Recap.IT – Aldi Vivid 32″ LCD TV

One of these Aldi TV’s came in – a Vivid 32″ LCD TV with a problem where the screen display goes out from time to time. I had seen it happen a couple of times, and could see that it was the backlight that was the problem. However, the backlight circuitry appears to be working fine most of the time, so the fault could be due to the power supply.

The power supply is a Megmeet MP-116A – a search of google shows that a lot of people have had problems with this power supply. Also searching for Vivid 32″ shows some similar faults but no real fix.

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I checked most of the electrolytic capacitors, they all appear to be within tolerance as far as ESR is concerned. ESR or Equivalent Series Resistance is a measure of how well a capacitor is working, in addition to its capacitance. Power supplies tend to get quite hot, and when capacitors get hot, they can start to dry out – and noticeably it’s ESR tends to increase, until eventually it is unable to filter out any ripple currents effectively.

I also used an oscilloscope and looked at the 12V output of the power supply – it was showing some 200mV of ripple, not a lot but could be causing a problem if the ripple increases during higher loading.  The large main capacitor ESR was not high, at 0.66 ohms, but usually this is one that would go first.

In any case, it is usually best to “recap.it“, i.e. replace all of the electrolytic capacitors – so this is what I did. I got the parts yesterday and replaced them today. After removing the parts, I also checked the actual capacitance and found a couple that appeared to be slightly low, but most were within tolerance. I will test it further during the weekend and see if the backlight fails again – if it doesn’t, then it could be that replacing the capacitors in the power supply did the trick.

[NOTE]  The replacement 68uF 450V main capacitor as a comparison had an ESR of 0.2 ohms – this means that its power dissipation for ripple currents will be considerably less than the original capacitor, and hopefully run cooler.

Retell.IT – 2013 – Faulty capacitor on Toshiba laptop

Speaking about faulty capacitors reminded me of a Toshiba laptop that I repaired in August 2013.  This Toshiba laptop belonged to a friend’s landlord – if my friend could get it fixed economically, he could score brownie points – always an advantage to be on good terms with your landlord, especially when the rental climate looks like rents are rising.

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This is with the laptop taken apart – see the massive amounts of dust in the cpu heatsink heat distributor and fan just above.  The laptop would fail to boot and from research on the internet, the cause was a failed capacitor – the rectangular NEC TOKIN inside that metal plate near the bottom left.  This capacitor was a bugger to remove – I tried Infrared heating – it just started to cook the plastic top and did not budge.  These are often glued down, but because it had a large contact area on the board, it was impossible to remove without more specialized equipment that I don’t have, i.e. dark infrared reworking station with under board heater and good temperature controller.

I had to effective destroy the capacitor piece by piece, layer by layer.  Actually capacitors are fragile and easy to destroy especially if I am wielding a scalpel.  Eventually it was removed, and I replaced it with four smd capacitors.  I had to use a fibreglass pen to remove the green coating from the board in order to do the soldering of the replacement capacitors.

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Now a closeup of the board with the replacement capacitors.  The motherboard was reassembled in the laptop, after cleaning all the dust, of course – then powered on, and it worked.  Total cost to the friend’s landlord was $60 plus a lot of brownie points.

[Note]  I have since learned that I could have used my existing infrared rework station by using the under board heater, then heating the capacitor with the infrared, and use my hot-air rework station to add additional heat – to keep the temperature steady.  But that comes with a risk of damage to the board due to the heat being there for much longer – my scalpel was much better.

Reveal.IT – Faulty capacitors on Presario SR5120AN motherboard

Last night, when I was swapping out the Corsair HX650W power supply from my Compaq Presario SR5120AN desktop computer to use in my VMware server, I noticed a bad or failed capacitor.  So, you might ask – what are the obvious signs of a failed or failing capacitor that it is possible to see with the eye? Here is a photo of the motherboard that I removed from the desktop this morning.

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The electrolytic capacitors that quite often fail are those with aluminium cans – these are the common types that fail due to over-heating.  What happens is that the dielectric material inside the capacitor is an electrolyte that will dry out either with time or through over-heating causing vents to open.  The venting of capacitors can occasionally be an explosive event with a loud bang – if anyone is around to here it.  Sometimes if you are lucky – you might hear a fizzle.  Can anyone with sharp eyes see anything unusual on the motherboard?

Sounds of suspense here – tick tock tick tock…  Ok, see that there is one capacitor on the top left – which has a bulged and blackened top, then another four in a row beneath the cpu?  Usual signs are bulging of the top – a sign of over-pressure caused by over-heating. If the vent opens, then electrolyte is released – this is that black stuff.  The bulging with or without dark/black discolouration is how a bad or failed capacitor will “reveal.it“self. On the top of the other capacitors, you can see something like a cross or a T shaped logo – this logo is actually the vent, where the aluminium is deliberately weakened so that failure occurs along the crease marks.  These capacitors are 1800uF 6.3V rated up to 105 degrees Celsius.  There are another four of these that would appear to be intact on this motherboard.  I don’t have any of these in my parts stock so will need to order.

Usually we would replace all of the ones that fail and including those of the same brand and type that have not failed, so I will need nine of them to be ordered.  I don’t have to replace the other four as they haven’t yet appeared to fail, but generally speaking, it is just a matter of time – so best to get the hard work done now instead of doing it again later. Why hard work? Motherboards are multi-layered board that can be very difficult to work on even with specialized desoldering equipment.  Sometimes no matter what, a capacitor may stubbornly resist being desoldered – and usually when this happens, we have to get the cutters out and cut it off the board, then use a soldering iron on one side of the board and desoldering iron/tool on the other side.  I have had to do this once or twice.

You may also notice that I had removed the cpu heatsink.  The thermal material has gotten quite old and is brittle and flaking off – I have cleaned it off and put in Arctic Silver 5. Then reinstalled the motherboard since it is still working, with the occasional freeze and blue screen – I am typing this right now on that desktop.  Anyway, that’s it for now.

[Note]  Desoldering equipment should be maintained regularly.  Due to the higher temperatures involved, the desoldering tip can go black from oxidation and then it will not transfer heat well, which requires increasing the temperature, which makes it worse, etc, etc.  I use a Chemtools Tip Tinner – it is something that has powdered flux and solder that reactivates the desoldering tip – also used for soldering irons.

[Note2]  When the electrolyte dries out, the capacitor’s capacitance will be reduced – which means that its function in the circuit, for reducing ripple, will be degraded.  Also the drying electrolyte will have a higher ESR (Equivalent Series Resistance).  This higher ESR also causes the capacitor to dissipate more power which leads to more heating – this is the mechanism that means that once it starts, it will continue until failure.  I have an ESR meter and with this I could test the other four capacitors to see if they are still functioning well – and then decide to replace or not, but the five failed ones definitely require replacement.