Replacing 900uF cap with 880uF?

[spleenharvester]

Hey all.

Posted this in the L300 thread but it seems pretty dead in there, and I have two of them waiting for a repair job. So gonna crosspost here if that's okay.

Anyway - for those not aware, these, along with many other Toshiba models, suffer from a failing proadilizer capacitor.

The cap in these two laptops is an OE907 - 2.5v/900uF/1.5mOhm ESR.

The usual procedure is to replace both these caps and the OE128s with 4x 2.5v/330uF/<10mOhm ESR.

I cannot find a single one matching on any of my scrap boards - all either have a too high ESR or 2v. I could always buy them but I cannot fathom why 4 tantalum caps cost £13 (~$19).

I do however have some 2.5v/220uF/9mOhm tantalums, quite a few.

Tl;dr: Surely a capacitance of 20uf lower isn't a major issue? Especially when there are people using a total of 680uF without an issue?

Cheers!

[spleenharvester]

Re: Replacing 900uF cap with 880uF?

Oh yeah I do have 4 of the 330uF 2.5v caps but the total parallel ESR would be 3.7mOhm which I guess is too high?

EDIT: Another idea, would 2x470uF do the trick?

[mariushm]

Re: Replacing 900uF cap with 880uF?

A slight change in capacitance won't matter that much. As long as you get close to the esr and ripple, you'll be fine.

[mockingbird]

Re: Replacing 900uF cap with 880uF?

I cannot find a single one matching on any of my scrap boards - all either have a too high ESR or 2v. I could always buy them but I cannot fathom why 4 tantalum caps cost £13 (~$19).

Because they're expensive to manufacture. You can also use Niobium Oxide caps, provided they are a series with a Low ESR. I've got a bunch of low ESR tantalums and niobiums I scored a while ago.

[spleenharvester]

Re: Replacing 900uF cap with 880uF?

Awesome, cheers.

Just one last question, the OE907 has ESR 1.5mOhm, would using 4x330uF tantalums with a total parallel ESR of 2.8mOhm be pushing it?

Seems a bit odd to me that the replacements have to be such low ESR, especially considering another board I have with the same CPU socket uses 4x15mOhm caps without a problem.

Cheers!

EDIT: Kinda tempted to just grab a few electrolytics and wiring them in, they're dead cheap.

[mockingbird]

Re: Replacing 900uF cap with 880uF?

You'd be hard pressed to find something with such low impedance... 2.8mOhm is more than fine. You're right, many people have used parts with much higher impedance, but I would say go no higher than 15mOhm to be safe. Electrolytics won't fit...

[spleenharvester]

Re: Replacing 900uF cap with 880uF?

FFS, replaced them today and it's gone wrong. One side seems fine, the other seems to have continuity, so I guess I'll be back out in the garage tomorrow.

At least it's proved the Tokin cap was at fault. Initially it would start up, display for one second then power off. Now it sits and stays on, but does nothing (0v in the tant caps).

Anyone got any tips for soldering these puppies? Seriously much harder than I expected, I've tried both a 15W antex and a 40W silverline iron and both are barely able to melt the solder, even with tonnes of flux.

[spleenharvester]

Re: Replacing 900uF cap with 880uF?

Just bought 4x EEFSX0E331XE 330uF 2.5v 9mOhm ESR caps for £8, finally a decent price.