components_desoldered

TV Repair

Samsung_TVfix smps

Samsung TV repair – Out with a BANG!

Of all the times you want a tv repair to be done, it is not when I want to finish watching a film in bed. When I powered it up I saw a bright flash, heard a loud bang, and it tripped out the RCD breaker.

I wasn’t going to be taking it apart there and then so, like most of my personal stuff, it sat around on the repair bench until I started to get through my back log.

I opened it up and started with a visual inspection. Major tripping out of the RCD points to a current surge, and in the middle of the TV is the switched mode power supply (smps). This was the first place I looked, and as soon as I inspected the main capacitor I saw the problem. This jumper wire had vapourised because of a current surge.

 

blown_component

Usually, when we see a problem like this, we have to find the root cause of the problem. Quite often on smps faults,  a blown fuse is caused by faulty switching transistors or bad capacitors. As soon as I looked at the capacitors it was quite obvious that they had caused the problem. Most of the capacitors were bulging out of the top, which means the electrolyte inside is drying up. Due to the high voltages present in a smps, they can fatigue quite easily.

high voltage test

A Note on Safety

Do yourself a favour before you attempt a TV repair: make sure it is UNPLUGGED from the mains supply and the fuse is removed from the power cord. If you are not comfortable with this then you need to consult an expert. DO NOT attempt this if you have no experience of discharging high voltages safely. Before you start tinkering with smps’ you need to be aware that even if they have been powered down, they may contain high voltages. The large capacitor is rated at 450V, which is enough to stop your heart and fry your central nervous system. If this has not been discharged, you may as well be working on a live circuit. It always pays to take a test reading on the capacitor to make sure there is no high volatage before you do anything. You should also short out HV capacitors with a resistor as they can charge off the air in some circumstances. The last thing you want is for this to energise while you are working on your TV repair.

Bad Capacitors

Having established that the circuit is safe to work on we can start to look at the capacitors. As we can see from left to right these are bulged out at the top. This means they are defective. The one under the coil is OK as it is flat topped.

bad_capacitor

These were supposed to be 68O uf capacitors but here I measured them at 191 uf. This is going to have a significant effect on the circuit and if you take 6 capacitors with bad readings then it is no wonder that a current surge occurred. You should always take capacitors out of the circuit to measure them to get an accurate reading.

bulged_capacitor

Next to this is a comparison of a good capacitor, notice the bulging on the left.

capacitor_blown

To be sure

Having removed all the bad capacitors then it was necessary to conduct further testing on the board. The high voltage may have been also caused by bad switching transistors. These take a lot of heat and are installed on a heat sink to dissipate some of the high temperature. The MOSFETS tested OK, and in this instance I was happy that it was caused by the bad capacitors.
components_desolderedI replaced 6 electrolytic capacitors, one high voltage disc capacitor next to the fried jumper, and then I replaced the jumper with the same gauge wire. You MUST use low ESR capacitors for this job or they will fail prematurely.

After reassembly it tested perfectly and luckily I did not have to go out and buy a new TV! Thankfully my TV repair job was straightforward and was caused by perished capacitors in the smps. TVs are prone to many problems but if it goes out with a bang, the smps is the first port of call.

fix_tv

 

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