Smoking transformer, burnt PCB - solder question

[gjames]

I am a noob electronics hobbyist and have been browsing EEVBlog for some months now, soaking up everyone's wisdom and humor. But now I'd like to ask a question.

I have a 9-year old Phillips LCD TV whose power PCB started smoking. I inspected the board and found that one of the main transformer's terminals had burnt a 12mm hole out of the PCB and scorched a film capacitor sitting next to it. The trace on the reverse side of the board was also partially lifted off. After searching the web for a replacement transformer and failing to find one, I ran across a repair service that said they would look at the board and repair it for US$70.

Today I got the board back and all they did to it was bridge the transformer terminal across the burnt out trace with a wire and sealed it with silicone glue. They did not replace the transformer or the capacitor. When I questioned them on why they didn't replace the components, they said that they still work, there's no supply of replacement transformers, and the root of the problem is bad solder on the PCB. The board was made by LG and is approximately 10 years old.

My question: Is it reasonable that solder on PCBs can fail like this, or is it more likely that the transformer is going bad and the repair service is just trying to make a quick buck? Since they obviously see more examples of these failures than I ever will, they are in a better position to know what's really going on. But from my naive point of view, I would have thought that without replacing the transformer and capacitor, the board's (and TV's) future would be in the re-cycling bin.

Thanks,
Greg

[james_s]

Bad solder joints are extremely common and they can arc if the voltages are high enough. Transformer failures are generally very rare. It sounds to me like they did a correct diagnosis and reasonable repair job although it's hard to say for sure without seeing it. It's important to grind away any blackened material though as carbon is conductive. They are probably correct that replacement transformers are not available, especially for a TV that old. It's pointless to replace parts that still work.

[Chris56000]

Hi!

Yes - the switch-mode power-supplies in modern equipment handle large and very rapidly changing currents on a continuous basis, especially as most TV sets are now used for long periods of time continuously with the advent of 24/7/365 programming, something that was unknown when I was young!

These currents cause substantial voltage-drops across joints and conductors which causes heat to be developed in joints and transformer windings, made all the worse by the "lead-free" muck that passes for solder in manufacturing these days!

Once heat has started to develop, it causes more losses, and more heat, etc., in a "vicious cycle" that results in serious damage to the PCB and components on it - by the time failure occurs and is noticed by the end-user it's already too late!

Since you sent the PCB away for repair, you'll obviously have it to hand already removed from the TV, so I would make a note of the identification numbers on it and start trawling eBay, Gumtree, Craigslist, etc., for a replacement PCB - if you can find a set with a smashed screen or power-supply failure, etc., then starting off with another PCB and transformer, etc., would be the best way forward as we can advise how to repair it reliabiy!

I agree that the repair service might well say that "spare transformers are not available", maybe not from Philips, but for things like TV sets if you can't find an exact replacement, there's always "parts mules" and you can get transformers remanufactured on new bobbins/cores altho' the cost of this is uneconomical for consumer products!

This Forum, and the UK-hosted Radio & TV Discussion Forum, is dedicated to the upkeep and repair of valuable and well-made equipment that should not, in my opinion, be adding to the landfill, recycling and plastics problems that already plague our planet!

Chris WIlliams

[james_s]

It's far more likely that what happened was a simple matter of the solder joint developing a crack, especially common with the lead-free solder used now. A small crack with a sufficiently high voltage and you end up with an arc, the arc quickly melts the solder making the crack larger and starts transforming the PCB into carbon which is conductive and can then self heat. I've seen this happen lots of times on all sorts of different equipment. It's especially common on high voltage stuff like CCFL backlights and switching power supplies.

[CatalinaWOW]

Since you identify as a noob I want to clarify a little on the lead free solder issue.  While I am not a fan of lead free solder, it (and the knowledge of how to use it correctly) has improved a lot in the 9 years since your set was made.  The diagnosis for your set as presented here is probably correct, but don't use that as an automatic indictment of good quality current production.

[gjames]

Thank you James, Chris, and Catalina for your responses. This was very informative.

[abraxa]

While I agree with the assessments made here, I still would prefer to see a picture of the region you speak of. Not just for curiosity sake but also to make sure the board you put back into your TV set is actually in good health.

[gjames]

OK -here are some pictures of the repaired board (Part 1 of 3).

[gjames]

OK -here are some pictures of the repaired board (Part 2 of 3).

[gjames]

OK -here are some pictures of the repaired board (Part 3 of 3 - for component identification if anyone is interested).

[Gyro]

I think I'd want to change that 220nF 630V capacitor, the side of it looks badly heat damaged (first photo in second batch).

[james_s]

Wow, yeah I would at least remove it and scrub all the carbon off to really assess the condition. I would also remove the transformer and scrub off or grind away any carbonized material before coating it with epoxy or silicone. Carbon is conductive and there is quite a lot of it, and that's a high voltage transformer for the backlight, that is begging for a repeat.

[abraxa]

Hmm... quite frankly, that's a little scary. The cap should've been replaced for sure and that's not a custom-made part. The charring should've also been removed, like others pointed out. I'm also not sure if that is really silicone. To me, that looks like hot glue, which could be flammable. Can you poke it? If it's soft and bouncy, it's silicone - if it's solid, it's hot glue.

[Relayer]

Hello gjames,
I have to agree in regards to replacing that cap, as it looks like the
side facing the heat has buckled the casing.
I also agree that you need to remove the transformer with the cap
so you can remove all carbonization.
You're also going to need to see if any of the semi's have been shorted.
Regards,
Relayer

[Ian.M]

+1
The marked isolation barrier zone round the live section of the PCB has been compromised by the charring, so IMHO its may be an unsafe repair.  At the very least the transformer should have been removed and  *all* the char in the isolation zone  under it should have been ground away, and any carbonised PCB round the rest of the hole should also have been removed, and the heat damaged capacitor replaced.

I certainly wouldn't be happy to have paid $70 for repair work that may be unsafe if the top side charring under the transformer has reduced the creepage distance to the secondary side below the original design distance.

[james_s]

Well since it works we know that the semiconductors are all ok, but the charring remains as an issue.

[shakalnokturn]

That's a very common  problem on those TV's, the transformer is mounted using through-hole studs, they usually provide a reliable solution for mounting heavy components. Not in this case.

The transformer is for the CCFL supply, charring is on the primary side so not quite as critical for insulation as on the secondary (voltages are lower).

All the same for 70$ I would have gone to more bother cleaning the burn.

[bd139]

Honestly I’d see if you can find a replacement board at this point or the same model with a smashed screen and extract the power supply from that.