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engle2308

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PC will not power up.

I have a self-built PC using an ABIT-KN8 SLI mother board. The PC became intermittent as to whether it would start or not and now it doesn't start at all. I follwed some advice from previous posts and found two things for sure: 1) if i jumer the green wire on the 24 pin connector to ground (Power supply to mother board connector), the system starts fine. I checked the power on switch that runs from the front panel to the mother board and it tests OK. I get continuty when I press the button. Release the button and continuity goes away, as it should. Next, I tried shorting the two pins that that power on switch connects to, still no power up. But, like I said, shorting the Power On pin (pin 14, green) to ground powers up the PC.

My question is, rather than pull the mother board to try and find the problem (and maybe create more problems), is it safe to just short pin 14 (green power on wire) to ground at the connector and leave it like that? A better solution, I think, may be to tie the power on switch to the Power on wire (pin 14) and ground. I may loose the functionality to be able to shut down the PC using the switch, but this PC stays on 24-7 except for an occassional reboot. I'm looking for an easy way out.

What is your advice?
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Andres Perales
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I would not try to short anything on your computer, unless you want to buy a new computer.  Take it all apart and put it back together and make sure everything is where it should be.
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engle2308

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I'm really for more technical info about the power-on and power hold circuits. I may have to go find a schematic but was hoping that there are some experts out there that have more detailed info about this circuit. Since shorting pin 14 to grd is a viable test solution, I'm not worried about having to buy a new computer. I have a background in electronic repair but haven't worked on PC mother boards and I'm not familiar with the circuitry for pwer-up/power-hold and the associatted shutdown commands that Windows or the hardware will pass.

A few more details, I'm running Windows XP on the box. The PS is 450 watts.
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aleghart
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aleghart: I'll give your suggestions a try tomorrow and post the results. Thanks the the details!

I was thinking that I have may have a hardware issue - a hair-line crack in the motherboard or a failed component in the power-up circuit. That's why I was wondering if I could bypass the circuit - which also gives up the hybernation function which is OK for this PC. I expect reset and shutdown would continue to work if I moved the switch between pins 14 and grd. I agree that this is not the best solution but if the mother board needs to be replaced anyway, I figured why not give it a try? But I better look at pulling the board if the first two suggestions don't work. Thanks for the details! I'll let you know what I find.
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nobus: Thanks for the comments. This is what I was hoping to learn.
But First...aleghart: I found a manual for  the KN8 SLI. It gives procedures for resetting the BIOS. I'll try them out as time permits and provide more feedback.  I'd rather do this before begin pulling the mother board.

I checked the bad caps site and will check for leaky caps. I've dealt with them in older TVs and Radar work so I'm familiar with what to look for based on what I found on the bad caps site. They mention a big problem with an ABIT cpu - an earlier model from what we have here but, none the less, a good warning to look for bad caps. These are good possibilities as being the culprits.
ok - have fun ! ...with hardware...
After receiving more information, I've decided to try replacing the Power Supply, as nobus suggested. I did not try to reset the bios yet as I found this out about the Antec power supply...

 A co-worker provided two pieces of information. One was a copy of "ATX12V Power Supply Design Guide", Version 1.3. Sections 3.3.2. PS_ON# and 3.3.3. +5 VSB are displayed in the code Snippet for reference. Also  this link provides some information about the Antec power supply (I'm not sure how reliable the information is so be careful how you interpret it):
http://www.pjrc.com/about/rambling/antec.html 

After getting this information, I checked the +5 VSB, which should always be present when the power supply switch is turned on (PC can be on or off). This is line that keeps the leds in the mobo lit and provides +5 V to the Pwr On# pin (the green wire, pin #14). The +5V VSB wire is the purple wire, pin #9 (sometimes labelled +5 V Standby) on the 20 or 24 pin connector going to the mobo. The voltage on the VSB read 3.3 volts under load and 4.8 volts when I unplugged the connector (no load). With that information and after reading the link to pjrc.com, I decided to buy a new power supply and try that first. The low voltage threshold explains why the system may work intermittently. I also noted that this voltage does increase to 4.8V after the sytem turns on (i.e. after shorting pin 14 (PC ON#) to grd momentarily).

