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Offline khaangaaroo

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Any way to identify a fried capacitor?
« on: Thu, 04 July 2013, 17:27:03 »
I'm trying to fix my dishwasher. After taking it apart and getting to the circuit board, I noticed that one of the (I think it's a) capacitors blew up. There's no more outer shell with any markings for me to identify it and find the right replacement. I don't know much about how these things work so I can't really make a guess to what capacitor needs to be there, or if I can just jump the pins and bypass having a capacitor. Any help would be appreciated. I can upload some photos if you guys think that would help.
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Offline Wildcard

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Re: Any way to identify a fried capacitor?
« Reply #1 on: Thu, 04 July 2013, 21:07:13 »
A capacitor isn't one of those things that you can simply jump over. I'd recommend looking for a schematic for your model/brand of dishwasher to replace it.
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Offline rowdy

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Re: Any way to identify a fried capacitor?
« Reply #2 on: Thu, 04 July 2013, 21:14:12 »
Is there any legend on the PCB?  On the back maybe?  Sometimes the cap rating might be printed thereon.
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Offline khaangaaroo

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Re: Any way to identify a fried capacitor?
« Reply #3 on: Thu, 04 July 2013, 22:26:38 »
I found someone selling the circuit board for my dishwasher on ebay for $130. Considering I paid $80 for the dishwasher, I won't be buying this guy's circuit board. But maybe I can get him to tell me what's written on the capacitor. :)

Here's a photo of the PCB from the listing. The red guy is the one that blew up on mine. Does VR300 mean anything?
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Offline rowdy

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Re: Any way to identify a fried capacitor?
« Reply #4 on: Thu, 04 July 2013, 22:38:21 »
Er, variable resistor???

Isn't it Rxxx for resistors, Cxxx for capacitors?  And Dxxx for diodes?
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Offline khaangaaroo

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Re: Any way to identify a fried capacitor?
« Reply #5 on: Thu, 04 July 2013, 22:43:56 »
Is that what it is? haha
I know almost nothing about this stuff.
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Offline rowdy

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Re: Any way to identify a fried capacitor?
« Reply #6 on: Thu, 04 July 2013, 22:48:32 »
I'm guessing that is what the VR part of VR300 is, but it definitely does not look like a variable resistor.

The numbers are just abstract, and probably refer to a circuit diagram or spec sheet somewhere.
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Offline Topre

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Re: Any way to identify a fried capacitor?
« Reply #7 on: Thu, 04 July 2013, 22:50:51 »
Variable resistor has three leads, that looks like it only has two. I think the 300 is just a label for a number. If you look at the diodes and resistors, you can tell some of them are exactly the same. The resistors also all have different numbers right after the R. So VR300 doesn't really mention anything to do with the part you are looking for, other than whatever VR stands for. It might be a variable reluctor?
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Offline rknize

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Re: Any way to identify a fried capacitor?
« Reply #8 on: Thu, 04 July 2013, 22:52:08 »
Its a varistor.  They are used for surge suppression.  They commit suicide to protect electronics from power surges.  A surge suppressor power strip will have a bunch of these in parallel.  Replacing it is easy.  The bad news is that it is not required for the circuit to operate.  You can simply remove it and the circuit will work fine...albeit without any surge protection.

What you are seeing is a PCB that took a large surge, blew up the varistor, and probably went on to blow up more of the circuit.  Has your house been struck by lightning?
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Offline rowdy

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Re: Any way to identify a fried capacitor?
« Reply #9 on: Thu, 04 July 2013, 22:53:21 »
Sorry, VR = voltage regulator: http://electronics.stackexchange.com/questions/25308/what-do-the-pcb-markings-mean

Or maybe not, having seen what rknize said ;)
« Last Edit: Thu, 04 July 2013, 22:55:52 by rowdy »
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"Because keyboards are accessories to PC makers, they focus on minimizing the manufacturing costs. But that’s incorrect. It’s in HHKB’s slogan, but when America’s cowboys were in the middle of a trip and their horse died, they would leave the horse there. But even if they were in the middle of a desert, they would take their saddle with them. The horse was a consumable good, but the saddle was an interface that their bodies had gotten used to. In the same vein, PCs are consumable goods, while keyboards are important interfaces." - Eiiti Wada

