Poor PCB design or implementation?

[andrew_c]

So guys, I've got a sweet tri-monitor setup which I've used for about 6 years now. Unfortunately a couple of the monitors have died along the way and I've managed to source cheap replacements of the bay of e. Well that could only last for so long. I've got 5 of these Mirai 19" LCDs now with three of them in pieces.

Without further ado, here are a couple photos.





Pretty obvious what is going on here. All three dead PSU boards in these monitors have bulging caps (Capxon) to go along with the charred AOP605 Mosfets. Am I right in thinking that the poor quality capacitors have gotten too hot, failed and caused ripples which the mosfets have 'struggled' with?

I think I'll get some replacement AOP mosfets and Panasonic FC caps on order unless anyone has recommendations for substitutes.

Cheers, Andy

[T3sl4co1l]

Maybe both.  Those things got toasty.  A race to the end?

And yeah, ripple reduces the average supply voltage, which would require it to draw more current (as long as it can hold regulation under that condition).  If it was marginal to begin with (which you can be guaranteed it was)...

And who even uses DIP MOSFETs?

Don't forget to check the controller.  FETs usually go three-way-short, easily putting a 200V+ transient into it.

Tim

[Alex Eisenhut]

Yes, I change electrolytic caps as a general rule in recent devices. Anything from the last 10-15 years, let's say. I have junk from the late '60s with big lytics that still work, so it's not a mystery how to make long-lasting caps; people choose to build crap.

I haven't done any measurements but I suspect as the caps age, they cause high leakage currents to flow in them. Currents which depend on the working voltage, ie you won't measure anything too bad at 5V, but at 400V you will.

Further, I think that the crimping of the pin to the foil inside also gives up, probably because there isn't enough/any corrosion inhibitors in the cap.

Cap goes, transistors fry, diodes short, fuses blow. Just make sure there isn't any charring on the pcb, that stuff is conductive. You can route out a channel, that takes care of that.

And who even uses DIP MOSFETs?

Tim

Low cost assembly, all single-sided stuff made with through-hole parts and wave soldering. You're not going to go out of your way to add a surface mount part in that case. ("case". ahah)

For example, in my Garrison fan repair story, the DC supply that feeds the controller uses a weird and wonderful part called the LNK362 in DIP.

You may be worried about thermal resistance of the DIP case, they get around that by using 4 pins for the source.

[andrew_c]

Thanks a lot for the replies guys, really helpful stuff!

Don't forget to check the controller.  FETs usually go three-way-short, easily putting a 200V+ transient into it.

Which controller are you referring to? After those FETs it's just a transformer then the backlight. All three broken monitors work fine when I swap out this power board.

Are we happy with the Panasonic FC caps? They look to be low ESR. Also, as far as I can see, the AOP605 has been discontinued.. is there a cheap replacement for it? I've found some on eBay which I could take a punt at, I just don't want this to happen again for a few years.



Cheers, Andy


P.s. Spot on sir

You may be worried about thermal resistance of the DIP case, they get around that by using 4 pins for the source.

[amyk]

What voltages does it need? If it's 5V, 3.3V, and/or 12V, then consider getting a standard ATX PSU (or bringing out the cables from the one in your PC) to power them -- no worry about any more bad caps in the monitors that way, and as a bonus they switch on with the PC. If the inverters are just free-running oscillators, you can replace them with standard CCFL inverters intended for case lighting etc.

[mamalala]

Yes, I change electrolytic caps as a general rule in recent devices. Anything from the last 10-15 years, let's say. I have junk from the late '60s with big lytics that still work, so it's not a mystery how to make long-lasting caps; people choose to build crap.

That's not completely true. You can not compare between a mains transformer + rectifier + smoothing cap, usually resulting in 50/60 Hz ripple from the sine halfwaves, and a switchmode supply, where you get frequencies in the high kHz (up to few MHz sometimes), with almost rectangle waveforms. The former is far less demanding on the cap, usually has lower ripple currents too. The latter is really demanding, and has usually high ripple currents.

It is only true insofar that choosing cheap caps in a switchmode supply is a receipe for "early" failure. Crap like Capxcon, Samwha, etc. are know to fail early. Use good Rubycon, Nippon Chemicon or Panasonic cpas, and rthey should last considerably longer. But even then, just the brand doesn't cut it, you need to use the proper cap for the given application. In switchers you want low impedance and high ripple current.

Greetings,

Chris

[andrew_c]

I've gotta admit, my knowledge of ccfl backlighting in monitors isn't too amazing. I've pulled all of the AOP605 microchips and all six have a short between pins 3, 4, 5 & 6. Not sure how to diagnose a p-channel mosfet though...

Any clues?

[johnwa]

If the inverters are just free-running oscillators, you can replace them with standard CCFL inverters intended for case lighting etc.

Yes, this is probably the way to go. I had a monitor with a dead backlight a while ago, and I did try new MOSFETs etc, but in the end I just put in one of these:http://www.ebay.com.au/itm/1pc-4Lamp-Backlight-Universal-Laptop-LCD-CCFL-Inverter-10-30V-For-15-26-Screen-/261440782697? They have on-off and dimming inputs, which were compatible with the existing circuitry.

At that price, it is not really worth trying to troubleshoot the existing circuitry. (though it would be considered a bit of a bodge if you were doing it for profit).

[andrew_c]

Nooo, certainly not for profit. I just want my monitors back!!



I placed my order for enough caps and AOP605 chips to "fix" all 5 monitors. May as well replace the caps on the "good" boards as well.

[andrew_c]

DIDN'T WORK! 

So, nothing has made any benefit at all. Replaced all the capacitors with Panasonic FC's. Replaced the AOP605s. Replaced a blown fuse (F200). Checked another fuse F201 and a resistor R100. Voltage to AOP605 chips is spot on at 19V. CCFLs work when I use a spare power board, so I must be missing something!

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