Samsung Television dead

I received a phone call from a customer who wanted me to go and check his television which has developed a problem and was not working.

So I walked to his residence for that mission with a few tools: a multi meter which in electronics world is actually the eyes of the technician, I mean you can’t see electricity (voltage) with your own eyes but the meter can see and tell you if there is voltage or not and a screwdriver to open up the set.

The set was brought to me and I asked a brief history of the set especially the last moment before it died, I was told there was some smoke seen coming out of the set before they switched of quickly.

I didn’t intend to confirm that by powering it on again and therefore I opened the set and started scanning around the power supply looking for a component with catastrophic failure because I figured that since there was smoke, there must have some fire also and where there is fire there must be a heat mark or component swollen or broken to pieces.

Yes! I noted one capacitor with the top part swollen, the capacitor was rated 100uF/160V, in electronics sometimes if you don’t have the schematic you can have an idea of the voltage expected on any line by using the working voltage of the main filter capacitor of that line, for example usually when designing circuit the rule of the thumb is that the working voltage of the filter capacitor should be double the expected voltage on that line.

Therefore a capacitor rated 160 volts like the one above expect around 90 volts to 120 volts on that line, usually in television this capacitor with the 160Volts rating supply the B+ voltage to the fly back and therefore it is the main power on the secondary side of the power supply.

Next I decided to check the state of the fuse and it passed the meter test with flying colors, so I decided to apply power and do some voltage testing.

I applied power and to my surprise nothing happened, no smoke so I concluded that whoever said that he switched off after seeing the smoke did it a little too late.

So I started my voltage testing, with my meter negative probe on the main capacitor negative pin, I started to test the voltage at the actual main capacitor and there were zero Volts …Nothing!

Hint: when you get zero volt on a circuit means there must be an open circuit or component but not a short, usually a short causes voltage to go low and pulsating(at least there must be some funny reading on the meter)

I checked the ac voltage across the power cable input connector on the circuit board and found 220 vac, so what is preventing the voltage from reaching the main capacitor.

So I checked backward from the main capacitor looking for any open component or circuit cracked, from the testing I noted that the surge limiter is open circuit.

After this I decided to take the television with me to the work shop for further testing, at least now I have some clues to the problem.

I replaced the two component and applied power using my homemade series bulb and I noted that the bulb was lighting with intense light.

I concluded there must be a shorted component on the supply and therefore I switched off and started touching component around the power supply looking for any which is getting very hot. I bumped on one high wattage resistor which was too hot to my fingers.

So I concluded that I have a bigger problem than I estimated, the TV was quite old and was using I.C STR50103A as the regulator and because I could not get the circuit diagram of this TV I rushed to my friend Google and I got this data sheet.

Before I went for the datasheet I had taken the voltage testing of this i.c and was as follows:

Pin 3 is the main dc in from the main capacitor, I was expecting around 300 vdc but I can only measure 252 vdc.

Pin 4 is the output of B+ to the horizontal circuit, please note this tv is very old and therefore is using series type of switch mode power supply. Here again I was expecting around 100 volts consindering the filter capacitor on that line is rated 160 Volts.

That why I decided to check on the datasheet for this ic to get the expected voltages on those pins and this is what I got:

Pin 3 is 282 Volts (around 300 vdc on 220 vac outlet or 155 vdc on 110 vac outlet) and pin 4 the output to the horizontal circuit is 118 vdc but this has gone up to 250 volts.

Now you can see why this B+ capacitor has to blow is due to this increased voltage on that line(from 118 volts to 250volts)

With that I concluded that this ic STR-50103A must have gone forever, looking at that ic internal structure you can see two transistors and I concluded that one must have shorted collector emiter and therefore instead of doing his work has a regulator it is acting just like a jump wire.

I replaced this ic plus the surge limiter and the blown capacitor and powered again the set via series bulb and the machine came back to life.

Just before I went into celebrations for the job well done I noticed that the picture was very clear(like digital picture but no audio)

I was not told about this before so I conforted myself that may be there is sound mute via remote control and started looking for one from the owner and they told me that it got lost over 10 years ago and therefore they have been using only the keys.

I opened the set again this time looking for the audio circuit of this set and it was not easy to locate because I was looking for an ic bolted on an heat sink only to find that this TV is using discrete components for the audio circuit.

I started doing some voltage testing not sure how much voltage to expect because I could not locate the circuit diagram for this television.

Then by coincidence when following a 110 volts from the underside of the circuit I noted a pin with 110volt and zero volt on the other leg. This component is in series with 110 volt so how can it be zero volt, I turned the board the other side and I saw the component was white in color and from the circuit it was marked as a fuse.

Took a resistance test on it and it turned out to be open and I was very sure this could be the end of my woes and indeed it was the cause and after replacing it and powering the set I liked what my ears heard…

Thank you guys

Let meet in the next class