As a Maker, we usually spend time making entirely new things or improving on existing things. But as important as those are, it is equally important to be able to fix things sometimes too. Whether they break on their own or (sometimes) by our own hands, we are frequently called on to do repairs too. So let me recount how the ability to fix things saved me $500 last month…
It started out fairly innocently – I had resolved to start running again every day on my treadmill. But I find this to be immensely boring and I know that I won’t stick with it for long unless I have something to occupy my mind while exercising… and so I keep a big-screen TV in my exercise room. Which brings me to my problem – the TV wasn’t working. It would turn on, and the power LED didn’t indicate any troubles, but sure enough there was no picture. I tried giving it a signal from multiple input sources (maybe the cable box was fried?) but nothing produced a picture.
Well I happen to know that this type of TV uses a backlight to produce a picture, and a fault in the power electronics circuitry is almost always the source of any failures. So let’s open this sucker up and take a look!
After disconnecting all of the wires (especially the power cable!), I removed the rear cover and quickly found the backlight electronics board. How did I know it was the backlight power supply? Easy – the backlight requires the majority of the power used in the TV set and so I just had to find the board with the large transformers, capacitors and mosfets with heatsinks…
Now, before we continue I would like to take a moment to bring up SAFETY. On this board you can clearly see two sections that are marked off with dashed lines. One of the sections is at the bottom of the above image and marked with the warning “High Voltage”. The other has a number of large transformers, fuses, inductor coils, and large capacitors. Additionally, if you pay attention to the electrical connections on the board (jumper wires or etched traces), you will also notice that wires/traces never “jump” across those dashed lines. The reason for this is that the lines demarcate regions of the circuitry that handle high-voltage or other dangerous (usually lethal) electrical signals. Pay heed to these sections of the board. NEVER touch these parts of the circuit while the device is plugged in (even when the power switch is off). Also, you want to “bleed” the large capacitors before working on the board. This is easily done by placing a high-value resistor (a few hundred kΩ) across the leads. Lastly, when possible you should always work on electronics on a workbench that has a proper dissipative mat installed (conductive, but with high resistance so that stray voltages bleed away slowly and safely).
So now I’ve warned against mucking around in the high-voltage regions of the circuit, and that’s OK because 90% of the time the problem in a broken power supply is due to failures in the mid-sized capacitors – which aren’t found in the high-voltage regions. This type of capacitor contains a fluid electrolyte, which evaporates over time and eventually leads to failure of the capacitor – sometimes catastrophic (they can explode!). This is normal (not the explosion part…), and if you look closely at electrolytic caps you might notice that the top is typically scored – which allows any evaporated fluid to vent out instead of exploding. Heat greatly shortens the lifespan of these caps, and power supply circuits tend to generate quite a bit of heat. So these caps don’t last more than a few years – especially if they are poor quality or part of a bad manufacturing batch.
And taking a closer look at the top left corner of that board which contains a number of this type of capacitor reveals that several of them are bulging at the top (and some are even leaking fluid). This is the smoking gun I was looking for!
Clearly these caps have given up the ghost and need to be replaced. First, we need to remove the bad ones and get replacements. Desoldering requires a little bit of technique – if done poorly you can permanently damage the circuit board and/or other nearby components. I find that the best way to do this is using some copper solder wick (held with spring-loaded insulated tweezers to avoid burning my fingers) along with LOTS of flux. For desoldering I prefer to use a paste flux (like ChipQuik) because it is very thick and stays put until the iron hits it. Don’t be bashful with the flux – it can really help draw the molten solder into the wick!
Another tip is to use something to hold the workpiece steady. I used to use a PanaVise Jr for this, but now I find that a PCBGrip works best. The PCBGrip system holds my board in place using clips on the board edges (instead of clamping down and possibly damaging components), and lets me quickly flip the board around to access both sides easily.
To desolder using wick, you first put a liberal amount of flux on the solder joint. You then place a clean section of the wick over the solder joint and press the soldering iron tip onto it – use the wick to transfer heat from the iron into the joint. When the solder melts it will be drawn into the wick via capillary action (it can be helpful to wiggle the wick to help draw out the molten solder). Don’t hold the iron in place for too long or operate it too hot/cold. And don’t be afraid to apply more flux and try again if all of your flux burns off.
Once the solder has been removed, the component should pop right out (you may need to straighten the leads with some needle-nose pliers first).
There will be some charred flux on the board that should be cleaned. This is easily taken care of using IPA (isopropyl alcohol). Be careful though – IPA can damage some sensitive components and you don’t want to aggressively scrub a circuit board (which can easily damage it). The best way I’ve found to clean the flux away is to use a horsehair brush (cheap from arts stores) and a cheap paper coffee filter. I cut out the flat center of a stack of filters and use one at a time as needed – hold the filter in place and brush the IPA over it. The flux char will dissolve and get absorbed by the paper without any scrubbing. It works amazingly well actually.
