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[Articles Home]  [Add Article]  

PCB Fault Finding

Daniel Bartlett (VK4TDB) on September 1, 2001
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Kit construction can be one of the best and most enjoyable aspects of hobbyist electronics, however, it can also be one of the worst! The feeling of “I made it myself”, is often drowned in the realisation that “It doesn’t work” and leaves you wondering why you even bought it in the first place?

Fortunately, there is hope – these kits can be fixed – many of them quite simply too! To quote Colin Mitchell (founder of Talking Electronics and one of my many mentors), if a kit works first go, then you have learnt nothing. All you have to do is put a few parts in the right places and add a bit of solder – where’s the challenge in that? (OK, so it’s a bit more than just that, but it will do for the purpose of this article!).

One of the most common (and often funniest) problems I have come across is attempting to use the kit without a power source. Before you flick the switch, make sure you have a (charged) battery, or some other appropriate power source connected (the right way around!). This can often fix many problems for beginners and experts alike – and, thankfully, is one of the easiest to solve.

One of the most common kit construction faults (and most obvious) would certainly have to be soldering. To check for shorting within the kit, connect an ohmmeter between the positive and negative inputs to measure the resistance. If the measurement is very close to a short-condition (under 1Ω - usual reading 0.00), check for solder bridges (where two tracks have been connected together with solder). A small amount of solder-wick can be used to fix this problem in seconds.

Other soldering problems include: forgetting to actually solder a certain connection (often found in large projects, where, say, all the resistors are placed in position before soldering) and dry joints (where the solder doesn’t make connection with the lead). Dry joints can be extremely hard to find and are caused by dirty leads, “old” solder (which has been burning on the iron for too long) and not using enough solder around the lead to make a decent connection.

If you are sure that all of your soldering is of a high standard, proceed to checking whether all of the polarised components (such as electrolytic capacitors, ICs, transistors and voltage regulators) are actually around the right way. A common trap for those new to electronics is the electrolytic capacitor – the circuit symbol shows where the positive lead should be, whereas the negative lead is the one marked on the capacitor. With ICs, pin “1” is usually marked with a “dimple” or other small imprint, and the end of the IC closest to that pin is “notched”.

When placing these components back the right-way-around, remember that they may have been damaged when the circuit was powered and therefore may have to be replaced altogether. So try and get them right the first time – you don’t want to have to replace a $15 MOSFET device out of a $35 kit do you?

Some components (such as resistors) are easy to put in the wrong places on the PCB, due to their colour coding. Sometimes, orange looks like red, or gold looks “yellowish”, so if you are unsure, always check with an ohmmeter (or similar device for other components). Simply repositioning these components into their correct areas on the board will usually set things right here.

Another common problem (relating back to soldering), is heat damage on components. This often happens with ICs and transistors and, luckily, there are many devices to stop this happening. IC sockets are cheap and invaluable when building kits (not just from a heat damage perspective, but from a general failure one also). As well as these, an investment in a pair of heatsink clips for people just starting their kit-building career is also an easy way to prevent heat damage to components such as transistors.

So it really isn’t all that hard to fix you problematic PCBs – if you are still having problems after trying everything in this article, consult a fellow hobbyist, or take a look on the Internet for advice. One of the best tips I can give to people building kits is to learn how to solder! I believe that over ¾ of all kit-building problems can be solved by good soldering.

Member Comments:
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PCB Fault Finding  
by K6SBA on September 1, 2001 Mail this to a friend!
Dan:

I wish you had written this article last week! I built an LDG Electronics QRP tuner (my first kit in 15 years!) and sure enough, it dit not work.

After connecting LDG and attempting some rudimentary troubleshooting, I decided to send it back for repairs. Sure enough, the culprit was cold solder joints -- many of them. I bet most non-functioning kits are suffering from the same problem. Many PCB's with small components make it hard to track these joints down.

My advice (which I am going to follow): practice, practice, practice. Some kit suppliers -- such as Vectronics -- even sell a soldering training kit. If you have been out of the kit business for a while it might be a good idea to invest in such a product.

