EEVblog #763 – Dumpster Plasma TV Bad Cap Repair

Dave & David2 replace the bad caps in the old LG 50PF3DF 50″ plasma TV found in the dumpster HERE
Will it fix the problems?
Also an impromptu tutorial on how to replace bad/leaking/bulging capacitors, and measuring capacitor ESR in-circuit.
Plus bonus banter.

Teardown HERE
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Forum HERE

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  1. Hi Dave, First let me say thank you for your very informative videos (that are also funny). Thanks to you and other great guys out there I’m starting understand some of the physics of electricity, magnetism, radiation and lots more about working with electronics.

    I remember getting in trouble as a kid because I had a drawer full of disassembled toys. I’d pulled them apart simply because I was so curious to figure out and understand how things work. I loved it in you your video with your son and the cheap soldering iron how you said something like “don’t plug it in and turn it on to see if it’s safe, open it up and check what’s inside”. Good on ya mate for encouraging pulling things apart to your son as I think it’s a great way to encourage a curious mind at a young age to get involved and learn how things work and have fun!

    Anyway what I wanted to post about about the the smoke detector you asked about. I’m no expert by any means but but those small round fittings look like the same residential type that are usually either using ionizing (radiation), optical (photoelectric) or carbon monixide sensing or a mix of say a mix of more modern techs like carbon monoxide and optical. I’m guessing maybe it’s tied into some kind of commercial system that alerts the fire department. I have no experience with commercial systems.

    Anyway I just thought I’d say that from what I do know is that it seems that the old common ionizing smoke detectors are copping a lot of criticism from those in fire fighting fields and scientists as not being as effective as optical smoke detectors. Ionizing smoke detectors are also a common cause of nuisance unnecessary alarms such as cooking or using a toaster in the kitchen. They use an alpha radiation source (Americium 241) to ionize a small passage of air. When only a tiny amount of smoke or particles go though this area they disrupt the flow of current and the alarm goes off.

    I think it may only be a matter of time before they get rid of these ionizing smoke detectors. One very interesting and useful item though it has that you can salvage is the Am241 pellet inside, if you want a fairly low risk “check source” for your Geiger counter such as for testing the µRad Monitor you have. Either replace an old ionizing smoke detector at home with a newer one sich as an optical and then you can rip the old ionizing one apart or you can buy one labelled as ionizion to rip apart from a hardware shop for around 10bucks. It’s great to test the Geiger Tube is still good especially as they ware out over time and I think they stopped producing the SBM-20 and SBM-20U tubes after the early 90’s. Obviously make sure the ethernet is pulled out or the uRadMonitor when using a check source as otherwise you’ll be broadcasting a high radiation ammount for a few minutes that will look strange to someone online looking at the uRadMonitor Website map.

    Other easily obtainable cheap check sources usually have higher amounts of gamma radiation and if not enclosed like for example depression glassware has the ‘depleted’ uranium inside the glass then there can be secondary by products. One example is the old Thorium containing Colman gas campling lantern mantles, will make the geiger counter go nuts from the gamma but I’d be more concerned about the breakdown product Radon gas which can be inhaled and being an alpha emitter you never want it to enter the body. (Alpha emitters are 20x more dangerous when eaten, inhaled ect than a gamma or better emmitter). Alpha particles from an alpha emitter (when outside the body) are stopped by a piece of paper or the outside layer of dead skin on your body.

    Best rip the ionizing smoke detector outside. Although the Am241 is in a solid metal pellet it’s never hurts be be extra cautious with making sure you cannot get possible boken tiny bits anywhere near your living space that get eventually on your food ect. Do this outside on the grass on some news paper you can thow away. Use disposable gloves. You’ll get a little aluminium pellet with a visible tiny more goldern piece inside which is the Am241. I put it in a small clear lab grade (borosilicate) glass 3ml vial. They sell 10 glass vials for $3.70 total on ebay with free shipping.

    Using my Gieger Counter kit build and configured with the SBM-20 I am getting around 20-30 CPM (counts per minute) as background in my house. I get around 490-500 CPM after holding the Am241 steady for a few minutes on top of the SBM-20. I leave it inside the glass vial during testing and storage. Am241 emits some gamma which is what you detect with the SBM-20. If you want to detect alpha then Am241 from a smoke detector is a good check source but you need an alpha, beta & gamma detecting geiger tube such as the high quality (but fragile) LND-7317 pancake tube.

  2. What blows up in a cap? Most likely it’s the liquid boiling and smoking.

    You have to remember the capacitance is not from the paper separator and dielectric fluid – the fluid is actually *conductive*. What happens is that there’s a very thin oxide layer on one of the plates that is non-conductive. That thin oxide is what gives electrolytic caps their huge capacitance in a small volume (remember capacitance increases the closer the plates are). The thin oxide layer is the dielectric and its thinness is what gives the huge capacitance.

    So you have one plate, followed by a separator soaked in conductive electrolyte that contacts oxide on the other plate. Add another electrolyte-soaked separator and roll it all up.

    The oxide layer is what gives the capacitor its polarization – reverse the cap and the oxide layer dissolves and it will try to form on the other plate, but oops – when it dissolves, you now have a nice short because both plates are now in contact with the electrolyte. The cap heats up from the nice short and the liquid boils/burns and the cap explodes. So you now have vaporized liquid and oxide and other capacitor guts in the air.

    That’s also why ESR goes up as the capacitor ages – the liquid dries out and it makes a less conductive contact with the oxide and generates more heat because all the charge is stored on the oxide interface, and charge has to move through this resistor that’s now suddenly in there. And capacitors can move a stunningly large amount of current for very short periods of time, so per the IIR power equation, the power dissipated will increase by the square of the current. A power supply under load can have many 10s of amps going in and out of the capacitor constantly – for very short periods of times.

  3. Blown cap? Once I have used a dirt cheap 150 watt car inverter to power my notebook on the go. It was summer and hot in my car (no aircon, that was a Moszkvics Aleko Luckily I pulled off the road to check the map (around 2002 I have had only PC-based GPS). I have heard some sizzling and blowing noise, and realized that was the inverter. It looked like the self-destructing message from the mission impossible at 1:30.
    I tried to throw out from the car, but it was extremely hot, so I grabbed by its cable. Of course the switching FETs shorted in it, but nothing too serious. The caps soldered in reverse usually pops but not smoke much.

  4. Haha, smoke detectors? They’re using a LED, a maze where the light can die in and some sort of light detector *at the side* of the light beam. Once smoke gets in, the smoke lights up, the detector sees that, and then the thing goes “weeeoooouweeeoooouweeeoooouweeeoooouweeeoooou”. Dead simple. Some uP may be in to recalibrate the thing from time to time. Wireless connections and 10 year batteries are the latest craze btw.

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