Repair/Test Furnace Control Board

Thread Starter

timbaker0365

Joined Aug 11, 2020
32
So my sister's landlord had to replace the control board in her furnace. I'm taking an electronics class and thought maybe I'd tinker with this for some practice with desoldering, testing, etc. It appears it's blown a relay - a 24V which I think I found a replacement for a few bucks on Ebay. Anyway, I was thinking of ordering it and replacing it, but I'm not sure how to go about testing the board (outside of the furnace) afterwards. I tried a brief search to see if I could find some sort of schematic for the board itself, but all I could locate were some wiring diagrams to wire it to the furnace. It's apparently a common board, model 62-24084-82 or ICM288.

Couple of questions: What are the probabilities that something else is wrong with the board if I go ahead and try and replace the relay? Is there a way to confidently test this board if I do "fix" it with just a MM? I do have a 30V 5A power supply coming soon. If I could fix/test it, I was thinking of putting it on Ebay cheap maybe to get some part money back but only if I'm sure it would work. Or, I could give it back to the landlord as a backup. Either way, I'd just be doing it for some experience - a little fun in my shop. I realize that for all practical purposes this probably isn't worth the bother to try and fix, but again, I'd be doing it for the practice. Thanks for any help.

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dl324

Joined Mar 30, 2015
12,684
It appears it's blown a relay - a 24V which I think I found a replacement for a few bucks on Ebay.
What is a blown relay? Did the contacts go bad? Did the coil open?

I used to be a fan of buying components and parts from eBay, but not so much in the last 10+ years. Counterfeiting is so bad that it's not worth trying to save a few dollars. Just buy from a reputable place.

I'll still buy used equipment, but not much else.
Is there a way to confidently test this board if I do "fix" it with just a MM?
No. You need to be able to exercise every feature before you can be confident of a repair.
 

Thread Starter

timbaker0365

Joined Aug 11, 2020
32
What is a blown relay? Did the contacts go bad? Did the coil open?
Well, the thing had big holes blown out the side and soot spread all around, so my sub-novice definition is "it just blew up".
I'm not sure exactly what blew up inside it, or why, as I didn't attempt an autopsy on it, and I don't know how the circuit works with the furnace in general to attempt a guess. I guess that's why I posted here hoping someone might have some experience with these particular boards.

No. You need to be able to exercise every feature before you can be confident of a repair.
Ok. Well, that's probably not practical/feasible since I guess I'd have to take out the new board and replace it with the "repaired" one to test it, which I'm not going to bother with. So maybe I'll wait and see if I can find a simpler practice project. Thanks. I'll keep watching the the thread though and see if anyone has any ideas as to what/why it might have 'blown up' in the first place. Thanks.
 

dl324

Joined Mar 30, 2015
12,684
Well, the thing had big holes blown out the side and soot spread all around, so my sub-novice definition is "it just blew up".
I'm not sure exactly what blew up inside it, or why, as I didn't attempt an autopsy on it, and I don't know how the circuit works with the furnace in general to attempt a guess.
Replacing defective components isn't guaranteed to fix the problem. A damaged component might be due to some other defect.

If a schematic isn't available, you can draw one by tracing the board. Seeing that the board doesn't have plated through holes and has jumpers, it must be a single sided board which will be straightforward to trace.

The two ICs don't appear to have markings. That will make it more difficult to understand how it works. That's one way companies try to protect intellectual property.
 

Thread Starter

timbaker0365

Joined Aug 11, 2020
32
Replacing defective components isn't guaranteed to fix the problem. A damaged component might be due to some other defect.

If a schematic isn't available, you can draw one by tracing the board. Seeing that the board doesn't have plated through holes and has jumpers, it must be a single sided board which will be straightforward to trace.

The two ICs don't appear to have markings. That will make it more difficult to understand how it works. That's one way companies try to protect intellectual property.
Yes, I was kind of looking it over more and actually thought I might be able to try and trace the board, make a schematic, but that seems like a lot of work/time for something that might not work anyway, and although it would be good practice/experience, I do have other things to do, like class stuff.

And you are correct, I did look at the ICs closer before I read your post and there are no markings... I was just wondering today in fact, is that why schematics for electronics are so difficult to find in the day of information, because of illegal reproduction?

