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820-3330 Dead short on PPBUS

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Ryanlen2

Member
Hi. Dead short on PPBUS and PPVIN_S5_HS_COMP. No liquid. Both rails beep to ground, OV. PPVCORE_SO_CPU, 1.8OHMS. CPU does not get warm when power is applied. I have PP3V42 - 3.42V. PPCDCIN - 16.25.
Should I hunt for the short or you think dead CPU already?

Thanks
 
smiba

smiba

New member
There is a fair chance the CPU is toast, if you have time for it you can try to hunt down which mosfet is shorting and replace accordingly, I've had about a 20% success rate on CPU mosfet PPBUS shorts where the CPU was not killed
 
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Ryanlen2

Member
Hey Smiba. Thanks for the advice. Good to know. How would you go about this? 1 volt voltage injection? Or just remove components until short is gone? Thanks man.
 
larossmann

larossmann

Administrator
Staff member
The common fault on this are the CPU mosfets for PPVCORE_S0_AXG, Q7550/Q7561/Q7551 area. If you inject voltage you will kill the HD4000. Just remove them. this is ivy bridge not haswell so it won't get killed from a bad MOSFET the same way.
 
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dukefawks

Administrator
Of course it will beep to CPU Vcore as that is low resistance too. The correct question is: Is resistance from PPBUS to Vcore lower than the short to GND on PPBUS? That would confirm a shorted high side FET on Vcore.
 
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dukefawks

Administrator
Well that means there is nothing wrong with CPU FETs and you don't need to pull them for that. That is just another risk factor added now.

Inject voltage on PPBUS and see what heats up. Also check GPU Vcore for short to GND and short to PPBUS as a short there is common too.
 
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Ryanlen2

Member
Hey Duke. Injected up to 2 volts on PPBUS to find Q8360 is getting hot. Didnt want to do anything else until I posted. Thanks, have a good night.
 
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dukefawks

Administrator
Well that sucks cause it is shorted and PPBUS will have gotten to the GPU and VRAM. So expect a dead GPU, 4x dead VRAM, dead U8350, Q8360/61. Minimum that will need replacing is U8350, Q8360/61. Pull them all off and measure if there is still a short on L8360.
 
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Ryanlen2

Member
Hey Duke, how are ya? I pulled components you advised and I now have .100 in diode mode on L8360. PPBUS in diode .436. Thanks man, have a good day.
 
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dukefawks

Administrator
Probably normal as there are a few voltage dividers still present on that line. Put back all known good components.
 
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Ryanlen2

Member
OK. So I should assume the CPU mosfets are fine and just put them back on. or get replacements from a donor? Thanks Duke.
 
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Ryanlen2

Member
PPDCIN fluctuating 16.78 - 16.3V. Causing PPBUS and S rails to fluctuate as well. PP3V42 is steady. Sometimes green light on charger blinks. 0V on PPBUS with only battery.
 
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dukefawks

Administrator
I would say PPBUS is being pulled down by excessive current. Could be bad soldering on the replaced FETs or any other line pulling this down. Liquid?????
 
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