Acer M230HML repair, white power LED but no image, bad caps in Main

[Chungalin]

This Acer 23" LED FullHD TV/monitor (M230HM) shows white LED on power button and it seems turned on, but no image on screen and it doesn’t respond to power button if you want to put it to Standby (orange LED).

All caps in PSU look fine and there’re 5V and 12V, but in this case the problem seems to be at Main board. There’re two small caps that are slightly bulged: C296 and C268 (220uF/10V Jamicon). They’ve even leaked something from below (only visible when you remove them). If check ESR while they’re installed they doesn’t look very bad, but same measurement once they’re unsoldered and it’s about 50Ω.

Replaced these two and it’s fixed.

[tom66]

Re: Acer M230HML repair, white power LED but no image, bad caps in Main

Nice fix. SMD caps do seem to fail from time to time. Are you sure that the stuff at the bottom wasn't glue? Sometimes big caps like that need to be glued down during reflow.

[Chungalin]

Re: Acer M230HML repair, white power LED but no image, bad caps in Main

They’re not SMD, they’re regular radial electrolytics.

Two years ago I had another unit of exactly same model with same symptoms, same bad caps, same leak below... but there was something else that prevented me from fixing it, some short elsewhere in Main board. I was afraid of the same story, but I’ve had better luck this time.

[tom66]

Re: Acer M230HML repair, white power LED but no image, bad caps in Main

Ah fair enough. It surprises me how many consumer electronic devices still use electrolytics. I avoid them as much as possible, and it's often cheaper to build devices that way as the small SMD electrolytics are quite expensive compared to ceramic capacitors.

[dskall]

Re: Acer M230HML repair, white power LED but no image, bad caps in Main

Are electrolytics and ceramic capacitors interchangeable?

[tom66]

Re: Acer M230HML repair, white power LED but no image, bad caps in Main

Are electrolytics and ceramic capacitors interchangeable?

Nope.

Ceramic capacitors have much lower capacitance typically, it requires a unique design philosophy to avoid electrolytics.

They have effectively zero ESR and ripple current isn't a significant issue until you get into the double-digit amp range. But their capacitance reduces as applied voltage increases, which makes stabilising some types of power supply difficult, especially ones with a variable output voltage. If assembled incorrectly, or if the reflow profile is wrong, they can crack during usage, which causes a short and often brings down the product. This is rare nowadays, as manufacturers are more aware of the handling and solderability of these types of capacitor.

With modern buck/boost regulators operating at very high switching frequencies ceramics work well. Because the frequency is so high, the larger capacitance offered by electrolytics is unnecessary. Electrolytics have pretty much been eliminated from most of my designs except where necessary. You will still see them on the output of AC "off-line" power supplies, and as bulk capacitors, as the frequencies there involved are much lower. A typical off-line SMPS power supply is operating at less than 100kHz so to get ripple down you need to use electrolytics. Nothing necessarily wrong with this, provided you design them right, electrolytics can last 20-25 years.

Tantalums used to be the go-to replacement for electrolytics in high-reliability applications, but they have been all but dropped from most electronics nowadays. They're a pain to design for, and are very fond of combusting if abused, which presents unique design challenges. You won't find many in consumer electronics. You might see them in high-reliability electronics (automotive, military, medical, etc.)

[dskall]

Re: Acer M230HML repair, white power LED but no image, bad caps in Main

Is failure of ceramics due to physical stress as opposed to electrical?

[tom66]

Re: Acer M230HML repair, white power LED but no image, bad caps in Main

Is failure of ceramics due to physical stress as opposed to electrical?

Usually. Mishandling during production or if the product is dropped/hit, it is possible for the cap to fail. They tend to be quite rugged to most force, but they are made of ceramic so are vulnerable to forces that would break glass or other types of fragile material. I've heard of smartphones that have been dropped with failed ceramic caps.

There are some cases of electromigration which causes failure sometimes. Basically high voltage stress causes highly charged particles to migrate through the dielectric layer, which can eventually cause a short. This is very rare, and only ever an issue if the rating of the capacitor is exceeded for some time, or if the capacitor is subject to an extreme environment like high humidity and high temperature.

On the whole, ceramic capacitors are extremely reliable, even electronic devices using electrolytic caps will generally contain hundreds of them in tiny values and large values alike. Failure is very very rare, and almost always due to bad design. Ceramic caps could easily last 100+ years. And as a further bonus, they're often cheaper when designed with modern chipsets which use high-speed switching frequencies and have fast control loops to accommodate them. The last time I used an electrolytic on a design for a power supply (aside from bulk filtering) must have been a good 5 years ago.

Fun fact: ceramic caps can "sing" when current passes through them, I have a piezo sounder circuit at work which uses one and when the main siren isn't connected you can still hear a sound coming from the ceramic cap near the output.