Thanks Dennis
 

dl324

Joined Mar 30, 2015
12,684
I was just wondering today in fact, is that why schematics for electronics are so difficult to find in the day of information, because of illegal reproduction?
China is a big intellectual property thief. If the original manufacturer has product manufactured in China, they can count on their manufacturer, or someone associated with them, becoming a competitor at some point. It's a big business in China...
 

MaxHeadRoom

Joined Jul 18, 2013
22,691
I have found in many cases, the foil to relay contact connection blows, but in your case it sounds like the relay contact blew internally.
In most cases, board manufacturers do not remove the venting 'pip' on the top of the relay, this can often cause ionization internal to the relay, this can cause arc over due to this and premature destruction of the relay.
This pip is there for any de-fluxing process of the board, unless the board is used in a dangerous environment, it should be removed.
These can be seen in the centre or corner of the relay top.
Max.
 

sagor

Joined Mar 10, 2019
406
R15 seems to be blown/burnt. I would trace that component to see where it leads to. That will point to a device that is possibly shorted (yes, it could be that relay, but could also be another component). It is also possible that some external component is "bad", and the new board may only last a short while until its relay goes too. A large amount of arcing inside the relay can cause large amounts of heat, which can melt the plastic housing, which then contacts the relay contacts and it goes from there until all the smoke is let out.
 

Thread Starter

timbaker0365

Joined Aug 11, 2020
32
R15 seems to be blown/burnt. I would trace that component to see where it leads to. That will point to a device that is possibly shorted (yes, it could be that relay, but could also be another component). It is also possible that some external component is "bad", and the new board may only last a short while until its relay goes too. A large amount of arcing inside the relay can cause large amounts of heat, which can melt the plastic housing, which then contacts the relay contacts and it goes from there until all the smoke is let out.
Examining the board a little closer it seems that R15 is more covered in soot from the explosion of the relay than some of the others in that line. When looking down the plane of the board it appears it is more covered because it sits higher maybe...? On edit, I just checked the R value and it appears its the same resistor as R12 and they both read around 8k+ so it might just look blown because of the extra soot. Thanks...
 

debe

Joined Sep 21, 2010
1,230
This is where it all started. Dry solder joint on relay pin as marked. Also dry joints on the socket pins along side that carry the same volt/ current. Once the relay connection becomes a dry joint there is arcing & eventual over heat of the contacts inside the relay, until theres a melt down. R15 is just soot from the burnt relay. Have seen this fault in alot of Grid Tie Inverters, with destroyed Relays. I have repaired a few GTI with this same problem, just cleaned up the board & replaced the relay.BURNT RELAY.1.jpgBURNT RELAY.2.jpg
 
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debe

Joined Sep 21, 2010
1,230
This was my Kelvinator rev cycle aircon that had smoke coming out of it from arcing dry solder joints on fan speed relays, & how i repaired it.PCB.1.JPGPCB.2.JPGPCB.6.JPGPCB.7.JPGPCB.8.JPGPCB.10.JPG
 

Thread Starter

timbaker0365

Joined Aug 11, 2020
32
Thanks debe & Max (and everyone else of course). I guess at this point I've decided the farthest I would go with this is to clean up the soot a little and replace the burnt relay and give it back to the landlord to try as an emergency backup in case the new one ever fails.

And even though I was hoping to be able to test the board somehow afterward, both for a learning experience and for the satisfaction/confidence that it would work properly, all is not in vain because I did pick up a couple of very useful tips - the very importance of a proper solder joint for one, and the use of solder wick braid to repair foil as another, as well as some other things I've picked up from this post, so it's been a useful learning experience. You all are some smart, talented and experienced guys (and gals out there) and this is a great resource and I expect as my learning as my learning/tinkering goes further I'll call on you again - much gratitude, admiration, and thanks to you all...

debe, Max, after reading about the possible foil damage I did look at the board more closely and it looks ok and I did do an ohm test between the one burnt joint and the other solder pad connected to it and it seems good. If I had to attempt a foil repair at this point I'd be passing on that as I definitely don't have the expertise/confidence for that yet to repair for someone else to try on a critical appliance but a great tip and I'll keep in mind for potential future use... thanks.
 

dl324

Joined Mar 30, 2015
12,684
I did pick up a couple of very useful tips - the very importance of a proper solder joint for one
A good solder joint can fracture with temperature cycling. I had a BMW lamp control module that I had to reflow solder on twice to get the headlights to work again.
 
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