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G4 Cube VRM Smoke

mattsoft

Well-known member
I picked up a G4 Cube and plugged it in and smoke started coming out of the top! No video on the screen, and the power light was flashing. I yanked the power and the smoke stopped. I pulled the core out of the case and on the back of the VRM, one of the tantalum (?) capacitors had blown clear off the board -- I later found it inside the case. Check out the pictures below.

So anyway, I cleaned off the damage with some alcohol and put the VRM back in because, why not? To my surprise, the Cube powered right up and appears to work OK. No more smoke. Unfortunately, the VRM has a massive crater in it -- 1 of the pads is blown clear off and there is some exposed copper.

Anyone have any idea what those caps are for? The Cube is running, is that capacitor even needed? Curious if I should be worried, or try to repair this, or just try to find a replacement VRM.

IMG_6236.jpgIMG_6235.jpg
 

ried

Well-known member
Bummer! Is there a CPU upgrade installed? These are known to fail when there's more power draw from upgraded components (CPU, GPU...).

Fortunately, used VRMs are readily available and you can still purchase a new Artmix VRM upgrade, if you're willing to pay and wait for it.

If there was no additional damage, you are lucky!

P.S. I would not run that Cube with a damaged VRM, because it can potentially take your logic board and GPU down with it.
 
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mdeverhart

Well-known member
I’m not familiar with the design of the Cube VRM, but best guess is that it’s bulk capacitance on the output of a switching voltage regulator, to help smooth the output voltage and respond to current spikes from the downstream load.

Unfortunately, the normal failure mode for tantalum’s (if that’s what it was) is to short, so you probably had a short from VCC to GND on that rail, hence the smoke and flashing power light. Once the cap blew off, the short was gone and the VRM could operate (mostly) normally.

Everything might be OK now, if the cap was just bulk capacitance. It’s possible you could see stability problems if current loads exceed the (now diminished) capacitance (causing monetary voltage droops). But, it’s also possible that the VRM is damaged and could damage the Cube. Given the relative scarcity of Cubes, it’s probably worth replacing the VRM if you can find one.
 

mattsoft

Well-known member
This is a stock 450MHz cube, so the VRM should not be overloaded. $200 for the Artmix VRM is crazy! That's more than the price of the Cube! Guess I'll try to find a lone, working OG VRM to replace this one. Bummer!
 

Unknown_K

Well-known member
Tants fail from over voltage which causes them to short so there might have been other issues in the VRM or just something like moisture or conductive material shorting it out (loose screw?).

Also when you bring something into a warm house in a cold winter let it sit 30 minutes or so before powering it up to let condensate evaporate.
 

Franklinstein

Well-known member
Looks like that was a ceramic cap, actually. They can also develop shorts as seen in several YouTube videos where people diagnose computer components that won't power on. Usually the short causes the power supply's protection circuit to trip before damage is incurred and the easiest way to find the culprit (unless it has gone so far as to have a visually obvious fault as is the case here) is voltage injection and playing "find the hot spot."

Anyway you could probably do with just a stock replacement VRM or possibly even repair the one you have if damage isn't too severe. You don't need the additional capacity of the Artmix VRM if you're staying mostly stock or with a single <1GHz G4 upgrade.
 

herd

Well-known member
I agree with the other posts so far. It was a blown ceramic cap. I would just keep using the Cube as is. The VRM upgrades are more for people who want to put high power CPU and GPU upgrades into a Cube.
 

mattsoft

Well-known member
Thanks all. just some clarity, there was nothing loose inside the Cube (like a screw) and no condensation, so no obvious short. That's not to say something else isn't faulty and caused a short, but it since it does boot fine now, I'm going to assume a failed tantalum. I still wonder what that cap is for and find it interesting that the VRM appears to be regulating power OK to the system. I've played some 3D games using the DVD drive with no issues, so the VRM appears to be providing the needed DC voltages OK. Anyway, if anyone has a spare VRM, let me know! :) Thanks!
 

cheesestraws

Well-known member
I still wonder what that cap is for and find it interesting that the VRM appears to be regulating power OK to the system

A lot of systems have more capacitors than they would normally need to accommodate various worst cases, or just because it's cheaper to only use one kind of cap even if it's bigger than necessary. For example, if you've got a good PSU, you'd be amazed how many of the caps on a logic board can be non-functional or just missing and the machine will still be basically fine. If I were you I probably would be trying to replace it, but that's honestly because I'm neurotic: if it's still working as it is you are probably realistically OK to keep using it.
 
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