NEC APC-H4100E | Ducky DK9008 Shine MX blue LED red | Ducky DK9008 Shine MX blue LED green | Link 900243-08 | CM QFR MX black | KeyCool 87 white MX reds | HHKB 2 Pro | Model M 02-Mar-1993 | Model M 29-Nov-1995 | CM Trigger (broken) | CM QFS MX green | Ducky DK9087 Shine 3 TKL Yellow Edition MX black | Lexmark SSK 21-Apr-1994 | IBM SSK 13-Oct-1987 | CODE TKL MX clear | Model M 122 01-Jun-1988

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Offline Endzone

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Re: Any way to identify a fried capacitor?
« Reply #10 on: Thu, 04 July 2013, 23:07:31 »
I think rkinze is right.  It is a 300V surge protector tied in parallel with the output to the post just to the right of the surge protector.  My guess is that this is some kind of DC power supply that applies voltage to a certain part of the dishwasher when those 2 relays (or one relay) are enabled.   But that is just a guess without seeing the schematic.  Is the back side of the board burned bad around R301 and R302?  Are the traces still good?

For now, just take a pair of small dikes and carefully cut out VR300 from the top of the board.  What is the Part number of the board?  Maybe we can do a Google search on it.  What is the model of the dishwasher? 

Do you have a multimeter?  What wire does the terminal just to the right of VR300 go to on the dishwasher?  Find this wire and do a continuity test between this wire and ground on your dishwasher.  Also do a continuity check between the terminal right next to VR300 and ground on the printed circuit board.  What resistance do you get? 

If you can get the part number of the printed circuit board, maybe we can find a schematic on the internet. 

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Offline khaangaaroo

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Re: Any way to identify a fried capacitor?
« Reply #11 on: Thu, 04 July 2013, 23:12:06 »
Ah, definitely a varistor then. The black wire of the 120v power line connects directly to the pin to the right of it, so it's the first thing that gets power on the PCB.

What happened to the dishwasher was that the power cord coming from below the machine up into the control panel got wedged in the hinge of the door and probably got spliced together. I soldered the wires back the way they should be and zip tied them out of the way so they don't get caught and chopped again. So now the dishwasher turns on, but I was afraid of running any operations in case it ruined the circuit board even more.

Does this mean the varistor did its job? It doesn't look like anything else on the PCB got blown. What should I look for in a replacement varistor?
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Offline khaangaaroo

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Re: Any way to identify a fried capacitor?
« Reply #12 on: Thu, 04 July 2013, 23:25:21 »
Quote from: Endzone on Thu, 04 July 2013, 23:07:31
I think rkinze is right.  It is a 300V surge protector tied in parallel with the output to the post just to the right of the surge protector.  My guess is that this is some kind of DC power supply that applies voltage to a certain part of the dishwasher when those 2 relays (or one relay) are enabled.   But that is just a guess without seeing the schematic.  Is the back side of the board burned bad around R301 and R302?  Are the traces still good?

For now, just take a pair of small dikes and carefully cut out VR300 from the top of the board.  What is the Part number of the board?  Maybe we can do a Google search on it.  What is the model of the dishwasher? 

Do you have a multimeter?  What wire does the terminal just to the right of VR300 go to on the dishwasher?  Find this wire and do a continuity test between this wire and ground on your dishwasher.  Also do a continuity check between the terminal right next to VR300 and ground on the printed circuit board.  What resistance do you get? 

If you can get the part number of the printed circuit board, maybe we can find a schematic on the internet. 

The power comes from the wall outlet, into a switch in the latch of the door (closed circuit when the door is closed), and then into the terminal to the right of the varistor. It receives power now that I reconnected the wires. All the traces are ok, and no burn marks on the bottom side.

The dishwasher is a Maytag MDB7100AWS, and the circuit board is I think 60D20960401. Here's a photo of my circuit board and toast varistor:
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Offline Endzone

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Re: Any way to identify a fried capacitor?
« Reply #13 on: Thu, 04 July 2013, 23:25:32 »
Quote from: khaangaaroo on Thu, 04 July 2013, 23:12:06
Ah, definitely a varistor then. The black wire of the 120v power line connects directly to the pin to the right of it, so it's the first thing that gets power on the PCB.

What happened to the dishwasher was that the power cord coming from below the machine up into the control panel got wedged in the hinge of the door and probably got spliced together. I soldered the wires back the way they should be and zip tied them out of the way so they don't get caught and chopped again. So now the dishwasher turns on, but I was afraid of running any operations in case it ruined the circuit board even more.

Does this mean the varistor did its job? It doesn't look like anything else on the PCB got blown. What should I look for in a replacement varistor?

