Author Topic: Furnace control board black burn - it has been that kind of week!  (Read 5317 times)

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Offline alank2

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Hi Everyone,

Yesterday was a dead microwave, today no heat! :)

First I checked the fuse box and the breaker was tripped.  Turned it all the way off, then back on.  Tried to run the heat again.

The blower was not running, so I turned off the heat and tried to turn on simply the blower, but no blower.

Opened up the furnace and started poking around.  24VAC present, 120VAC present.  I disconnected the black (full speed) line from COOL and measured the terminal with the FAN=ON or FAN=OFF setting no change.  I didn't think about the blower having more than one speed at this point and decided to try the fan by disconnecting the 24VAC transformer hot input and connecting the blower there instead.  Turned on the breaker and the blower ran great.  Then I figured out the blue fan wire was connected to the HEAT terminal and retried the FAN=ON, FAN=OFF test.  I could hear the breaker firing, but in this case it was millivolts when off and 0.5VAC when on.  I decided to pull the board and this is when I found the black mess on the back.

What could have caused this?  I don't want to just replace the board if this was an over current event from the blower or its cap or something else.  I did NOT test the blue wire, but I could do the blower test with it connected to the hot directly to see if the blower runs or not... 

Thanks for the help!!!

Alan
 

Offline alank2

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Re: Furnace control board black burn - it has been that kind of week!
« Reply #1 on: January 23, 2017, 03:41:04 pm »
Could this simply be a case of high power trace on the pcb wore itself out after 15 years?  I saw this with a clothes dryer once...
 

Offline JFJ

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Re: Furnace control board black burn - it has been that kind of week!
« Reply #2 on: January 23, 2017, 03:43:49 pm »
What could have caused this?

Possibly, a bad solder joint to the Omron G8P-1C4P relay's Common connection (pin 5):



Arcing across the joint appears to have isolated the relay pin from the PCB trace.
 

Offline alank2

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Re: Furnace control board black burn - it has been that kind of week!
« Reply #3 on: January 23, 2017, 03:50:06 pm »
Will arcing over a bad joint cause the excessive current / relay trip all on its own?
 

Offline Seekonk

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Re: Furnace control board black burn - it has been that kind of week!
« Reply #4 on: January 23, 2017, 03:54:04 pm »
Check both relays for welded contacts.  Two speeds could have been on at the same time causing high current between the two windings. That trace was the weakest point and acted as a fuse.
 

Offline alank2

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Re: Furnace control board black burn - it has been that kind of week!
« Reply #5 on: January 23, 2017, 04:01:43 pm »
I've got a black wire connected to COOL and a blue wire connected to HEAT.

I cleaned the pcb up and soldered a wire to repair the burned trace and put it back in and it is working fine right now.

I've tested fan only mode and tested heat and both seem to work fine.

Do you still think I need to check the relays?  Any idea why are there two of them?
 

Offline Gyro

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Re: Furnace control board black burn - it has been that kind of week!
« Reply #6 on: January 23, 2017, 04:30:24 pm »
I've fixed a few boiler control PCBs with similar faults in the past. Dry joints during flow soldering due to high thermal mass pins on relays and terminals. Give it a few years of thermal cycling and vibration and you get results like than. Once the joint starts to arc it breaks down into a black mess very quickly.

It would be worth checking the other relay pins too. Suck off the solder blobs and check that the pins are properly tinned.
Chris

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Online Ian.M

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Re: Furnace control board black burn - it has been that kind of week!
« Reply #7 on: January 23, 2017, 04:47:25 pm »
This sort of board damage also used to be common in old CRT TVs.  Vibration from various sources including the internal speaker(s), mangetostriction in inductors or piezeoelectric deformation of capacitors would fatigue solder joints.  Eventually one or more would go dry and a barely visible micro-crack would form in an annular ring round the component lead.  If it was in a high current circuit, Joule heating due to the increased resistance at the crack would eventually melt the solder on the pin and the annular ring of fillet remaining on the pad, but due to the crack surfaces having been oxidised due to the slow heating, the joint would not reform, and surface tension would pull the solder on the pin away from the pad and the remaining solder on the pad away from the pin.  The crack would blacken and become far more obvious.  If enough voltage was available for the crack to arc over, the melting, burning and resulting track and component damage could become extensive, typically with severe laminate damage and carbonisation for typically up to 1/2" around the original faulty joint.   

