M1 ATX Problem 5v Rail Under Voltage

**uhohrobbo** · 05-16-2007, 10:12 AM

I'm having issues with my M1 ATX. It has been working for just under a year. However, recently my carpc has not been booting properly (only boots about 20% of the time), 'no boot device can be found'. I have replaced the Hdd and still had the same issue so I checked the rails of the M1 ATX.

Both rails stayed constant when the engine was on, off and reving. However the 5v rail sometimes starts and stays at 3.9v to 4v resulting in me not booting. But sometimes it starts and stays at 5v.

I'm using a Via EPIA-M Mobo, and Lilliput 7" touch screen and a 2.5" Hdd. So I am well under the max 90Watts. I estimate my peak consumption to be no more than 25W.

Has anyone had this problem before?
does anyone have the schematic so I can find the problem?

Any help is really appreciated!

Thanks!

**MibMib** · 05-16-2007, 12:46 PM

I think it's a problem of dryed polar capasitors of 5v rail - find them,on PCB and try to change - use same voltage and capasitance , also see at marking - if they "low esr" type - use the same, if you'll change it by new same -be attentievly - check the polarity before istalling them !!!.

**Beehphy** · 05-17-2007, 10:42 AM

I've attached a PDF of the best "Low ESR" caps I've found. you can order any of them on digikey.com. just use the PN in this doc to search.

I found some info on the web about many batches ot bad electroltic capacitors being sold to computer manufacturers, causing premature failure of the products. Something about a stolen manufactureing process that's missing some inportant info causing them to fail early.
http://www.badcaps.net/

This may be a contributing factor...

**uhohrobbo** · 05-18-2007, 03:44 AM

Ok thanks for your suggestions. I have done a visual inspection of the board and everythink looks ok. Unfortunately as you probably know the board is multi-layered so determining which caps are on the 5V rail is tricky. Does any one have a schematic or pcb layout for the m1 ATX?

cheers

**MibMib** · 05-18-2007, 09:34 AM

Oh ...... you can simply use a multimeter to determine which one of caps

**uhohrobbo** · 05-21-2007, 04:49 AM

I have located the 5V caps. But the capacitance meter I have only goes up to 200uF which isn't very much considering the caps are 1500uF. But at least I was able to test that none of them had shorted.

I did notice however that the board ticks! Only very very quietly you would need all fans turned off etc but it definately ticks when it is turned on! Anyone noticed this before?

I guess this is the PWM feedback loop but I just want to check that all m1 ATX's make this noise?

**uhohrobbo** · 05-25-2007, 06:26 AM

Just to update. After probeing the board I found that Q8 MOSFET transistor has a 5v input, a 5v switching and only a 4v output. So it looks as though the MOSFET may have popped. I have a replacement IC on order and I shall let you know if i fix it!

I replaced the MOSFET over the weekend and it fixed the problem. Woohoo!

Thanks for you help!

**Fabius** · 04-20-2008, 03:35 AM

what type the mosfet are used to replace it??
i have the same problem, an q8 it's broken....

**uhohrobbo** · 04-20-2008, 07:15 AM

I just replaced it with the same mosfet. From what I remember the part number was quite clearly marked on the IC. One thing to note though is that I also had to replace my hdd. I guess the hdd died and started to pull too much current from the psu and popped the mosfet.

**Fabius** · 04-20-2008, 11:50 AM

yes, same conditions...
my MB doesn't work too, the HDD are fault and the mosfet q8 it's exploded!

from circuit i know that it's N channel mosfet and i temporary replaced it with irfz48V and it's work....but to make a better work it's wil replaced with same mosfet original.

what sign have IC marked?

FDS--????

thanks