Or know a good forum to get input on?

I have this old pellet stove that kicked the can and testing has pointed me to the circuit board as the likely culprit. Unfortunately, the stove is so old that they no longer make parts for it and used parts are all dried up it looks like. There does appear to be a service to repair the circuit board but cost is $300+ so I figured I'd take the board out and see if I could fix it myself.

Looking at the board it's pretty obvious one resistor is burnt up. Unfortunately, I cannot readily tell what the resistor value is. The colored banding is either faded or burnt up. Under light and by eye, the bands all look brown, possibly purple. With a light and zoomed in picture however there appears to be some color variation.

Testing with a multimeter it shows 22.12 kilo-ohm but I don't think I'm supposed to be testing while it's still on the board...? Anybody know if I am testing correctly or what the correct procedure should be?


No flash. Bands look brown.



With flash. Bands appear slightly different colors.

 

Looks like brown-brown-brown-gold. 110-ohms, 5% tolerance. I'd use a DVM and measure the resistance across the leads (you need to lift one leg to get an accurate result). May actually be close, at room temp. Then, may drift when the unit is turned on, for awhile. BTW... that is a metal oxide resistor. They are usually have white, blue, or light green colored bodies.

But, what is more important is why that resistor fried. Plus you may have circuit board (tracings) damage on the backside. These are nearly impossible to fix and keep the unit reliable for any length of time. In addition, a heat sensitive IC chip and transistor are nearby.

You have a schematic of the unit?

 

The above is correct. It appears, if the colors are right, to be 110 ohm resistor. And as he said you need to pull one leg out of the circuit to check the resistance. However, if those colors are correct and you are reading higher than that in circuit, that resistor is damaged. If you place any two parts in parallel, the resistance will be less than either of them. So regardless what is in the rest of the circuit, if it is indeed a 110 ohm resistor you should get that or less when reading it in circuit. Again, we are assuming heat has not caused a change in color in the bands.

The traces can actually be repaired quite easily (especially in this type of board) if you are good with a soldering iron and have a decent one. I have repaired traces on many boards, including modern PCBA's with great success. However, I also have spent a lot of time with a soldering iron in my hand and have actually gone to classes for just soldering alone, so take that as it is But this is a very simple 2 sided PCBA with huge traces compared to some of the stuff I have worked on so this would be relatively simple to fix. Normally on something like this that is a non RF board, the easiest is to add a wire from the pad of the component to the pad of the one the broken trace connects to. That wire becomes your new trace and as long as you solder well and add a drop of hot glue to keep the wire from hanging out in space (a good bit of insurance for when handling the board) it will be fine.

The trick to any soldering is to only apply heat as long as you need and to apply enough heat to get a good joint. Soldering iron temp setting is key for that as is cleaning the tip and tinning it properly.

The other question though is what caused this to burn up to begin with. Like a fuse, something caused this to happen. So you may repair it to find out it does the same thing again.

 

Thanks guys! I knew there'd be some smart folks up in here!

The stove is at least 25 years old and the stove is an import (Austrian). I managed to find a wiring schematic of the stove unit but unfortunately, from my research, it appears the circuit boards were likely farmed out and the board diagram is not available to even the stove manufacturer.

I'm no pro with a soldering iron where I can do teeny tiny stuff but this board is huge in comparison to stuff these days so I'm pretty confident I can at least put a new one in. I'll try pulling the resistor tonight and measuring.

Thanks!

 

Are there used stove electronics for this model, on eBay or CL? Maybe, a swap would be easier?

FWIW, a small vise works great for holding, soldering, removing parts off a board. Minimizing damage. Some type of clamp, rigged-up to hold the board would be nice.

 

Unfortunately there's nothing available. I've been keeping an eye on thE bay and nothing even close has popped up.

Finally had a chance to take the resistor off the board and measure it. While I think you guys read the colors correct, I fear the colors may have been discolored when it burnt up.

It's measuring ~22 kohm from one end to the other. It's kind of odd though. If measuring in the middle to one end, it measures ~14 kohm. From the same center to the other end it measures ~8 kohm. The end that measures lower has discoloration so could be that its supposed to be ~14 kohm as well...? Resistance shouldn't be variable across the resistor right?

Now, I'm thinking of going with a 26-28 kohm resistor and see how that runs...?

Wish I had more time on my hands, so I could try playing with an arduino or raspberry pi controller, lol.

 

When you say middle, do you mean on the lead close to the body of the resistor? You should just be clipping onto the leads at the ends and reading a consistent resistance. You may be seeing oxidation on the leads, so try cleaning them with some Emory cloth or fine sandpaper to make sure you are getting a good connection on the leads

Those bands really look to be the same color, even if they have changed color. So that would mean if they were originally red then it would be 2.2K or if all orange 33K

 

Sorry, I should have clarified.

I scrapped the coating off the middle so that I could actually touch the resistor underneath. Cleaned both ends with wire cloth and also cleaned the whole thing with some alcohol and re-measured with similar results.

Here are pics checking resistance:

Measuring resistance across both leads.


Measuring resistance from middle to top lead.


Measuring resistance from middle to bottom lead.

 

OK, duh. I should have realized, unless I'm EXACTLY in the middle, of course the resistance is going to differ from one side to the other. Stupid me.

So ~22 kohm? The third band does seem slightly lighter in color. red-red-orange-gold?

 

It could be red red orange which would be 22k. Yeah it is hard to measure exactly in the center on those like that so not easy to get much from that.

I would get a 22k and try swapping it in. But, if it reads 22k now and was supposed to be 22k then that would change nothing. If that still does not work, you could try a 110 ohm (Brown ,Brown, Brown) in the case the original is just almost burned out. But in that case I would have an IR temp gun and be watching the board very closely when powering up to make sure it is not drawing too much current somewhere and be ready to shut it off quickly! Very hard to figure out without the schematic for sure.

It is also possible that the large power resistor next to it had a lot of current through it and heated that part of the board up, meaning this resistor is fine and is not the root cause.