Motherboards Polyphase buck converter.Who start it?

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Hi ,
please,forgive the form of my English

i have to repair an industrial motherboard, model ITOX G4S300,
it is necessary to make a machine working,capacitors were already changed  

in recent past.
Disks diconnectd,CPU and memory replaced,tried with onboard and AGP video  

card,no video outputs,beeps neither
,the only sign of working are spinning fans .
+12V,+5V and +3V are present on ATX connector
The most noticeable thing is the lack of any PWM activity on the gates of  

each of 6 MOSFETS forming the buck converter, commonly placed close to the  
  
CPU.
The question is:
is this dependent only by the PWM controller itself or something wrong  
 from the BIOS (that of course i can't check) or from other hardware could  
  
stop the buck converter from working?
In other words,the PWM should run in any condition or not?

Many thanks for your help and your time spent reading this

Diego
Milan,Italy

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http://www.avg.com


Re: Motherboards Polyphase buck converter.Who start it?
On 1/21/2018 2:24 PM, blisca wrote:
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Were ALL the caps changed?  I once had a cap over by the video circuit
fail and cause the system not to boot.
Did the system function after that?
What were the circumstances leading up to the current failure?
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Did you check those supplies with an oscilloscope looking for spikes?
I've had circuits destroyed or weakened causing later failures
from voltage spikes allowed by the caps previously replaced.
I had one repeat failure from a cold solder joint on one of the
previously replaced caps.

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I don't think there's any chance that the cpu can run with no supply  
voltage.
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First thing I'd do is look up the specs on the PWM controller
chip, look at the circuits in the app note to see if there's
a control input and look at that with a scope.
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Did you check for source volts right at the PWM controller with
an oscilloscope so that you can see if the supply is trying to start?
Fuses in the path?

Re: Motherboards Polyphase buck converter.Who start it?

scritto:

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Thanks Mike
Many,but let me check again
  I once had a cap over by the video circuit
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Yes ,it did for months
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Normal use,the board is mounted in a rack,not much air,but 2 minifan  =

helping cooling
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Yes,no negative spikes tirggers the oscilloscope
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I will keep it in mind
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s
e

g


I'll check it,thanks


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Yes i tried but even at start i can't see any PWM attempt to start the  =

buck converter



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