[dskall]

Re: Acer M230HML repair, white power LED but no image, bad caps in Main

Thanks for info. Really appreciate your input

[Chungalin]

Re: Acer M230HML repair, white power LED but no image, bad caps in Main

Perhaps I haven’t searched with enough interest, but I’ve never found a clear comparative matrix of features between all kinds of caps: electrolytic, ceramic, MLCC, tantalum, metalized polyester film, polystirene, polypropylene, etc.

I’m not electronics engineer, I just repair things. I try to replace capacitors with those I believe that are most compatible (keep same type and temperature rating, equal or a bit higher capacity and/or voltage rating), but that’s all what I dare to modify from original design.

[tom66]

Re: Acer M230HML repair, white power LED but no image, bad caps in Main

Perhaps I haven’t searched with enough interest, but I’ve never found a clear comparative matrix of features between all kinds of caps: electrolytic, ceramic, MLCC, tantalum, metalized polyester film, polystirene, polypropylene, etc.

I’m not electronics engineer, I just repair things. I try to replace capacitors with those I believe that are most compatible (keep same type and temperature rating, equal or a bit higher capacity and/or voltage rating), but that’s all what I dare to modify from original design.

I guess I can contribute a few ideas...

Electrolytic: High capacity and high voltage ratings for small size. Relatively low ripple current rating. Highest ESR of all types, including "low-ESR" electrolytics. Usually polarised: must be installed correct way around. Poor lifespan compared to other technology but if sufficiently over-rated these devices can last 30,000 hours+. Generally fairly tolerant to short term abuse, including polarity reversal and over-voltage, but excessive ratings can cause damage and venting.

Ceramic and MLCC: Same names for both technologies, virtually all ceramics are multi-layer ceramic capacitors. Medium to low capacity for their size (modern dielectrics massively improving performance), generally lower voltage ratings for MLCC, some parts are made with 1000V+ ratings but they tend to use obscure dielectrics and different construction. Virtually zero ESR and negligible temperature rise in use. Not polarised. Excellent reliability, tolerant to abuse and failure is very uncommon.

Tantalum: Kind of like fire in a box. Medium to high capacity for size, but modern ceramics are very competitive. These used to be the compromise when you need reliability but can't afford space or cost of lots of ceramics. Nowadays ceramics have almost completely replaced these parts. Very sensitive to abuse. Must be derated for temperature: a 16V tantalum capacitor can only withstand ~10V at high temperatures. Soldering can cause the capacitor to combust if done incorrectly. High ripple current, excessive voltage or reverse bias can lead to fire. ESR is lower than electrolytics typically, but higher than ceramics. Very good reliability if not abused, but completely intolerant to significant abuse with failure being extremely likely.

MPF & Polypropylene: High voltage devices intended for mains filtering generally. Rarely seen outside of applications requiring high voltage and safe operation.

Polystyrene: Obsolete technology, rarely seen nowadays. Generally very low leakage current but low capacitor values. Might see applications in precision oscillators and timing.

[dskall]

Re: Acer M230HML repair, white power LED but no image, bad caps in Main

So at least one of the Samsung main boards have failing 47uf tantalum as common failure. What would you use if you didn't want to replace with tantalum. Kind of high uf for ceramic I think. Promise this will be my last question

[tom66]

Re: Acer M230HML repair, white power LED but no image, bad caps in Main

Tantalum unless I analysed the circuit carefully and found I could use a ceramic (and you can get 47uF, if not two 22uF in parallel). The ESR of tantalum capacitors can form part of the stability of a regulator, so replacing that with ceramic could stop the circuit working. For better reliability I might consider a better manufacturer, higher rated voltage or high rel version.

[budm]

Re: Acer M230HML repair, white power LED but no image, bad caps in Main

The bad Tantalum caps that failed in SAMSUNG are NEC-TOKIN which also know for going bad in the PC also.

[attainteddragon]

Re: Acer M230HML repair, white power LED but no image, bad caps in Main

this is awesome when the info comes out so fluidly.

some of us are taking notes as it goes.

thanks guys.

[Chungalin]

Re: Acer M230HML repair, white power LED but no image, bad caps in Main

MPF & Polypropylene: High voltage devices intended for mains filtering generally. Rarely seen outside of applications requiring high voltage and safe operation.

These kind of caps are usually described as "safety", but why? They fail by slowly losing its capacity. "Safe" means that they rarely short?

[tom66]

Re: Acer M230HML repair, white power LED but no image, bad caps in Main

These kind of caps are usually described as "safety", but why? They fail by slowly losing its capacity. "Safe" means that they rarely short?

Effectively never short, always fail open. Tolerant to mains transients and much abuse.

Few reasons for this. Sometimes they are before the fuse, so short would be bad. Or in the case of class Y, a short could energise the chassis. They are designed to always fail open.

[budm]

Re: Acer M230HML repair, white power LED but no image, bad caps in Main

Safety caps: Read and enjoy.
http://powerblog.vicorpower.com/2013...-y-capacitors/
https://www.tecategroup.com/capacito...capacitors.php