all, my name is Chris and I'm the average guy who likes to repair his own stuff on his spare time. Very basic knowledge on electronics and programming. I've just joined this forum because I'd like to discuss something very weird, at least to me, that I came across. As I'm attempting to repair a quite complex oven PCB, while removing an electrolytic capacitor (to have access to solder an IC ) I realized that both its legs were soldered on the same track. Although I have very little experience on electronics this doesn't make any sense... Or does it?

The PCB is double sided and the polarised electrolytic capacitor legs connect both sides of the PCB, it's like it's acting as a jumper. Has anyone ever seen something like that?
Just to be clear, the capacitor has been placed there by the factory who developed the board (CWJ) as no one fiddled with it before. Also, the tracks on opposite sides, where the capacitor is placed, are already connected in several other points by solder points. I'm attaching pictures as it's easier to understand what i mean...
1.jpg 2.jpg 3.jpg 4.jpg

I'd appreciate some comments. Thanks!
 

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Thread Starter

ChrisRep

Joined Mar 24, 2019
9
Thanks. Now that I think about it, no label on PCB, more likely a mistake but I would not expect that on a well built PCB. (Made in Europe for what I understand)
 

iONic

Joined Nov 16, 2007
1,650
I'd say that the cap may have been desired, but the PCB was faulty. The circuit functions without it, but it could function better with it. It's anybodies guess though.
 

Tonyr1084

Joined Sep 24, 2015
3,584
There is no purpose to putting a capacitor in that position. As noted, there is no component marking and as you've shown the far side of the board (solder side) it appears to be either a ground plane or a power plane. A capacitor would work best across both planes, not on just one.

That board is nearly 10 years old. It appears U8 has blown its cover off. Chances are decent there are other issues with the board as well. If your goal is to repair this board I'd suggest you identify ALL failure points before you begin. You may end up fixing one thing only to have it blown out again by something else that is defective. Always fully diagnose a problem before you begin fixing it.
 

dl324

Joined Mar 30, 2015
8,920
A shorted capacitor would serve no useful function.

If the wire jumpers shown in the last picture were from the factory, they also have a quality control problem.
 

spinnaker

Joined Oct 29, 2009
7,815
There is no purpose to putting a capacitor in that position. As noted, there is no component marking and as you've shown the far side of the board (solder side) it appears to be either a ground plane or a power plane. A capacitor would work best across both planes, not on just one.

That board is nearly 10 years old. It appears U8 has blown its cover off. Chances are decent there are other issues with the board as well. If your goal is to repair this board I'd suggest you identify ALL failure points before you begin. You may end up fixing one thing only to have it blown out again by something else that is defective. Always fully diagnose a problem before you begin fixing it.

And the poor design may have lead to its demise.

I would just contact an appliance repair parts store and see if you can buy a new board. There might be updates to the design.
 

Thread Starter

ChrisRep

Joined Mar 24, 2019
9
A shorted capacitor would serve no useful function.

If the wire jumpers shown in the last picture were from the factory, they also have a quality control problem.
:) That was somebody's repair as the first time the board malfunctioned several tracks were blown. The PCB was well built to the eye... As far as design goes might have had issues as this part was discontinued by the oven manufacturer just after two years I bought it. :(
 

MisterBill2

Joined Jan 23, 2018
3,883
As I looked at the photo I think that I see a narrow air gap ringing around the one terminal of the capacitor. So my suggestion is to use an ohm meter and observe that it is not a short circuit across the capacitor. My thinking is that the cap is not short circuited.
 

Thread Starter

ChrisRep

Joined Mar 24, 2019
9
There is no purpose to putting a capacitor in that position...

...That board is nearly 10 years old. It appears U8 has blown its cover off. Chances are decent there are other issues with the board as well. If your goal is to repair this board I'd suggest you identify ALL failure points before you begin. You may end up fixing one thing only to have it blown out again by something else that is defective. Always fully diagnose a problem before you begin fixing it.
You are spot on. The oven is 10 years old and indeed there were other issues. I was only going to ask about the purpose of that capacitor but seeing that you were able to tell that much by knowing so little about the PCB maybe I'll explain my problem starting from the beginning. Any pointer would be much appreciated.

Just after the oven (SMEG s45mcx2) warranty expired, the PCB failed (2 years after purchase) I went to the manufacturer service and they told me the part was discontinued :eek:. Initially I gave up (had a spare oven), but after maybe a couple of years I googled the part and found on a website outside my country (Italy) there was a board replacement but cost around $400. The board was a new design, quite different from mine. As I d'idnt want to spend that much i opted to try and repair it with the help of an acquaintance (my teacher in electronics many years ago) only problem was that U8 was blown and couldn't make out what it was. It had 7 pins though and after a bit of googling I decided that it could have been a tinySwitch III ..274-280. after reading the specs I went for the 280 with higher characteristics (tny 280pn). Also the four diodes (bridge rectifier?) were replaced as some were burnt. It was a long shot but it worked... and after years i was able to use the oven again.. Every single day... But only for a few months. :( :(
This time the capacitor next to the diodes exploded with quite a bang burning several tracks went back to my teacher who replaced diodes with higher specs ones, a new 400v 33uF capacitor a new tny 280pn and as a precaution added a small fuse just before the diodes. Oven worked again for a few months and then it just stopped. As I don't feel like bothering my teacher again I'm attempting to repair it by myself.