Selecting Replacement Parts:
With the bad caps removed and the board cleaned, we need to find replacement parts – capacitors are easy to identify since their ratings are almost always printed on the canister. It just so happens that in this case, I found five failing capacitors – all exactly the same brand, size, voltage rating and capacitance value. Smells like the TV manufacturer bought a crappy batch of parts from a vendor (happens often to save a few cents here and there)! Anyhow, the important thing is to replace the bad parts with new ones. You need to select a replacement part with the SAME capacitance value (typically rated in Farads – F, or microfarads – uF). You also want to select a part that has the same physical dimensions – especially the lead separation distance so you can actually solder it in place. Select a replacement part with equal or higher voltage and temperature ratings – never lower. Finally, if you want your repair to last then buy the parts from a reputable source. I always buy mine from DigiKey because they are trustworthy and shipping is cheap (and fast).
Oh, and also always buy extras! Most suppliers offer price breaks as the quantity increases, and it never hurts to have spares in case you need them again or in case you mess up and damage one during the repair.
Total cost for this repair – less than $10, and I bought twice as many caps as I needed.
Installing Replacement Parts:
When installing an electrolytic capacitor, almost all are polarized – so it matters which direction you install it. If you look closely at the capacitor, you will see that one of the leads is marked with a stripe that indicates it is the “negative” side of the component. The circuit will similarly be marked in some fashion to identify the negative pole. Match these up when inserting the part into the board for soldering.
When soldering in the new parts, again use a generous amount of flux. For soldering I use a flux pen instead of the goopy paste that I use when desoldering. The pen is a bit less aggressive – and for freshly soldering a new part you don’t need as much. For repair jobs, the best solder to use is almost always going to be standard Sn63/Pb37 0.025″ with rosin core. Yes, this has lead in it. So don’t eat it. Lead-free is significantly harder to work with, and you only need to use lead-free solder when making something that you intend to sell. So go with lead-bearing solder here and make your life easier.
The proper way to solder is to do it quickly, and do not dwell. The longer you apply heat to the circuit board, the greater the chance that you’ll damage the board or other components on it – heat travels very well through metal wires and copper traces. It should only take from two to three seconds of applying heat to fully melt and “flow” the solder into the joint. The iron should be clean and freshly “tinned”, and the proper method is to press the iron tip lightly onto both the wire lead and the surrounding eyelet on the circuit board – heating both at the same time. Bring the solder to the opposite side of the joint – the solder shouldn’t directly touch the iron tip, it should touch the lead wire and the board and melt from the heat conducted through them. This ensures that the metal is hot enough for the solder to fully flow and form a solid bond. With practice, this process is very quick and dependable.
And invest in a decent soldering iron. Not a Radio Shack Special. I’ve been using a Weller WESD51 and it works great. It is very dependable, can dump a lot of heat into the tip when needed, and is reasonably priced. Take care of it, and your soldering iron will last forever. Clean and tin the tip after every use, and always use an appropriately sized/shaped tip for the job at hand (almost always a small chisel or conical tip).
The resulting joint should be smooth and fully bonded to both the lead wire and the eyelet where the lead wire passed through. It should be shiny and not dull. If the solder has a dull appearance then you probably did one of several things wrong:
- You used old or improperly selected solder
- You moved the lead wire while the solder was cooling and it caused the metal to crystallize as it hardened
- Your soldering iron was too hot and this caused the joint to cool too slowly
- Your soldering iron was too cold and the solder did not fully bond with either the lead wire or the eyelet trace
- You forgot to add flux prior to soldering, and the solder failed to flow properly between the metal surfaces
Because so many things can go wrong, I recommend practicing your soldering technique on some scrap boards before relying on your skill before working on a project that you actually care about.
After soldering the replacement capacitors in, all that is needed is to clip the leads, clean the flux residue again (same technique as before), and put the TV back together.
After powering the set back up, SUCCESS! I just saved myself from having to buy a new TV set. And I also gave myself an excuse to not exercise for another week while completing this repair project! Double Win!
The parts for this repair project only costed me about $10, while a new TV of the same size would easily be $400 to $500. It’s repair projects like this that makes being a “maker” not only a great hobby but also economically smart. Last year I performed a similar repair on the amplifier component of our home audio system that stopped working. That repair was also just a matter of replacing bad caps – total repair cost was under $30, and it avoided having to spend $2,500 on a new amplifier unit.
And that’s money I can instead spend on more gear for my workshop!