Something else I learned (see snippit) is this: pin 14 is held hight (+5V) in its normal state when the system is on or off. It is only driven low to power on the PC. It also has protection for a mechanically operated switch. Therefore, I believe that the power on switch could be used as a temporary solution but I would not tie pin 14 to grd (i.e. not use the switch), as it is supposed to be held high by TTL logic. For a failing power supply or mobo, this perhaps may only have provided a temporary fix in my situation.

Also, when researching which power supply to buy, I read the reviews at Newegg.com. The Antec Neo Power 430 430W which sells for about $90 showed several reviews where the unit failed after 6 months. It only had 52 reviews in all.  I'm using an Antec 450W unit. Perhaps the higher end Antec is more dependable, I don't really know. Anyway, I settled on ordering the Thermaltake TR2 W0070RUC 430W ATX for about $35. This unit has 1849 reviews, 76% of which were 5's.

Will provide an update next week after the power supply arrives.





Source: 
ATX12V Power Supply Design Guide
Version 1.3
 
3.3.2. PS_ON#:
PS_ON# is an active-low, TTL-compatible signal that allows a motherboard to remotely control the power supply in conjunction with features such as soft on/off, Wake on LAN*, or wake-onmodem. When PS_ON# is pulled to TTL low, the power supply should turn on the five main DC output rails: +12VDC, +5VDC, +3.3VDC, -5VDC, and -12VDC. When PS_ON# is pulled to
TTL high or open-circuited, the DC output rails should not deliver current and should be held at zero potential with respect to ground. 
 
PS_ON# has no effect on the +5VSB output, which is always enabled whenever the AC power is present. Table 13 lists PS_ON# signal haracteristics. The power supply shall provide an internal pull-up to TTL high. The power supply shall also provide de-bounce circuitry on PS_ON# to prevent it from oscillating on/off at startup when activated by a mechanical switch. The DC output enable circuitry must be SELV-compliant. The power supply shall not latch into a shutdown state when PS_ON# is driven active by pulses between 10ms to 100ms during the decay of the power rails.
 
3.3.3. +5 VSB:
+5 VSB is a standby supply output that is active whenever the AC power is present. It provides
a power source for circuits that must remain operational when the five main DC output rails are in
a disabled state. Example uses include soft power control, Wake on LAN, wake-on-modem, intrusion detection, or suspend state activities. The +5 VSB output should be capable of delivering a minimum of 2.0 A at +5 V ± 5% to external circuits. The power supply must be able to provide the required power during a "wake up" event. If an external USB device generates the event, there may be peak currents as high as 2.5A lasting no more than 500mS. Overcurrent protection is required on the +5 VSB output regardless of the output current rating. This ensures the power supply will not be damaged if external circuits draw more current than the supply can provide.

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Here's a follow-up. The power supply arrived and is almost completely installed. I got to a point where I was able to turn it on and the PC fans started running immediately. Prognosis looks good but the final answer will come soon. Will provide final update - hopefully later today or tomorrow.

Here's clarifiation to a paragraph in my last post:
Something else I learned (see snippit) is this: pin 14 is held hight (+5V) in its normal state when the system is on or off. It is only driven low to power on the PC. It also has protection for a mechanically operated switch. Therefore, I believe that the power on switch could be used as a temporary solution but I would not tie pin 14 to grd. (clarification in upper case:) INSTEAD, I WOULD PUT THE SWITCH BETWEEN Pin 14 AND GROUND SINCE Pin 14  is supposed to be held high by TTL logic. For a failing power supply or mobo, this perhaps may only have provided a temporary fix in my situation.
Solution.
The power supply was the cause of the problem. I installed a new one and everything is now working. I noted that the old power supply used a fan speed sensor wire that was not included on the new power supply. This is not an issue. So we have one connector but all is well. I did see the fan speed lead included in the other power supply offerings either. The technology may have changed and this connector does not appear to be necessary in the newer PS. Anyway, I running without the connector just fine.

The complete solution to this problem is contained in all the comments and I suggest reading them all if you have a problem. Input from aleghart and nobus present excellant comments regarding this issue so I feel that I should split the points between both experts.

Many thanks to all. This has been a team project!