Tnx for a great article es 73 de K6SBA
David in Santa Barbara, CA
 
PCB Fault Finding  
by W4CNG on September 1, 2001 Mail this to a friend!
I've built many a kit, and even more home-brew projects (read that Repeater Controllers) in the past several years. My most recent was a PSK-31 interface to my Yaesu FT-100 (into their external data cord). All went well, soldered all components (about $18 worth my shop is still well stocked), connected up and the TX keyed all the time! Checking the circuit, I discovered that I had swapped the base for the collector lead on a 20 year old 2n2222 transistor. I have used these things years ago, but not in the past 5-7 years. Memory sometimes fails, and so do directions. Fixed it, PSK-31 alive and well on my new FT-817. It does not cost $$$ to build simple radio to computer interfaces, but sometimes you do have to get it right the second time.

Good article. 73 Steve W4CNG
 
PCB Fault Finding  
by G3RZP on September 3, 2001 Mail this to a friend!
I've frequently found that on equipment that has failed, resoldering the PC can clear the fault much faster than going through a proper fault finding procedure. I start with the proper procedure, and isolate the area of board, then if the fault isn't easily found, I try a resoldering exercise. It's surprising how often that cures things.

73

Peter G3RZP
 
PCB Fault Finding  
by VK2GWK on September 3, 2001 Mail this to a friend!
The other day I built a digital display meter on a PCB board. And of course it did not work first time round. No dry solder joints but after a lot of searching and testing I discovered hair fractures in some of the tracks. Probably because of some fault in the manufacuring proces. With a powerful magnifier I could see them. So beware of that sort of thing.
 
PCB Fault Finding  
by N9KWW on September 3, 2001 Mail this to a friend!
some of the bigest pain in the ... problems can be sloved with good old and cheap isopropyl alochol. it's cheap and does a real good job cleaning the leads on all parts. the other big investment is a sponge, wet it down and clean the tip of the soldering iron, re-tin and you are set. these two little items will save you a ton of time hunting down those bad solder joints.

i found out about these two items after i burnt up a $50.00 part, thank god for soldering school !!!
ron
n9kww
 
PCB Fault Finding  
by KC8OWL on September 4, 2001 Mail this to a friend!
Something to always keep in mind when soldering components to a PC board ... always heat the component lead, not the solder. Simply heating the solder and 'melting' it onto the component lead is asking for trouble when you power up the circuit.

When soldering a component to a PC board, always place the tip of the soldering iron on the components lead at the point where it passes through the pad (hole) in the PC board. After heating the lead / pad area for a few seconds (depends on what you may be trying to solder and the temp of the iron), touch the solder to the component just at the point where it passes throught he PC board. When the component lead and solder pad are at the right temperature, the solder will melt and flow onto the pad and around the lead.

This will help ensure that both the solder pad and component lead will be joined with a solid and reliable solder joint. When using a PC board with plated through holes, this is even more importaint.

When you have finished soldering a section of the board it is always a good idea to dip an old tooth brush into some isopropoal alcohol and scrub the area to remove any left over flux. Once the area is clean, you can easily inspect the quality of the solder joint.

73's

Mike - KC8OWL
 
PCB Fault Finding  
by N8PKN on September 6, 2001 Mail this to a friend!
Great article, good hints.
A couple more points to remember, many of the current kits have options you can add requiring additional components. This means there will be holes in the basic board not populated in the basic unit. Putting parts in the wrong holes is a sure way to have an in-operable kit.

You should also use good equipment when soldering.
A soldering pen or gun that is too small or large makes for lots of grief when assembling any project.
Undersized guns make you keep the heat on too long, overheating solid-state devices and pc boards. Too large of a gun or pen much gives you excessive heat too. Get a good one, sized correctly, or with variable heat, it will make life easier.

Also be aware that many ic's and transistors are static sensitive, use a grounded tip soldering pencil when working around these devices.

Good luck, have fun building.
 
Improving soldering technique...  
by KC8PBS on September 7, 2001 Mail this to a friend!
It's not black magic, it's soldering technique, as has already been said. But here's how I built confidence: I took hopelessly broken VCRs, old computer motherboards, and anything with a variety of large and small components, especially surface mount components, and carefully desoldered as many parts as I could. Sure, at first I burned even large resistors, but after some practice on these old "doorstops," I aquired the skill to install and remove surface mount components with a needle point soldering iron. It's a cheap way to learn and it gives you a real feel for the way solder melts, hardens, dissolves flux, coagulates on dust, flows onto leads... also, buy a small variety of fine rosin core solder and learn each types "personality."
 
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