Netvil DVB-T Receiver no power on repair
A few months ago the transmission of the analog TV signal has stopped in my country and the new era of the digital (MPEG 4) transmission had already begun.
In order for the old types of analog TV sets to be saved instead of being thrown away, we use these Digital Video Broadcasting Receivers, which reproduce clean analog video and sound signals after processing and demultiplexing the digital one, reproducing thus both of these original signals at their respective “scart” connector terminals located at their back side. Most of these devices are also capable of providing a HDMI output signal as well.
Although these devices are very cheap to buy them and thus perhaps not worth to repair them according to the opinion of many of the readers and apart from the fact that this case I faced really deserved repairing it, you already know that a defective device always triggers me for repairing it… For this one particularly you will see for yourself and understand why, by further reading…
Well, this unit you see below belongs to my close friend Mario, whom I visited recently and he told me that it suddenly stopped working. He could just see the green light behind its power switch flashing and nothing more.
Well, I took the unit at home to see what its problem was. You can have a general view of its internal circuitry below:
When I opened it the first thing I noticed was a slightly bulged electrolytic capacitor, which I replaced immediately. Its place can be better seen below:
It’s the one with the biggest diameter, a 1000μF/10V cap. Below you can see the original cap I replaced.
Well, as I was very busy that day, I asked my wife (who is familiar with these receivers as we use three of them at home) to connect it to a set of ours and perform a proper functional test on it. Then I left home. When I came back, my wife seemed to be scared from the experience she had with the task I delegated to her…
“This thing, she told me, in less than one minute of operation made a strong “bang” noise that scared me to death, producing also smoke and terribly bad smell”…
I thought momentarily that everything in its power supply circuit had turned into ashes, although the static tests I did before closing it didn’t show me anything else wrong with it.
During these thoughts of my mind, I heard her also saying in addition… “But it kept working normally after the “bang” and that smell. I just closed it and removed it from power because I could not stand the bad smell of it and of course I didn’t want to expose it further in a possibly catastrophic condition by keeping it working”…
What a relief I thought! But now the question of how it had survived after all these symptoms was wide open, making my curiosity about the cause of these symptoms soaring.
I re-opened the unit and it was really very easy to identify what the trouble was. Look at the picture below and you will understand:
They put during manufacturing that terrible glue to support the main filter cap in its place, which is conductive even when it dries out. If you can see in the picture, the part of it which touches the No 1 pin of the I.C is already black and this is what first caused the “bang”, starting to arch right afterwards and producing that bad smell!!
All of this petrified glue was standing above the tracks which connect the mains filter capacitor with the rectifier bridge!! The perfect way to kill the power supply circuit!! Given that on one hand this is certain to happen, being only a matter of time, on the other hand it is both a funny solution of supporting the filter cap in such a way and ironical too as it will end up in a complete destruction of the entire PSU section…Nevertheless it is a fact…
Many times in the past I found this kind of glue to be either a direct or indirect cause of many failures. Normally I always remove it from the PCB whenever I see it, but this time I didn’t because I was in a hurry!! And although this (hurrying) has always its own costs as a rule, fortunately it worked as an exception in this case…
You can see the tracks after I removed the glue using a sharp cutter.
Right afterwards I reclosed the unit and connected it to a TV set for a new test. There were no more problems with it and I believe Mario will keep enjoying TV-watching by using it, hopefully for a long time to come.
This article was prepared for you by Paris Azis from Athens-Greece. He is 59 years old and has more than 30 years’ experience in electronics repairs, both in consumer and industrial electronics. He started as a hobbyist at the age of 12 years and ended his professional carrier as a senior electronics technician. He has been a specialist in the entire range of consumer electronics repairs (: valve radio and BW TV receivers, transistorized color CRT TV, audio amps, reel and cassette tape recorders, telephone answering and telefax devices, electric irons, MW cooking devices e.t.c) working in his early stages at the official service departments of National-Panasonic first and JVC afterwards, at their premises in Athens.
Then he joined the telecoms industry, working for 20 years as field supporting technician in the sector of DMRs (: Digital Microwave Radio transmission stations), ending his carrier with this subject. Now he is a hobbyist again!
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Erik
October 28, 2015 at 8:15 am
Hai Paris,
Conductive glue , is that possible?
So that is not a electrical isolating of a kind.
What kind of materials are then used?
Strange.
wrt
erik
Paris Azis
October 28, 2015 at 4:20 pm
Hi Erik
Of course it’s possible! There is a high D.C voltage of a magnitude of 320V beneath it and things are critical in such cases. In any case this glue is not an epoxy type (see also my answer to Rui as well, shown below). The best supporting medium for these caps (in my humble opinion) is the hot silicone glue. (That one which needs a thermo-gun to use it).
Best Regards
Robert Calk
October 28, 2015 at 8:39 am
Good job, Paris. I always remove the glue also. I heat it up with my heat gun, then pinch it with needle-nose pliers to pull it off.