Just tell Radio Shack that you want something equivalent to a 300 volt rated MOV or surge protector.  If they don't have it, go to an electronics parts place like Mouser or Allied Electronics.  Make sure you cut that surge protector out of the circuit now.  It is probably shorted out, but I'm not exactly sure how they work.  Or, just blow it off.  Many millions of dishwashers worked just fine for their entire life before MOV's came along around 1985 or so.  But if you have a soldering iron or gun and want to replace it, that is fine.  I don't think polarity needs to be observed with them does it? 
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Offline Endzone

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Re: Any way to identify a fried capacitor?
« Reply #14 on: Thu, 04 July 2013, 23:29:03 »
Quote from: khaangaaroo on Thu, 04 July 2013, 23:12:06
Ah, definitely a varistor then. The black wire of the 120v power line connects directly to the pin to the right of it, so it's the first thing that gets power on the PCB.

What happened to the dishwasher was that the power cord coming from below the machine up into the control panel got wedged in the hinge of the door and probably got spliced together. I soldered the wires back the way they should be and zip tied them out of the way so they don't get caught and chopped again. So now the dishwasher turns on, but I was afraid of running any operations in case it ruined the circuit board even more.

Does this mean the varistor did its job? It doesn't look like anything else on the PCB got blown. What should I look for in a replacement varistor?

Cut that varistor completely out of the board.  See if the dishwasher will work normally now. 
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Offline khaangaaroo

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Re: Any way to identify a fried capacitor?
« Reply #15 on: Thu, 04 July 2013, 23:36:00 »

Quote from: Endzone on Thu, 04 July 2013, 23:29:03
Cut that varistor completely out of the board.  See if the dishwasher will work normally now. 
 

It had already been blown off when I got to it. Just reconnecting the wires where they got cut at the door hinge got the dishwasher to turn on. I'll try running a cycle tomorrow after the dishes that are in there now get a chance to dry. I've just been using it as a drying rack for the last couple weeks :p


I don't quite understand how the varistor works if the machine still functions without it? haha
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Offline Endzone

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Re: Any way to identify a fried capacitor?
« Reply #16 on: Thu, 04 July 2013, 23:44:37 »
Quote from: khaangaaroo on Thu, 04 July 2013, 23:36:00

Quote from: Endzone on Thu, 04 July 2013, 23:29:03
Cut that varistor completely out of the board.  See if the dishwasher will work normally now. 
 

It had already been blown off when I got to it. Just reconnecting the wires where they got cut at the door hinge got the dishwasher to turn on. I'll try running a cycle tomorrow after the dishes that are in there now get a chance to dry. I've just been using it as a drying rack for the last couple weeks :p


I don't quite understand how the varistor works if the machine still functions without it? haha


Look at SIOV in this simple schematic.  This is the MOV or high voltage spike voltage protector on your board.  Think of L1 being the black wire going to it and N being the white ground or neutral wire.  Your 117VAC power going to the board has a peak voltage of 170 volts and 117 volts RMS.  This is a 300 volt surge protector, so it wont break down and short out under 117 volt line voltage.  But when a voltage spike higher than 300 volts peak should come across L1 & N for some unknown reason, SIOV will breakdown and short that high voltage spike right back to the line voltage instead of it doing damage further down the line to the printed circuit board. 

http://www.electroschematics.com/5224/metal-oxide-varistor/

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Offline khaangaaroo

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Re: Any way to identify a fried capacitor?
« Reply #17 on: Thu, 04 July 2013, 23:59:36 »
Ah ok, I see how it works now. Thanks for the explanation!

I'll head to Fry's tomorrow. It looks like the carry NTE varistors.
So model 2V115 from this list should do it right? http://www.nteinc.com/Web_pgs/MOV.html
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Offline Endzone

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Re: Any way to identify a fried capacitor?
« Reply #18 on: Fri, 05 July 2013, 01:48:52 »
Quote from: khaangaaroo on Thu, 04 July 2013, 23:59:36
Ah ok, I see how it works now. Thanks for the explanation!

I'll head to Fry's tomorrow. It looks like the carry NTE varistors.
So model 2V115 from this list should do it right? http://www.nteinc.com/Web_pgs/MOV.html

Yes, you're right.  I was looking at the wrong column on the chart. 
« Last Edit: Fri, 05 July 2013, 01:57:52 by Endzone »
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Offline khaangaaroo

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Re: Any way to identify a fried capacitor?
« Reply #19 on: Fri, 05 July 2013, 20:05:42 »
Thanks for all the help everyone! I replaced the varistor and ran a wash cycle this morning.
and then I blogged it ;D   http://www.khangfixesit.com/2013/07/dishwasher.html
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