How do I find the matching solder points?

[susb8383]

Hi,

I've soldered water pipes but never electronics before and I apologize in advance that I don't speak the language 

I have an air purifier that stopped working. People online with the same problem with that model have said that they replaced a capacitor and it was good as new, so thought I'd try it.

I opened it up expecting to find an obvious dome but I don't see anything amiss to my untrained eye. I bought a simple ESR meter so I could test them in circuit. But I can't figure out which points on the bottom of the board go with the capacitor I want to test.

Here are some pictures. The four capacitors in the clump are what I'm trying to test. People said it was usually the 470uf 16v but one person replaced all four shown.
Also attached is the bottom of the board.

Is there any trick for figuring out which goes with which? I took my best guess but the ESR meter didn't come up with anything so I'm not sure I was using the right points.

Thanks in advance.

[NE666]

First off, you can't just jump to the conclusion that a particular problem is down to bad caps. It's true that they can and often do give problems but it's also true that this is often just being repeated as a mantra, in a cargo cult fashion, by those with no better understanding. But you have an ESR meter to hand, so you're up one on the game already.

What you need to understand is, it's often impossible, at least misleading, to measure such component values "in-situ", i.e., when they are mounted in the PCB and therefore affected by other components in their circuit path.  There are meters which claim this capability but reality is a different country. In many cases you really do need to unmount them from the board (at least, lift one lead off) and test them in isolation against their data sheet values to be certain whether or not they are really the source of the issue. And definitely not all bad capacitors explode and/or show outward visible signs of failure.

If you're having trouble working out which leads belong to which components on the board when you're looking at it from the track side (and there is an art to it, gained with experience), it might help to photograph the top side and print it out, scaling it to the size of the board. You can then offer the print out to the bottom side, reversed, as a visual guide. Alternatively, depending on the thickness and material of construction of the board, you could also try shining a strong light through from the component side.

[PGPG]

When electrolytic capacitors dry (after years of being used) you don't see it.
They are cheap so replacing all of them is generally good idea, but you have no guarantee that the source of error are certainly they.
For me ESR means Equivalent Serial Resistance and I have never heard of ESR meter.
Looking at PCB from top and bottom you simply see where are capacitor pins.

[susb8383]

Looking at PCB from top and bottom you simply see where are capacitor pins.

Well, for a total beginner, I don't know what capacitor pins look like. I see blobs, lots of blobs, lol.

[susb8383]

First off, you can't just jump to the conclusion that a particular problem is down to bad caps.

True. I figured it was probably the easiest to replace.

What you need to understand is, it's often impossible, at least misleading, to measure such component values "in-situ"

Fair point. I'm ok with removing the component to test it if I can find the matching lead.

That's a good tip about using a print out. I'm a Photoshop person so I'll probably take the two images I posted and overlay one on the other, then play with the layer opacities. That should  be a similar method.

Thanks.

[Xena E]

Looking at PCB from top and bottom you simply see where are capacitor pins.

Well, for a total beginner, I don't know what capacitor pins look like. I see blobs, lots of blobs, lol.

I note the bottom picture is flipped vertically.

To answer your question directly see the attached.
The two topmost caps are connected in parallel...

[susb8383]

Thank you!!!!!

[golden_labels]

Some things to observe.

Mind the polarity of capacitors, when placing new ones. For aluminium electrolytic capacitors negative terminal is marked with a thick line on the side of the cylinder. You can see that on your photos. Use them as a reference.

The board seems to be covered in conformal coating. This will give you pain. You’ll have to remove it for soldering. For small area around the solder joints you can just scrape it off with something very sharp. Carefully, both to protect your fingers and the board. On the forum you can find threads discussing the topic.

If you’re going to desolder the capacitors, you believe a capacitor is a culprit, and don’t wish to do in-depth debugging of the problem, just replace them all. They are cheap and if one of them is actually dying, others are likely not in their prime either.

You did mention you soldered pipes before. Note that the solder used for pipes isn’t going to be good for PCB. It has no flux and usually is too thick for comfortable use in electronics. 0.7 mm to 1 mm solder, with flux core will do. Sn60Pb40 alloy is a popular one with low melting point.

[BennoG]

Also do not use S39, this is often used in soldering pipes, but will kill your electronics.

Benno

[susb8383]

Thanks. Yup, was not planning on using plumbing solder. I didn't know about the coating, so thanks. I do have solder I used once for some audio wires. Will that work?

Yes, I've seen videos about the polarity of the caps.

Where is a good place to buy them? I've seen Digikey mentioned but since they're in Canada and I'm in the U.S, shipping is high. There must be a better place for the U.S. I know I can get some on eBay but I don't want to end up with cheap caps that will fail in a year. We use this purifier every day. I did look at Mouser but I had no idea what to look for. I found their site very confusing for a beginner.

I assume I want aluminum electrolyte?

Thanks.

[PGPG]

Where is a good place to buy them?

Should be in any shop with electronic components.
At Mouser you select:
- Passive Components,
- Capacitors,
- Aluminium Electrolytic Capacitors,
- Aluminium Electrolytic Capacitors - Radial Leaded,
Then Capacitance, Voltage and dimensions.
If it is linear supply then other parameters are practically 'don't care' but if it is DCDC then 'the lower ESR the better'.

[Doctorandus_P]

When reverse engineering tasks get more complicated, it's quite common to make pictures of the PCB (form both sides), then use a photo editing program to do some corrections (barrel, parallel, etc) and mirror the bottom image. You can then lay the layers on top of each other, make layers semi transparent and more. KiCad also has the ability to load such pictures in the PCB editor, which is also a way to make reverse engineering easier.

[susb8383]

Something else I realized. Again, remember total beginner.

When I just looked at the bottom of the pcb I saw lots of blobs. But when I looked at it a little sideways I could see only some of them had the ends of the wires sticking out from the components, so that narrowed things down.

[pcprogrammer]

Also do not use S39, this is often used in soldering pipes, but will kill your electronics.

Benno

Good one.   

And also do not use your torch with an open flame to de solder the components. Works well when you want all the components for reuse, but the PCB most likely dies due to burning.

A simple cheap soldering iron will do for something like this. The solder can be lead/tin alloy or lead free and should have a flux core for ease of soldering.

[susb8383]

I've ordered a cap from Mouser and Sn60Pb40 solder from AZ so I should be all set. I do have a solder iron.

[MathWizard]

Looking at that PCB, is there a clear coat/conformal coat on there, to keep out the moisture ? That might throw people off when trying to measure on the PCB. Some solder also get a pretty thick oxide layer, and you really have to stab into the solder, just to get a decent reading.

I tried cleaning some coating off an dishwasher PCB, with ISP alcohol, it didn't work that good. So soldering into it the whole time.

Even with rosin solder, having some flux to add to the solder joints, makes things way easier, just make sure to clean it good afterwords, some fluxes have to be cleaned, they can be full of conductive sludge.

I didn't have flux for a few of the early DIY PCB kits I soldered up, and the solder never took to most of the via's. or pads. I'm sure I would have over heated them in my attempts too, so it wasn't that.

[JustMeHere]

Using equally scaled, flipped photos works a treat.  But trust your fingers.  If you touch one side of the mobo with your left hand, and try to touch the same spot on the other side with your other hand, you will get right quite often.

[susb8383]

All fixed! Got the bad capacitor out (hardest part. Small, hard to reach, cheapo solder iron), replaced it, it all works.

Thanks for everyone's help!

[Xena E]

Good job!
Thanks for letting us know