Paris Azis
October 28, 2015 at 4:07 pm
Hello Robert
Yes I remember you has written that somewhere else in your comments. It is very strange that they didn’t notice the problems it causes so far…
Regards
Gerald
October 28, 2015 at 9:19 am
Nice story Paris, very well written as usual. Thanks for sharing.
Cheers,
GM
Paris Azis
October 28, 2015 at 4:08 pm
Thank you too Gerald.
Rui
October 28, 2015 at 10:28 am
Great article! With great English! I would have never thought that the glue/epoxy would be conductive.
Paris Azis
October 28, 2015 at 4:12 pm
Hello Rui
Thank you for your positive comment. This glue they use is not epoxy at all. It looks like shoe-repairing glue to me, at least if I only judge from its appearance when it is dried up.
Best Regards
Derek
October 28, 2015 at 4:51 pm
Conductive Paint is available, so I see no reason why glue can be conductive.
I am surprised an electronics manufacturer applies glue that may have conductive properties.
Paris Azis
October 28, 2015 at 9:56 pm
Hi DereK
I feel the same about this glue. Nevertheless I have seen it used by many manufacturers and it caused problems even used in low voltage circuits. It is totally improper for electronics. I don't know of course what type of chemical substance it is and it is not my job to know that, but I remove it systematically whenever I see it on PCBs.
Best Regards
Muftah
October 28, 2015 at 1:34 pm
I thought this glu is not conductive, i will take care of it in the future. Thanks Paris
Paris Azis
October 28, 2015 at 7:15 pm
Hi Muftah
Unfortunayely this glue is improper for this use in electronic equipment. It always leads to unexpected surprizes...So remove it whenever you will see it and secure a trouble-free operation of the device that came to you for repair.
Thank you too for your comment.
Best Regards
Suranga Electronics
October 28, 2015 at 1:54 pm
Good Repair..
Thanks you, for Article.
Paris Azis
October 28, 2015 at 7:15 pm
Thank you too Suranga.
Best Regards
Anthony
October 28, 2015 at 2:40 pm
Hi Paris,
Nice photos and a good narrative to explain what is happening. I always enjoy reading your articles.
Kind Regards
Paris Azis
October 28, 2015 at 7:19 pm
Thank you too Anthony. You have always a warm comment to put whenever you see a new article of mine and I appreciate that.
My Best Regards!
Albert van Bemmelen
October 28, 2015 at 5:34 pm
Thanks Paris. In our country we have DVB-T transmissions for a long time now since about 2003. Analogue transmissions already long ago stopped. And while living here around 3 country borders, we can also recieve the transmissions from Belgium,Germany and even the France channels (for instance: La Une, La Deux and La Trois, EURONEWS etc.). But most channel transmissions are still in older compressions and or resolutions, and all our current DVB-T recievers here probably soon will be useless when DVB-T2 starts. It would be a real shame if the old transmissions disappear in favor of DVB-T2.
Paris Azis
October 28, 2015 at 10:01 pm
Shame or not Albert, things are changing rapidly no matter if we agree or not...
Besides none will ask your opinion about that. You simply follow...
Greetings!
Albert van Bemmelen
November 1, 2015 at 7:51 am
I am no follower of Fashion (haha). But the consumer also decides what stays and what not. VHS won, Betamax lost. Blu-Ray won, HD DVD lost. But I heard that in our Country, maybe even in whole Europe DVD-T2 will come in 2017. So DVD-T ends in 2017.
Yogesh Panchal
October 28, 2015 at 6:08 pm
good job! Paris.
yes here you can use "HOT GLUE GUN" am i right Paris?
Paris Azis
October 28, 2015 at 10:10 pm
Yes Yogesh
It's absolutely trouble-free method. And you know something? This endless effort of the manufacturers to reduce the manufacturing costs by any means without even thinking about how to do this in compatibility with reliability on their products, this hits my nerves! They are blind in seeing limits to it. And I find this to be one of all these abnormal things happening in our modern way of life...
But I cannot finally help it!
Best Regards
Robert Calk
October 29, 2015 at 6:46 am
As I understand it, they use the glue to keep components from vibrating loose during shipment. Also, the glue only becomes conductive after it gets old. It's easier to remove it when it is new, but few devices are ever opened up unless it stops working, and by then the glue is usually old, dried up, and of course, conductive.
Paris Azis
October 29, 2015 at 5:43 pm
Hi Robert
Yes they put the glue in order to protect generally tall and relatively heavy components from vibration. Usually this glue will be found at the bases of el-caps and at the bases of ferrite transformers. In the first case (el-caps), this glue is put there to protect them in case that they even slightly bend for any reason. There is always a risk in such a case of a break in the internal connection of their output terminals. In the second case (transformers) it is put there in order to avoid any breaks in either their terminals or the ferrite core itself in case the device is dropped down. Usually in this case the plastic core holder breaks at its base and then the terminals soldered to the PCB are found broken in pieces with their wires on top of them completely cut and the transformer’s body just hanging there. I have seen both cases many times so far.
