HOT Keep On Shorting Immediately After Replacement Now Repaired
21 inch Nash CRT Colour television HOT keep on shorting immediately after replacement now repaired.
I got a referral from a previous customer whom I repaired her television long time ago and now had a friend who inquired from her where to get a good technician for her television. After the phone call we connected and visited her home and was shown the television which was China made by Nash consumer electronics.
Brief history was that the television just stopped working and I was almost certain that this is a power related problem. I took the television with me to the shop and once again decided to apply the power and see if there is any sign of life and I confirmed no sign at all even the power ON led was off.
I opened the set and went straight to the safety components and I noted all tested okay. The 300 volts’ dc was present at control i.c and the start-up voltage also present but not stable around 15 volts’ dc. I was expecting around 18 vdc here and I almost suspected this ic was the problem. I checked the output lines and noted were all zero volts but one line has 0.5 volts’ dc and I was surprised if my meter was faulty or not.
I decided to test the state of the HOT transistor and found it dead short. I soldered out the middle leg out of the circuit and after applying the power I noted all other voltages are now present. This means the shorted HOT was pulling the B+ voltage plus other voltages. This also explain why the startup voltage was oscillating
Rarely does the HOT transistor short without a cause and therefore it is always good practice to check related components on the horizontal output circuit, after checking I did not get any suspect but bumped on capacitor C561 rated 47uF/160v which had very high ESR reading, this is the capacitor which filter the B+ line (109v in some TVs) and is highly regulated and any problem on this line can cause all manner of problem, this early catch almost makes me put a smile on my face.
I also found resistor R551 and R55a both open and replaced both plus the capacitor.
After replacing the three components I decided to apply power and see the progress and I heard chaa! Sound as the HV fired and then dead. It was too fast and I just wondered what really happened and going back to the board I tested the state of the HOT transistor and realized I have lost it through sudden dead.
If you have been electronic repair for sometimes you will agree with me that the problem of H.O.T getting shorted immediately or after sometimes of being replaced has given many technician sleepless nights trying to figure out what is happening here!!
This problem is so stubborn that I did a whole chapter on how to solve it on Chapter 12: in my e-book CRT television repair course now on sale on Jestine yong blog here.
After thorough investigation I noted Diode VD435(FR309) was failing under load, thanks to my analogue meter.
Bottom side of actual position where this diode was located.
After replacing this diode which to me was highly suspect to cause instant short to H.O.T transistor I decided to install a new H.O.T and see if there is any progress and once I powered up the television after replacement.
This time round the H.O.T did not short and the television came up beautifully as you can below.
Thank you friends
Humphrey Kimathi is from Nairobi Kenya and the author of:
1) Lcd-Led television repair guide
3) CRT Television repair course and
4) Basic Electronics course.
He is also a blogger at Electronicsrepairmadeasy.com
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https://jestineyong.com/khind-tv-no-power-repaired-and-make-your-own-series-lamp-limiter/
Parasuraman S
March 13, 2020 at 5:45 pm
CRT REPAIR IS ALWAYS A THRILL TO DO. Great trouble shooting and repair there! Well done and thanks for sharing!
Albert van Bemmelen
March 13, 2020 at 8:00 pm
Finding that bad VD435 diode no doubt was a difficult one. But without schematic also a very hard to find culprit. And for any engineer a real time-consumer to solve. Most of my previous crt tv's are now disassembled and used for scrap parts to save space and flat screens are preferred.
Henrique J. G. Ulbrich
March 13, 2020 at 11:39 pm
Excellent, Humphrey. Indeed, this maintenance was not easy. Best regards.
Gassan Ali
March 14, 2020 at 3:19 am
Great repair!
Waleed Rishmawi
March 14, 2020 at 2:42 pm
good investigating work and a job well done on the repair. have a blessed day
Yogesh Panchal
March 14, 2020 at 3:45 pm
Excellent! Fault Finding Sir.
Very critical to find culprits which get fail under load.
beh
March 15, 2020 at 12:52 pm
Hi Humphery
thank you so much for your very well written in technical fluent English words as well as very clear pictures and not very detailed.
cheers
Behzad
Tito Kanshulu
April 7, 2020 at 5:48 pm
Very well explained and experienced. HOT is a thorn on the fresh of many technicians especially when it is blowing time and again. It really gives headache.
Robert Calk Jr.
March 19, 2020 at 5:05 pm
Good job, Humphrey! I have got into the habit of checking every diode in the suspect circuit.
subhash
June 14, 2021 at 10:34 pm
I am facing h out hot get shorted immediately at the same time diode ru3am also get shorted. Sony 21 inch crt tv. Lot replaced osc tr replaced. Tried on similar picture tube suspected your.No use, can u suggest me some tips.