HOT transistor is continuously getting shorted after replacement

For those who have been in repair business for a while you will agree with me that this is a common phenomenon.

You receive a TV in your shop and you diagnose the problem and found the HOT is shorted and you begin to celebrate how you got the culprit effortlessly.

You pull a new HOT transistor and replaced it and powering on the TV came up beautifully and before picking the phone to call the owner to collect his machine you realized that the TV is down again.

You check again and find the HOT is shorted again. If you have never experienced such a problem lucky you, but for Abdul Haleem from Sri Lanka has not been lucky... Lets us hear his story.

Hi Humphrey,

WESTPOINT TV 26" Model TV-9863M2, the HOT transistor is continuously getting shorted. Initially I found safety/hold down capacitor dry soldered and re-soldered the joint after confirming the cap is good. But when I powered up the TV the HOT got shorted again. HOT is BU508AF without internal diode. Checked the external damper diode and found OK. When I apply power to the board outside of TV nothing happens. Yoke windings are OK. I suspect CRT shorted internally, because along with two heater pins another pin on the CRT neck shows continuity with heater pins. What else could be the cause?

 Thanks in advance,

Abdul Haleem-Sri Lanka

From that letter you can feel Mr Abdul is frustrated with this problem, you can also note that he has done a lot of detective work on the cause of this problem and still not got the culprit.

Very much impressed by the work he has done so far to trace this problem. For the purpose of those who have encountered such problem I am going to highlight a few things which can cause H.O.T to short continually.

  

1.Snubber capacitor faulty.

This capacitor is also call safety capacitor, why? Because its work is to

maintain the safety of the H.O.T transistor, so once this capacitor lose its value(capacitance) or open then the HOT transistor will also go down.

According to adbul he said he tested it and found its okay, for me I will advice that whenever you find HOT shorted don’t even test this capacitor, just replace it.

Looking at the picture below you can see two capacitors which look the same, so how do you know which one is snubber capacitor?

Snubber capacitor usually have working voltage between 1200volts to 2000volts.

The one on this TV is rated 1400 volts, the next capacitor which look the same but rated 200volts is the S-correction capacitor.

 

2. Dry joints around the horizontal drive and horizontal output components.

This is another cause for HOT to short, so I advice that before you even start testing any component around this area be sure there is no dry joints especially around the pins of drive transformer, fly-back pins or even dry joints on actual HOT transistor and drive transistor.

3. Drive transistor.

This transistor after continuous working also lose gain, this in turn will cause weak drive which may cause the HOT get hot and eventually short.

4. Horizontal drive V.C.C Filter capacitor,

This capacitor is used to filter the supply to this circuit and if it fails or loss capacitance then this will cause unclean dc to the circuit and this again can cause the HOT to get hot and eventually short.

 5. Excessive voltage at HOT collector (B+ high)

This happens when there is a problem with feedback circuit, so it is important when you find HOT is shorted, confirm if the B+ is normal, if you find its high than expected check out for bad feedback component or dry joints around that area, if still high then change the opto- isolator directly.

6. Horizontal yoke,

Horizontal yoke if shorted can cause instant short to HOT transistor, the best tester for horizontal yoke is fly-back tester.

Note that this happens if the yoke is shorted but not when open; when open you will have only a vertical line on the screen.

As you know yoke is a coil and therefore will test low on ohmmeter and this will be true if shorted, that is why I am insisting that you can only know if it is shorted or not by using fly-back tester.

7. Fly-back transformer

This is the most suspected component whenever HOT is shorted but in most cases it is innocent but to be sure it is okay then you can only use fly back tester to confirm if it is okay or not.

Sometimes when fly-back fail you see physical sign or stress on the body of the fly-back and therefore you should on lookout also.

8.CRT shorting internally.

This also can cause HOT to short though not common

Thank you guys

Stay safe