In preparation for my shed (slash new workshop) being finished, I wanted to get my hands on a small LCD TV for testing my machines whilst I’m working on them, which I could mount in the upper corner of one of the back walls.
Annoyingly, most current LCD TVs on the market start at 19″, and can be exorbitantly expensive to buy new (>= £100) – a cost that I can’t really justify, given the workshop environment that it would be used in.
Luckily, my parents were throwing out a Technika SC520529 15.4″ LCD TV with a built-in DVD player and FreeView – a perfect candidate for this application – which had stopped working following a long period of storage. So, despite not having worked on a modern LCD previously, I decided to try and fix it up.
The TV was completely dead – no red power LED, and no response from any user input (from either the built-in buttons or the IR remote) – which indicated a probable power issue. The TV requires a 12 Vdc centre-positive PSU capable of supplying 3A – the PSU it came with was suitable and tested OK when unloaded, outputting exactly 12Vdc.
Testing the voltage input into the TV itself required cracking it open. This was a relatively easy task, requiring the removal of the stand and front bezel, then unscrewing the LCD and moving it out of the way.
With the PSU connected, the supply voltage into the mainboard via the DC jack (bottom left in the above picture) tested correctly at exactly 12Vdc, meaning that the PSU worked OK under load, and that the DC jack was OK – I originally thought that the jack may have been the problem, as mechanical stress from the PSU during storage could have damaged the jack or led to cracked or dry solder joints.
After a few minutes with the PSU plugged in, I started to smell the typical acrid smell usually associated with a component running hot – I checked all the SMD ICs, FETs, and voltage regulators, but everything seemed cool to the touch. Shortly after, one of the electrolytic capacitors in the power section of the mainboard blew its lid and started to leak smoke, so I disconnected the PSU immediately.
Success! A potential problem found with minimal effort. Usually, I’m not blessed by the causes of problems making themselves so immediately apparent.
The offending capacitor was a 470uF 16V rated part from some cheap, off-brand manufacturer, which had failed dead short. This was fitted across the 12Vdc power input rail for filtering, so there’s not surprise that 3A through it made it fail so spectacularly.
Luckily, I had a suitable replacement in-stock (a Panasonic 470uF 35V radial electrolytic capacitor) – fitting this required the removal of the mainboard from the TV, which is held in by several ribbon cables and four screws, and the removal of the original shorted capacitor using my desoldering station.
After replacing this capacitor, the TV seemed to start up OK – it displayed a picture correctly on all video inputs (including FreeView), and responded to all user inputs, however it wasn’t outputting any audio. I checked all the software settings to no avail; I re-checked that I’d reconnected all the internal cables correctly, and everything looked fine; I checked both speakers to make sure they hadn’t failed open, but both tested fine at around 4 Ohm.
I decided to pull the mainboard again, this time to check ALL the electrolytic capacitors. After all, if one had failed short in storage, and as they were all the same brand, multiple failures would not be unlikely.
Sure enough, another capacitor had failed short – another cheap capacitor from an off-brand manufacturer, this time a 47uF 16V part, which seemed to be used for filtering in the audio output stage, hence the lack of audio.
Luckily, I had a suitable replacement in-stock (a Panasonic 47uF 25V axial electrolytic capacitor) – again, fitting this required the removal of the original shorted capacitor using my desoldering station.
Following the replacement of these two electrolytic capacitors and the TV’s reassembly, it is now working perfectly again – an excellent result, and better yet, saving it from the scrap heap only cost me about 30p in electrolytic capacitors.
*UPDATE* The TV is now mounted in my new workshop, ready for testing old computers, watching Salvage Hunters on FreeView, and watching Wheeler Dealers on DVD. *UPDATE*