G4 Cube VRM

According to the specs the LTC7819 has a maximum output voltage of 0.8 Volt to 99% of the input voltage Vin (4.5 - 40 Volt). So I consider 12 Volt not " high voltage". = 12 / 0,99*40 = about 30% of max.

Any suggestions for the best way to test an original VRM board? I have two replacements now for my “magic smoke” VRM I posted a bit ago. Should I look for shorts on the connector voltage pins in case there is a short on the motherboard? Or any way to test a board in isolation? I’d rather not plug it in and just hope for the best. Thanks everyone.

Thanks for the reply JDW. I believe evanboonie's start of a VRM replacement has the pinouts for the connector but need to reinstall Kicad to check.

View of the DC-DC board's connector with the plug holes facing towards you. A1 is in the top left corner of the drawing.

__A1_________connector___________ | 2 0 0 3 3 3 3 0 0 0 0 5 5 0 1 | | 2 2 0 3 3 3 0 0 0 0 5 5 5 0 e | ================PCB===========B15

Legend:
0: GND
1: 12V
2: 28V
3: 3.3V
5: 5V
e: enable

Look for a replacement for my blown C3. It looks to be a 150 uF 6.3v capacitor? Does anyone have a digikey/mouser/newark link for the proper replacement?

Tested out my blown VRM and the 5v rail is shorted. Not sure which MOSFET is bad since they are in shorted with a DMM. Hopefully it didn't nuke my motherboard or some other component.

Thank you!

someone on Reddit answered my question:

Panasonic TPB Series 150uF 6.3V Polymer Capacitor:

https://docs.rs-online.com/4155/0900766b814dbdb1.pdf

Kemet PS/K Series Neocapacitor, jE8 = 150uF 6.3V:

https://web.archive.org/web/2018012...uctCatalog/Attachments/728/neocapacitor_e.pdf

Hi everyone, I'm sure you're already familiar with this link, but it's a cheap and easy modification.

https://forums.macrumors.com/threads/open-source-power-mac-g4-cube-vrm.2405896/

I did it on a Cube and it works very well. It's powering a Sonnet Encore ST 800 and a Radeon 7500. For European users, you can find the parts at TME, and the heatsinks I used are these:

22x8x7mm With Pad
https://es.aliexpress.com/item/1005...st_main.5.21ef194dYANSFA&gatewayAdapt=glo2esp



It's worth the modification to prevent damage to the stock VRMs.

Thanks indibit! Do you have a surface mount soldering set up with a hot plate? Or did you manage to make these modifications with just a regular soldering iron. Evan talks about how hard it is to get the board hot enough. I was going to try with just a soldering station since the SMD parts are so large comparatively. I did order all the parts from Digikey recently to try to fix my smoked one.

jasperman said:

Thanks indibit! Do you have a surface mount soldering set up with a hot plate? Or did you manage to make these modifications with just a regular soldering iron. Evan talks about how hard it is to get the board hot enough. I was going to try with just a soldering station since the SMD parts are so large comparatively. I did order all the parts from Digikey recently to try to fix my smoked one.

Click to expand...

Hi, I initially wanted to do it with a preheater and a hot air soldering iron, but I didn't want to overheat the components I'd be keeping, so in the end I used a technique that might not be as professional, but one I've used many times to remove components. I set the soldering iron to about 310-320°C. I applied flux to all the pins and added leaded solder until the four legs on each side were joined. Then I grabbed the MOSFET with tweezers and quickly switched the soldering iron from one side of the MOSFET to the other while gently pulling upward until it pops out. I cleaned the pads thoroughly and finally added the new MOSFETs, also using the soldering iron.



The photo shows a different heatsink, the one I bought first, but when I saw that a taller one would fit, I opted for the one I showed you in the previous answer, but I don't have a photo.


Just a detail, from what I've read, when a Cube VRM fails, it usually takes down the motherboard, and I don't know if the processor as well. I upgraded preemptively to avoid that. I'm mentioning this because you might recover the VRM but find the motherboard isn't working. I hope this isn't your case and it works perfectly.


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