In your case, the vibration from the relay, and possibly the blower if both are in the same cabinet will have contributed to the joint going bad.  The relay should be replaced as the pin heating has probably compromised its mechanical support in the relay body, and if its loose, contact movement will fatigue your repaired joint.    Remove all carbonised PCB, make good with Epoxy (with a non-conductive filler) and repair the track.   Make sure your repair is well secured as if the fault recurs, the wire or foil track repair could detach and cause a short circuit.

If there was a wave soldering process problem, its very common to see suspect joints across much of a board.  If you are trying to actually locate an intermittent dry joint fault rather than 'shotgunning' it by bulk re-soldering, and not being certain you've actually found all the faulty joints, try heating the suspect pin, on the pin, not the joint fillet with a freshly tinned bit, without applying any side force.  If the solder on the pin reflows without the annular ring of solder on the pad melting, the joint had a complete full-circle crack and was certainly bad.  If the whole joint reflows it was probably O.K. - touch it up if it needs it and keep on looking for the fault.   Once the fault is located, repaired and soak tested, any remaining dodgy looking joints can be resoldered with reasonable confidence the fault wont recur in the near future.
« Last Edit: January 23, 2017, 06:45:13 pm by Ian.M »
 

Online pmcouto

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Re: Furnace control board black burn - it has been that kind of week!
« Reply #8 on: January 23, 2017, 05:32:40 pm »
As other have already mentioned, this is a very common failure observed in PCBs with relays switching medium/high power loads.
Vibration caused by normal relay operation and mechanical stress caused by temperature variation will eventually crack the solder joint.
On a cracked solder joint, arcing will burn the PCB and damage the relay.

Give the PCB a good clean and replace the relay and it will be as new.  :)
 

Offline alank2

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Re: Furnace control board black burn - it has been that kind of week!
« Reply #9 on: January 23, 2017, 06:31:15 pm »
Thank you everyone.  Just so I understand, the high current/relay trip situation is caused by the arcing?
 

Online Ian.M

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Re: Furnace control board black burn - it has been that kind of week!
« Reply #10 on: January 23, 2017, 06:43:16 pm »
If its an AFCI breaker, it would trip on the spiky current flow due to arcing as well as its normal trip on over-current function.
 

Offline lilshawn

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Re: Furnace control board black burn - it has been that kind of week!
« Reply #11 on: January 23, 2017, 07:25:42 pm »
looks like it could have been a cold/broken solder joint. vibration. lots of things. it is a fairly high current carrying leg. if the solder around the pin was cracked, it will heat up and fail much like this (large load break... poof)

i'd just clean it up and patch it with some solder braid and go for broke. it should be fairly obvious if the blower moter is the cause of the failure.
 

Offline Dwaine

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Re: Furnace control board black burn - it has been that kind of week!
« Reply #12 on: January 23, 2017, 07:32:33 pm »
It's nice to see this message about furnace repairs.   I have seen people pay over $800.00 for a new controller board installed.  Usually, the caps on the board are the cheapest.
One person I know got an estimate for over $800.00.   I replace the bad caps and the furnace is still working after three years.

Used Nichicon UBT2V220MHD caps which are rated for 125C.   They are nice caps....

Dwaine
 

Offline Gyro

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Re: Furnace control board black burn - it has been that kind of week!
« Reply #13 on: January 23, 2017, 08:55:03 pm »
Quote
Usually, the caps on the board are the cheapest.

