Flatscreen Monitor Repair

Hey guys,
I have an LG flatron monitor that the power supply crapped out on. I ended up changing out some capacitors and wah-la I now have power. Unfortunately, I get an error message saying that I have exceeded the frequency output of 60hz. It is now about 67. I didn't have a datasheet for the old caps, but I did honor the values (ex.1000uf/25v). I'm curious as to why the new caps would change the frequency output.

Thanks, David

I have no idea why this would make a difference in what I assume is the vertical [refresh] frequency, but, it's the only thing I can think of that is filter cap related (assuming the capacitors in question are filter caps (you left a LOT to the imagination ))

A capacitor has more "values" than just capacitance and max voltage. There is also ESR (Effective Series Resistance). For a linear supply this is not a critical value (unless what it is supplying has some sort of high frequency, high current demand. But, for a switching supply (which, again I'm making an assumption, here, based on limited information), the ESR is a very important parameter.

So, one theory is, that you didn't use low ESR cap(s) and thus the power supply is either not supplying the proper voltage(s) at the proper current(s) and that is causing some oscillator in the monitor to not oscillate at the proper frequency.

But, again, that a HUGE shot in the dark!

Try Changing the refresh rate of your computer. Most flat panel monitors only operate at 60 Hz..

It is my understanding that most caps that die in applications such as flat screen monitors, die because they have too high an ESR. I recall doing an accelerated lifetime study with a power supply I designed using various ESRs on the filter caps. The ones setups with higher ESR all failed earlier.

If the OP really did replace the caps with higher ESR, wouldn't you expect them to die sooner?

That makes sense. With a higher ESR I would expect the caps would get hotter (in a SMPS), thus the failure mode, right?

But...

"Sooner" than what?

And, what's an "OP"?

Sooner than the previous caps. That is to say, he might be doing this repair again, in the near future.

OP = original poster? Sorry, I will refrain from internet slang.

No problem, now I know what 'OP' means

Thanks for the insight guys. The caps in question are on the output side of the power supply. I have been unable to find datasheets for the originals and ended up replacing with some generic panasonic caps. Based on my limited knowledge and everything I've read, I think Steve has hit the nail on the head. I'm going to try some better caps with a lower ESR and see how it goes. This is more or less a learning experience so discovering the how's and why's makes for a good lesson. I think it would be interesting to compare the outputs with current caps to the outputs of caps with a lower ESR and see what the differences are. The originals lasted about four years with daily use.

Thanks again!