A friend of mine called me to go and check on his TV, Due to my busy schedule I was not able to go immediately and therefore he decided to ask for assistance from a different source (technician).
According to him the tech was coming to check on his fridge, he also mentioned to him about his TV and he said he also repair TVs and hence took on the new assignment.
According to the owner he replaced two components, the STR-S6707 and the posistor. After installing the two components the TV came up very well and after 10 minutes the TV was down again.
The technician figured out that the IC-STR-S6707 might have been poor quality and hence he was back again to the market for another one.
Back again and installed it and fired the TV on and after 5 minutes the TV was down again.
Off to the market now more determined than before to get a real genuine I.C and back and installed the new I.C.
Pressed the power ON button and this time they only heard the high voltage fire and then dead,After this third attempt the Fridge/TV technician left in hurry and never came back again.
After hearing this story I made one conclusion, there must be something which is causing the I.C STR-S6707 to misbehave and the most likely cause for this must be a capacitor with has its ESR gone to the roof.
So the first thing I did was to do an ESR test on all capacitors (electrolytic) on the power supply and I bumped on one C623 (220uF/16v) on pin 4 of STR-S6707. This pin is the sink pin of this I.C.
Below is the internal structure of this I.C
Looking at the internal structure of this I.C you can see that this sink pin is very important. It is internally connected to the base of the drive transistor and therefore need to be well filtered of any ripple.
When I tested this capacitor with an ESR meter, my hypothesis was confirmed and I replaced it with a new one.
Please remember in all this the i.C S6707 was not shorting so the fuse was intact, it just stopped working.
After replacing this capacitor i tried powering the TV with the dummy load attached to the B+ and nothing happened so I confirmed the ic was also gone.
I went to the shop and bought a new one and installed it and applied power via the series light bulb and the dummy load on the B+, just to confirm if the power supply is now okay.
After applying power with my eyes fixed on the B+ load bulb, to see if it will light up, Sadly nothing happened and I concluded that my problems are far from over.
One thing I was sure of, the primary power section still has problems,So I decided to do components test one by one and from experience, I decided to start diode test for shorts.
Having very few diodes on the primary side I got one which was dead short between pin 3 and 2 of the I.C STR-S6707 (D609).
I replaced the diode with a FR series diode which I salvaged from the secondary side of the power supply of another TV board in my workshop.
Now I was sure I have got the final culprit and applied power using the same method and to my surprise nothing happened.
Now frustrated I started doing voltage testing on the primary side and I started with the voltage to the control I.C STR S6707. A quick test on all the pins I was able to get voltages but I didn’t write them down which I should have done.
I disconnected the board from the Set and took it out to do some resistance test for any short on the board particularly on the primary side.
When sitting outside my shop with despair on my face I tried to recollect the voltage on the control I.C STR-S6707, and here is the actual circuit diagram of the power supply using this I.C STR-S6707.
It came to my mind a flash of small information of the voltages that all the pins of this I.C STR-6707 had voltage?
Then I asked myself, you mean that ic had no ground pin because I could not remember my meter reading zero volt, could it be there is something shorted to the ground?
After doing some resistance test between every pin of this ic to the ground, I was again not lucky.
Because this ic has been removed and installed many times I suspected there could be a loose joint and therefore I decided to do resistance test between each pin and the next component on that line.
When I reached the ground pin 2, I was expecting a beep but I didn’t hear any,I thought; wait a minute could it be my meter is loose at the connector? I checked it out and touched the two probes and I heard a beep.
Then I decided to be keen on this pin and using my magnifying glass I was able to pick a very thin crack on the pcb track to the ground pin.
After confirming this I felt peace in my heart and I said to myself…this must be the end of my woes.
I went back to the workshop and connected a link to the ground pin.
Then the moment of truth, I attached the dummy load and applied power to the board with my eyes fixed on the dummy load bulb,It took around 10 seconds and the dummy load bulb finally had light.
Truly that was the end my woes…I rushed to my audio machine and played my favorite song to celebrate job well done.
Later I was on my way to the owner to re-assemble the board back to the tube, I assembled the board and applied power.
The TV came up well and I was a happy man. This though did not take long before the TV picture disappeared in front of me and the owner. The screen is now black but audio is crystal clear.
I took a big breath and sat back on his very comfortable sofa sets. These sets are too comfortable for technician and I almost feel asleep.
As you all know technicians seat are not supposed to be very comfortable otherwise you may sleep on the job with your hands creating a jumper across the main capacitor…
After the nap I went straight to the small board at the back of the tube looking for the heater bulb. Looking at the heater I noticed that the bulb is not lighting.
I took a meter to test the voltage to that circuit and when I was putting my probe on the heater pin, I noticed a very big dry joint there...in fact the heater pin was just hanging there.
I re-soldered the dry joint and after applying the power I was more than happy to see the screen with full picture and audio…
Lesson learnt. Whenever you find voltage on the ground pin suspect an open circuit somewhere.
This repair history is pulled from the CRT TV Real Life Repair Experience Guide Vol.1, You can easily add this book to your workbench with less than $10 dollar today! Think I am kidding, click the book cover and let meet on the other side...
Thank you guys, see you in the next class.