I agree with your comment and moreover I think that this glue is already conductive from the time of its application on the PCB. In liquid form it is logically expected to be more conductive than what is when it dries out…But we are not a research & development authority. This job belongs to others and is their own responsibility. I only hope that they will read about these problems we report and publish and then take proper countermeasures…
Best Regards
Robert Calk
October 30, 2015 at 12:06 am
But the glue usually doesn't pose a problem until several years after the device has been operating. Anyway, the glue is a lot easier to remove after it's heated up first. Most of the time the heat will make old hard glue softer, and easier to remove.
Paris Azis
October 31, 2015 at 5:52 pm
Hi Robert
Although I agree with you, there is still no guarantee about this conclusion. If you also read the article Gerald suggested below, or if you already have seen it, there is a clear description there that this glue changes its color over time and comes to its worst condition when it is dark brown, almost black.
Now compare this with the relevant photo of my case. (This was an almost brand new unit). You will see that it had not even turned in brown color. It was still in a golden like coloring which means that it was in a premature stage, far earlier than its dangerous one.
Nevertheless it revealed the problem much earlier than expected (at least under the light of this thought you expressed). That’s why, in my opinion, it is not to be trusted and left there in the PCBs waiting its time to cause an explosion.
About its removal, I have always been trying it without heating the relevant area but I will copy your method since it seems to make things much easier than mine.
Best Regards
Henrique Jorge Guimarães Ulbrich - Brazil
October 28, 2015 at 11:39 pm
Aziz, thanks and congratulations for your maintenance reports, all of them written in a clean and correct English. Besides, the articles are very didactic too. It's interesting to note that many problems with electronic equipments are originated not eletronically, but due to mechanical, physical and chemical causes, among others. Things that the technical schools really do not teach... As regard to the "fantastic" solutions the manufacturers adopt, not only this glue example, but I can enumerate other vices, starting with the notorious case of low-quality elcaps and continuing with enclosures not screwed, but "crimped" in such a way that the attempt to open, even using the utmost care, sometimes renders the case to cleft or even brake. All of this hit my nerves too and have a damn name: planned obsolescence, which means: we are being robbed and the environment sacrificed.
Paris Azis
October 29, 2015 at 6:08 pm
Hello Enrique
Thank you too for your warm comment. I appreciate it. I also agree with you about the origin of many failures in electronic circuits and your saying about the didactic purpose of my references is right on the target. This is exactly what I would like from all the people involved in electronics repairs. It may encompass two components a) to use their imagination in the highest degree they can and b) to treat the device under repair with respect, with this respect reflecting to its owner. That’s all the prerequisites needed for a successful repair (as I humbly believe)…
About your specific reference on elcaps, hit “the capacitor plague” in Google and there is another great surprise for you to see there…
And finally your last phrase with that “planned obsolescence”, which I will keep permanently in my mind, just overkills me man! I really can’t stand it. This is really an embarrassment for the society we live in…
My Best Regards!
Agustin Lalu
October 29, 2015 at 2:30 am
Very nice infromation to be very careful at all time and not miss a thing congrats...
Paris Azis
October 29, 2015 at 6:09 pm
Thank you too Augustin.
Best Regards
Parasuraman S
October 29, 2015 at 6:53 am
A good satisfying job! As rightly mentioned by you, our job is to put back defective electronic gadgets to working condition! Well said! Best wishes!
Paris Azis
October 29, 2015 at 6:16 pm
Hello Parasuraman
Thank you too for your supporting comment. Yes this is our job. To bring dead equipment back in life. This looked miraculous to me right from my childhood, when a radio-technician came in our home to repair our ancient radio with those 90V huge BEREC batteries it used… Yes I remember it like it happened yesterday…And I believe that I was at the age of seven years old. Perhaps this event had sealed my professional future…
Best Regards
beh
October 29, 2015 at 1:14 pm
Hi Paris
thanks for sharing your experience.
regards
beh
Paris Azis
October 29, 2015 at 6:18 pm
Hello Beh
Thank you too for your supporting comment.
Best Regards
Gerald
October 29, 2015 at 5:41 pm
Hi Paris,
I found the following article regarding the "conductive" glue.
http://siber-sonic.com/audio/carnage.html
It explains it all!
Cheers,
GM
Paris Azis
October 29, 2015 at 11:27 pm
I have just finished reading it Gerald. Well, although unbelievable, it’s a fact. And the problem is that they keep on using it…Amazing!
Thanks a lot Gerald.
Greetings
Robert Calk
October 30, 2015 at 12:36 am
Excellent find, Gerald! Thanks.
Humberto
October 30, 2015 at 8:02 pm
Good job Paris, what a misfortune with the conductive glue. Thanks god the PSU did not stop working at all
Paris Azis
October 31, 2015 at 4:18 am
Thank you Humberto. Yes, I think that I was lucky this time...Otherwise I sould be waiting for long enough until a new PWM controller I.C comes from abroad after ordering it...
Greetings