I checked everything within my capabilities, diodes, electrolytic capacitor and everything seems ok. Replaced the tny 280pn but to no avail. Nothing lights up and the tny280pn gets really hot just after a minute, but this may be normal? unfortunately there are lots of SMD components which I'm unable to check, in particular the capacitors. For my little understanding the part of the PCB that is failing is the SMPS (?) which provides different voltages to the rest of the board. I see 24v and 12v DC relays.
The transformer has 12 pins, I'm attaching an image which indicates the voltages I get with respect to the ground.
IMG_20190324_120037.jpg 15534552026972256267546174422727.jpg

The four electrolytic capacitors on the left of the PCB are 25v and that side of the board gets no power. Can someone point me what to look for? What else can I test with a multimeter (that's all I've got) thanks to all of you who took your time to read this far :)
 

Thread Starter

ChrisRep

Joined Mar 24, 2019
9
As I looked at the photo I think that I see a narrow air gap ringing around the one terminal of the capacitor. So my suggestion is to use an ohm meter and observe that it is not a short circuit across the capacitor. My thinking is that the cap is not short circuited.
Hi, thanks for your help, are u referring to the second picture I posted? I can't see any air gap but I already removed the capacitor :) too late now. However both sides of the tracks are already connected by solder points even without the capacitor... So even if there was a gap it still would be short circuited.. besides, I checked and that track gets more than 100volts while the capacitor was only 10v . Thank you
 

spinnaker

Joined Oct 29, 2009
7,815
Here is a question. What are you using for a oven (I assume this is a kitchen oven) while you tray and figure this out? Or do you happen to have a extra oven lying around? :)
 

Thread Starter

ChrisRep

Joined Mar 24, 2019
9
Here is a question. What are you using for a oven (I assume this is a kitchen oven) while you tray and figure this out? Or do you happen to have a extra oven lying around? :)
:) Got an older one...
The only reason I'd like to fix this is that it's a combined oven with microwave function and have a very small kitchen... No room for a microwave and a traditional oven ;)
 

spinnaker

Joined Oct 29, 2009
7,815
:) Got an older one...
The only reason I'd like to fix this is that it's a combined oven with microwave function and have a very small kitchen... No room for a microwave and a traditional oven ;)

Where are you from? I really would just consider buying a new board if available .
 

MisterBill2

Joined Jan 23, 2018
3,883
The electrical repairs on microwave ovens that I have had to do were fuse replacements.. On several different ones, both ours and belonging to friends, a glass fuse, 7.5 or 10 amps, would fail. Replacing the fuse made it last for 5 more years and then I had to replace the fuse again, at least on mine.
Did you measure the resistance between the capacitor connections? If it really is zero, or close, there is a short circuit someplace, because there would not be an unused component installed.
This looks like an inverter type power supply for a microwave oven, so I am presuming that there is not a big heavy transformer some place. Or possibly there is and I am wrong. Please let me know. That would affect the diagnostic process.
 

Thread Starter

ChrisRep

Joined Mar 24, 2019
9
The electrical repairs on microwave ovens that I have had to do were fuse replacements.. On several different ones, both ours and belonging to friends, a glass fuse, 7.5 or 10 amps, would fail. Replacing the fuse made it last for 5 more years and then I had to replace the fuse again, at least on mine.
Did you measure the resistance between the capacitor connections? If it really is zero, or close, there is a short circuit someplace, because there would not be an unused component installed.
This looks like an inverter type power supply for a microwave oven, so I am presuming that there is not a big heavy transformer some place. Or possibly there is and I am wrong. Please let me know. That would affect the diagnostic process.
Thank you MisterBill2 for taking an interest. I don't think it's an inverter microwave, its got a big transformer with a big capacitor next to magnetron like traditional microwave ovens. 15534941032293287515534858146887.jpg
As for the capacitor, I already removed it before posting in this forum. 2.jpg
This above was an old picture where I show the capacitor in place. (PCB Bottom view) Those four solder joints go through the PCB, please look picture below.

Only after I removed it I realised it was shorted because soldered on the same tracks. That is, if I do a continiity test on the uncovered pads either side of the board, it's shorted (it's the same track as can be seen from the image) both of the tracks (top side and bottom side of PCB) are also shorted by soldered points3.jpg
Please look at the picture above where the capacitor was soldered, the big dark area is one single track. On the bottom of PCB there is another just as big, both connected by the four soldered joints u can see above. Forgive my poor English. No matter how u solder the capacitor it will be shorted... That's why I posted in this forum, I though maybe it had a purpose that I could not understand. Thanks for your time!
 

MisterBill2

Joined Jan 23, 2018
3,883
OK, that is an interesting situation indeed. Perhaps the capacitor was stuck in to confuse those who wanted to copy the design, or to impress the unknowing folks. I have glued spare parts onto a circuit board so that they would be available when they were needed next time. (that was a relay on a computer PCB in an ice cream machine).
 

Tonyr1084

Joined Sep 24, 2015
3,584
Nothing lights up and the tny280pn gets really hot just after a minute, but this may be normal?
Not normal.

Quite possible one of the many SMD capacitors may have gone short. That could be a reason for the chip getting really hot. In that case you'll never find it. May just be the most economical approach may be to replace the whole oven. After all, 10 years old - what's next to break? Why put a new engine in a car with a cracked and rusted frame?
 
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