China TV (Model SONNI-MAX) Dead-Repaired
Initial test: Television is totally dead.
I opened the TV and went straight to the safety components; I started with the main fuse, found to have had catastrophic failure that even the glass part of the fuse was broken.
Once I found the fuse has died catastrophically, next I moved to check the state of the switching transistor.
Upon testing it with my digital meter set to diode test, I got a beep either way, the resistance was near to zero.
Then I concluded this component has gone also-please note that if this transistor short expects the fuse or the surge limiter to open.
To confirm the switching transistor was actually shorted I cut its middle leg (collector) with a side cutter and tested across the C-E pins again, there was still a beep (short indication)
Next I moved to the secondary side, I tested across the secondary side diodes with the meter probe on one side and then reversed the probe.
I noticed when testing across the main B+ diode rectifier there was a beep either way, This is an indication that the actual diode is shorted or a component along that line.
To confirm, I cut one leg of this diode and tested across it again I got no beep. This confirmed to me that this diode is okay and therefore I should concentrate on the component after this diode.
The main suspect now is after this diode is the H.O.T, testing across the C-E junction of this transistor; I got beep on either way of the meter probe.
Again I suspected it is the culprit, to confirm I cut the middle leg from the circuit and upon testing the again the C-E pins with one leg (collector) removed from the circuit, the transistor was found to be okay.
Then I suspected the flyback as you can see from the above diagram it is also on direct path with the suspect line (B+) and to be sure it was the one shorted, I decided to remove it completely from the board.
This I did and after testing across the C-E (on the circuit track with the middle leg of H.O.T transistor lifted up from the circuit board) sadly I was again wrong. The short was still present.
So what next? I pulled my ESR meter to assist me to locate the shorted component.
With one probe of the ESR meter on the collector(C) pin of the HOT transistor, I followed the circuit track from the pin E(emitter) and going forward I noticed the ESR value was decreasing steadily and the lowest reading was recorded at both legs of capacitor C416 which was number 470p/2kV
I pulled my digital meter to confirm and indeed this capacitor was dead short.
I soldered one of it leg out and testing again across the C-E junction of the HOT, this time I never got the beep and this confirmed to me that indeed this was the culprit.
I got another from the junk board and I replaced it, I also replaced the switching transistor and re-installed the FBT and now was the time to apply the power which of course was via the series bulb.
After applying the power I noticed the bulb was very bright and was not going dim and this concluded to me that there is still a shorted component.
I tested the voltage across the main capacitor and the reading was 64 vdc.
This could point to a problem with the diode rectifier and I tested with a meter in diode mode and for sure I found one of the four (4) diodes was shorted and replaced it.
I was now smiling because I figured that this is the end of this assignment but again I was wrong.
After replacing the diode I tested across the main capacitor and still the voltage was very low and the series bulb was still very bright which is still a sign of present of a shorted component on the supply.
So decided to do component by component testing for the entire primary side components.
From my experience I decided to start with all the diodes and all diodes were found to be healthy according to my digital meter.
Next I checked the transistors and I decided to start with the transistor C3807 which drive the switching transistor.
This transistor has its collector pin connected to the base of the switching transistor.
I tested between the C-E junction of this small transistor and I got a beep, I reversed the probe and still got a beep.
I guessed maybe it is because I am doing the testing in-circuit and hence decided to remove this transistor out of the circuit and upon testing it out of the circuit still there was a beep and therefore I concluded the transistor is shorted.
I took another and replaced it and again applied power via the series bulb and this time the bulb was dim.
This confirmed to me that there is no other shorted component on the supply.
Later I went to the owner of this board and after re-installing the board back to the tube the TVcame back to life.
Lessons learn: whenever you change the switching transistor kindly change the drive transistor also directly.
Usually when the switching transistor get shorted this small driver transistor connected to its base short also or get a lot of stress and therefore even if you find it okay it is a matter of time before it succumb.
Conclusion: After doing replacement of all the parts in this television I figured out what could have really happened to cause this kind of damage to these components and this is how I figured out to have happened (postmortem).
First I suspect the problem started with small capacitor (470p/2kV) on the B+ line which developed a short.
After shorting the B+ voltage which is highly regulated at 109 VDC dropped drastically.
At this time the feedback circuit unaware of what has happened communicated to the primary side that we need more voltage on the secondary side and this then made the switching transistor to switch very fast to correct the short fall in the B+ voltage…this transistor was then overdriven and eventually surrendered (shorted), this also affected the driver transistor and also got shorted.
All this time the main fuse was still hanging on and then next one of the diode rectifiers also got shorted.
After this the main fuse had no choice but to also give in and because of many shorted component on the supply it has to go catastrophically…and hence the power was cut from damaging more components…the all process could have taken less than four (4) seconds.
All the best guys
By Humphrey Kimathi
This article was prepared for you by Humphrey Kimathi an electronic repairer from Kenya Africa. He had written 4 ebooks and you can click on the link above for more information.
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