laptop power fault: compaq presario C300EA

Perhaps the plague? No, not THAT one, THIS one:

Worth eyeballing since you already have the main board exposed.

The symptoms suggest a short on the mainboard, the PSU aren't able to regulate so their protection circuits are cycling them off as best they can (at the price-point of the design).

There isn't much you can do at this point except strip the mainboard down as much as possible. Disconnect hard drive, optical, memory, CPU (except if you take the heatsink off and it was heatsinking the chipset, you must put that heatsink back on and ensure it makes good contact with the chipset still), card reader, screen, inverter board. See if it will then "seem" (since you have no screen, watch the power LED(s)) to stay on. It may not, without a processor and memory. If necessary put those back in and retry. If it stays on, reconnect screen but not the backpanel lighting inverter yet. If it turns on and stays on, see if there is output to the screen by shining a strong flashlight on it.

If it works this far, an inverter failure is a common cause. If it didn't work at all up to this point, mainboard probably needs replaced. If it works up to some point in the middle, suspect the part (s) you added at that point. These days such a problem is typically handled by replacing the mainboard at a repair shop, then if that doesn't work they replace the next part and so on, till it's fixed or the customer refuses to pay for their diagnosed problem since often the laptop cost little more than the total repair cost.

Didn't think laptops used electrolytic capacitors...

What did you think they used for capacitors in the power supply?

They're surface mount, so you might not recognize them. Google is your friend....

In the external AC-DC brick/wart, yes there are certainly electrolytics.

In the interior of the notebook, only a cost-cutting design would have any, so yes it is possible but no better notebook would have any.

Yes, but, although surface mount caps can be electrolytic, in a notebook (none at all if it's a good design), very few caps are electrolytic, if any, except in the external brick AC-DC PSU.

Grasping at straws.

That's not remotely related to the problem is it?

I wouldn't buy any laptop using electrolytics other than in an external power brick.

The battery (or external supply) sources one voltage. The main board and peripheral components require multiple, regulated voltages. Therefore, somewhere there are several DC-DC converters to supply those voltages. Those regulators will likely use tants or low ESR aluminum electrolytics.

The OP is having problems with a "laptop power fault," therefore

Capacitors in the power stages are rather more likely to fail than inductors or (properly rated) semiconductors, therefore it's worth examining them for no other reason than crossing them off the list.

Check that it's not the Compaq model that used RAM for BIOS and dies when the backup battery gets low.

Bad ram.

That's complete nonsense, of course a laptop uses electrolytics, and lots of them to filter power throughout the unit. As jakedart said they're surface mount units. Take one apart sometime & look.

Jerry

Had a problem like that with an old Dell laptop. Measuring the resistance across the power input jack showed a direct short. After disassembling the unit I found an electrolytic across the power lines, after the rf filter & fuse that was shorted. Removed it and the laptop now works fine (so much for no electrolytics in laptops).

Jerry

I guess you'll never own one then ....

our

I have, several times. They use mostly if not entirely solid, (usually chip) caps, and ceramics with good reason. Electrolytics wear out too fast inside modern laptops because of the elevated temps, not to mention their height being a problem when engineering something as thin as reasonably possible.

Here's a picture of both sides of a quite typical HP laptop mainboard, under two years old and quite similar to what they're still using. Point out the electrolytic caps on it and BTW, this is the whole thing there is no separate power board:

The solid caps are black, and yellow. The ceramics of course are tan. There are plenty more pictures from 3rd parties available with a google search if you can't accept the above pics are typical:

By that I meant no internal power converter board that's separate, it does have a separate input jack board (also without caps, it's only a strip big enough to hold a couple jacks), and the brick AC-DC adapter separate as with practically all laptops in the last several years.

e:

our

False. The only place a typical modern laptop has electrolytic capacitors is in the AC-DC brick, and there typically 2 or 3. There might be one of those old build-your-own PCChips kits still running that has 'lytics, but that's because PCChips makes cheap junk and it was huge for what it was, IIRC.

rt

Now you know one of the reasons why they don't typically put electrolytics in laptops anymore. I never suggested no old laptop ever had any, but this is a pretty modern laptop not some ancient Dell.