21-inch CRT Samsung Television Model CS21C512CL Come In Dead Now Repaired!
I received this television from a client who wanted it checked, I asked him a brief history of the problem and told me they shut the TV using remote control to say a word of prayer before having the dinner and that was the end, never woke up again.
He also mentioned that I was the second person to open it having been to another technician about a year ago. Since I was busy I decided to schedule it the following day expecting an easy fix since dead equipment are easy to repair according to my past experience.
The following day I began to work on it but before opening it I applied the power to confirm what the client said and noted the television was dead as a dodo. Upon opening the set, I went straight to the safety components and to begin with, I tested the main fuse and according to my meter it passed the exam with flying colors.
This made me to change the tactic of approach and decided to use my series lamp current limiter and do some live voltage testing.
I started with the voltage across the main capacitor and I noted it was fluctuating very fast between 280 to 310 Vdc. Just when I was figuring out where next to attack, with the power off and disconnected from the board, I turned the board the other side and while doing it I incidentally touched across the main capacitor and got a nasty shock but I survived.
Besides learning my lesson from the shock, I concluded that the power has no output on the secondary side because these television discharge via the secondary side that is why you see the power ON led taking time to go out since it is also hooked to the secondary side. I removed the HOT transistor from the circuit and applied the power again and tested B+ voltage and still got zero volt output.
With this I concluded the problem is still on the primary side and I tested every component on the primary side and all were found okay. With this I decided to directly replace the control I.C which in this case was STR W6753. I did it and noted there was output but very low, less than 10 volts, Also I could hear some low noise like the control IC was trying to start then shutdown. Like I said I had already re-installed the HOT transistor because I figured the problem was only the control IC but I was wrong again.
I decided to focus on the secondary side and noted the HOT transistor area is darkened and this prompted to test with a meter set to diode mode between the collector and emitter pin and found it was okay, No beep.
I decided to use my sense of touch and upon touching it immediately after power off noted the transistor was very hot, I decided to subject it to further test using my multi components tester and this is what I got.
The transistor is reading a resistor?, for the output transistor since it has base-emitter resistor I was expecting at least two diode results like the one below.
The reason the meter is confused is due to base emitter resistor like I said and that is why it is not able to pick it as a transistor.
If you remember in basic electronics, we learnt that testing a transistor using a meter you consider the transistor like two diodes connected back to back like as shown below, picked from my basic electronics course book sold here.
I replaced the HOT transistor and was sure I have got the final culprit and after applying the power I noted the television was not generating the HV voltage, I tested the B+ and noted it was stable and decided to concentrate on the horizontal drive circuit, first I tested the Horizontal drive filter capacitor using my tester and I was not happy with the results as you can see here below.
A good capacitor value 47uF/100v should have an ESR reading of less than (1) one ohm and this is reading 13 Ohms.
Kindly note that the capacitance is within the range 45.87uF and that I why I advise techs never to trust capacitance test for good capacitor, ESR test is the way to go!
I replaced the capacitor and again applied the power and still no HV voltage was generated, I even begin suspecting the fly-back transformer but after testing it with my homemade fly-back tester I found its okay. You can easily make one here.
I decided to do some voltage testing around the horizontal drive circuit and testing on the horizontal drive transformer I noted only one pin has voltage and I was expecting two pins with voltage and the other two without voltage.
I removed the small transformer from the circuit and testing across same side pins did not get reading on one side and upon close observation I noted the small wire broken as you can see below.
Getting this part from the shop is practically impossible and I though this is the end of my effort, later I figured the idea of salvaging from a junk television of the same model and I was lucky I got one.
I did not mention that the first time I opened this television I noted the connector to the degaussing coil was not connected and decided to connect it via my series lamp current limiter and noted the bulb was too bright like the sun and with this I concluded the posistor was shorted and that is why the previous technician decided to leave it hanging.
I replaced it and now the series bulb was bright and then low and concluded the circuit is now okay and working well. I again applied the power and this is what I saw.
Later I called the owner with the success story and he pointed to me that the television had a problem of spots on the screen since last time it was taken to the previous tech and asked if possible I should check on that issue also. I assured him the issue is now sorted out and was very happy.
Thank you friends
Humphrey Kimathi is from Nairobi Kenya and the author of:
3) CRT Television repair course and
4) Basic Electronics course.
He is also a blogger at Electronicsrepairmadeasy.com
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