21inch samsung CRT TV intermediate Turning ON problem Solved

Hi all ,
Modern days CRT Televisions are becoming very less  popularity because of rapid introduction of LCD technology.

Some of the peoples are still stick on the CRT tv may be their mind set or the sake of elders memory I  don't know but, I am still getting for repair these TVs.

Hats off to CRT inventor in those days and technology which occupied nearly 5 to 7 decades in an industry.Even my generation people enjoyed with both B/W and colour television with awaited TV telecast-ed programmes. 

Come let us begin the story how I solved Samsung CRT TV intermediate  turning ON problem.One of my  customer brought the TV with is son and asked me to solve the problem.

Now TV is at my working bench as usual opened the chassis and visually inspected for any burning symptom. No any burning clue fuse and input components like PTC,Rectifier diode,MOV found good. Next checked the voltage across big tank electrolytic filter capacitor found ok  280V DC which is normal.By this testing it is concluded input section is quite good.Now my next step it to test for secondary section of TV smps. Checked the cold test LOT chopper transistor found ok and parallel y checked the 120DC  Voltage presence in power ON condition via series bulb at the center pin of horizontal chopper  transistor 1803DFX their is no voltage .
Came to conclusion SMPS is not functioning.

FIG 1. A Look in the rear side of the CRT TV

Checked all the possibility in secondary side for shorts and found ok. After this  as a trial just unplugged the 230V AC  inlet and re-plugged now the stand by LED started flashing and all the necessary voltages including 120 v dc released and LOT section fired the static and a fine blue back raster occurred and TV worked .Surprised to see this because nothing i have done but TV is working fine.Suspected on the customer site wall outlet 230V AC Loose contact same i conformed with costumer and returned back saying loose contact at your wall outlet there is no problem with the TV. Coustmer trusted me and taken back to his home.
After two days same customer called me and said Issue not resolved the tv complaint remained unchanged.Requested the customer to get it back to re-investigate the problem.
I reopened the TV and power ON there in no stand by led flashing again started to check the voltages in the test points,Main filter capacitor voltages ok 280volt DC,Secondary voltages checked no voltages guessed problem is due to Dry joints applied the flux in the primary section of the SMPS and re-soldered all the stress area.Now, retested the TV by powering ON now worked fine conclusion in mind the problem is due to Dry joint in the SMPS Primary stress area.I started to test run the TV continuously 4 hours worked fine after this i made power on/off by removing and re-plugging  230volts in ac  wall outlets.Now again TV is not turning ON.
Headache started me really its an Intermediate turning ON complaints power supply solving is not so simple, for this intermediate problem i should focus whole power electronic theory hear.

FIG 2 .SMPS Top switch STRX6750

FIG 3. STRX6750 SCHMATICS

The first thing what i have done is Concentrated SMPS Chopper STRX6750 and its data sheets,Started

to check individual components using my multi meter.All the components found ok including STRX6750.My next activity is to replace the Top switch STRX6750 and to make observation.

same is done but no use.The intermediate complaint remained unchanged.

FIG 4 .Zener 6.2 volts Zener diode leaky culprit

Really its very changeling Task for me.Already i have written a point in my previous repair article  we should not believe Multi-meter cold test readings in some cases remembered the same and implemented hear.

Started suspecting a defective in Zener semiconductors and started to find the culprit using my external DC Source and started to check Zener break down on each individual Diodes.

Ultimately i am not happy with one of the zener  which is connected to OCP(Over Current Protect )

Yes,Its 6.2 volts zener the break down voltage was not constant.Because of this it is controlling OCP and pulling STRX6750 to sleep mode for protection.

defective 6.2 volts zener diode

Replaced the new with 6.2volts zener and tested N number of time,Now the TV is back to re-life worked perfectly.Till now i have not received the call from that customer.

I request my repair friends if you face such situation in SMPS advised to change the active components directly, I repeat Please don't beleave your multi meter reading in some cases.You will be mis-leaded in repairing resulting unsuccessful. A defective/leaky semiconductor components changes their property on current flow and its very difficult to trouble shoot such cases. So, advised to change suspected active components so that you will be saved your valuable time.

Ultimately my repair story is success hope you enjoyed my article i will be back to you with one more interesting repair story.

FIG 5 .After repair found TV OK with Blue back screen

Best regards

Basavaraj