Actually, while you're in maintenance mode, it might be worth replacing that solitary and undersized looking Electrolytic next to the nice toasty looking resistor. At least they fitted a 105'C rated part, but I'd be tempted to replace it with a known good quality one and lean it off in the other direction. Hint: If you buy one with the same value but a higher voltage rating then you'll get a larger can size which will take longer to dry out.
Chris

"Victor Meldrew, the Crimson Avenger!"
 

Offline Dwaine

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Re: Furnace control board black burn - it has been that kind of week!
« Reply #14 on: January 23, 2017, 10:31:57 pm »
Quote
Usually, the caps on the board are the cheapest.

Actually, while you're in maintenance mode, it might be worth replacing that solitary and undersized looking Electrolytic next to the nice toasty looking resistor. At least they fitted a 105'C rated part, but I'd be tempted to replace it with a known good quality one and lean it off in the other direction. Hint: If you buy one with the same value but a higher voltage rating then you'll get a larger can size which will take longer to dry out.

I would also replace the toasty resistor with a 3w 1%.   Maybe upgrade all the resistors with 3w 1%   They will be a bit bigger, but raise it off the circuit board a little. 

When I bought my furnace, it came with the full schematics.  It's nice to have the full schematic of the furnace that is installed in your house.   

Dwaine
« Last Edit: January 23, 2017, 10:48:40 pm by Dwaine »
 

Offline ashley.mcdermid

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Re: Furnace control board black burn - it has been that kind of week!
« Reply #15 on: January 25, 2017, 12:33:38 am »
I had to do a similar repair on a rangehood above the stove. Much simpler board though....

Sent from my SM-G920I using Tapatalk

 

Offline james_s

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Re: Furnace control board black burn - it has been that kind of week!
« Reply #16 on: January 25, 2017, 01:33:17 am »
It's possible that an arc started from a cracked solder joint and then spread to a nearby terminal or grounded surface, the current from that could have tripped the breaker. I would clean all that carbon off the board and patch up the traces and give it a try. Carbon is conductive and will make it much more likely to arc again if it isn't removed.
 

Offline alank2

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Re: Furnace control board black burn - it has been that kind of week!
« Reply #17 on: January 25, 2017, 02:31:23 am »
I cleaned up the carbon and soldered a wire in place of the missing trace after cleaning up the terminal on the relay.  I don't have a replacement relay so this one will have to do for now.  It is back in position and has been running since yesterday morning.  I was eyeing over a 5 pack of the relay from a seller in Hong Kong for $19 or so, but then I got a guy on ebay with an entire new board to accept an offer of $75 shipped so I went with that.  I may put that board in and keep this as a spare if I need it.  I appreciate the help everyone!
 

Offline james_s

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Re: Furnace control board black burn - it has been that kind of week!
« Reply #18 on: January 25, 2017, 02:59:17 am »
It would not surprise me in the least if the board you repaired lasts the life of the furnace.
 

Offline Gyro

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Re: Furnace control board black burn - it has been that kind of week!
« Reply #19 on: January 25, 2017, 10:11:52 am »
Quote
It would not surprise me in the least if the board you repaired lasts the life of the furnace.

Nor me.

It probably goes without saying, but if you decide to put the new board in,  carefully go over its solder joints with a magnifier first - some faults are 'characteristic'!
Chris

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Offline lilshawn

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Re: Furnace control board black burn - it has been that kind of week!
« Reply #20 on: January 25, 2017, 06:19:54 pm »
It probably goes without saying, but if you decide to put the new board in,  carefully go over its solder joints with a magnifier first - some faults are 'characteristic'!

This is a big problem with RoHS stuff. the lead free solder is terribly brittle and can break due to vibration. this is why a lot of larger items often have some silastic or hot snot gobs holding things in place.

if you can, just go over it with some good old leaded 60/40 solder. it's a little more pliable and less likely to fracture.
 

Online Ian.M

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Re: Furnace control board black burn - it has been that kind of week!
« Reply #21 on: January 25, 2017, 06:29:48 pm »
Don't do that unless you suck all the old Pb-free solder off the joint first as some Sn rich non-standard alloys in the Sn-Pb system vastly increase the risk of a dry joint.
 


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