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picturevalve
9 Posts
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November 5th, 2012 12:00
Dell Dimension 5100 intermittent switch on
I have read all the posts on this topic for the past few years. In short, it will not power up and the green led on the motherboard is lit. A psu tester suggests all voltages are correct ( off load). I suspect something on the front panel where the power switch lives. It's possible there is a time element involved before it will allow itself to be switched on again after a shutdown or hibernation. I tried today to switch it on, it refused so I removed the side cover to have a look. The psu on volts read 4.13v, which seems to be normal. So I sat down at my other computer for a read and suddenly the lights on the front panel lit up momentarily. The computer then allowed itself to be switched on. I have noticed that the lights normally flash like this with a normal "power connected" before a successfull switch on, but when it's not going to switch on there is no flash although you do get the dell logo up on the screen for a moment. So a few questions, does the psu fan kick when the power is first connected? I was too far away to see if it did this at the time the lights flashed. As the switch on the front panel is a momentary device does anyone know where the latching device, which holds down the psu on line, is located? Is it on the front panel or the motherboard? I am toying with the idea of making up a simple relay latching circuit with a momentary toggle switch connected across the appropriate lines for when the pc refuses to switch on. There's lots of nice holes in the vetilation grid at the back to mount it. Thanks in advance.
picturevalve
9 Posts
637
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Posted November 6th, 2012 02:00
No problem there when it does switch on, I'm wondering now whether the fault is charge related, which would explain the time delay before the pc will switch on. I haven't totally ruled out the psu which is why I asked whether the psu fan in these kicks on power application before switch on.
picturevalve
9 Posts
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Posted November 6th, 2012 03:00
No, it doesn't. When the pc is refusing to switch on the fan does not kick when power is applied to the psu.
Yes, if you unplug the system and allow capacitance to discharge, then when you plug it back in, the fan will run momentarily. That's normal. Does it start every time if you unplug and hold the power button for a couple seconds, then plug it back in?
picturevalve
9 Posts
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Posted November 6th, 2012 15:00
That's certainly a possibility and I may as well get a new one as this one is now 7 yrs old. I managed to witness the lamps flashing on the front of the pc today as I inserted the iec power plug and the psu fan did not kick but as the front panel lights flashed I knew it was going to switch on. So hopefully a new psu will solve the problem, otherwise it's going to be a new front panel or a homebrew switch on relay circuit. Thanks for your input, I'll let you know how it turns out.
picturevalve
9 Posts
637
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Posted November 13th, 2012 01:00
Well, got the new psu - OCZ-CXS500W as it was the same size, but a pain to fit as the original dell psu has got cutouts in the case to engage with pressed-out flanges in the chassis. (It also has a 3 year swap out warranty.) But it's in and so far it's been turned on and off quite a few times with no problems. The trouble is the original works flawlessly on the bench, no startup problems whatsoever. So I'm no wiser and will have a look inside the old unit later. Hmmmm.
Thanks again for the conversation.
picturevalve
9 Posts
637
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Posted November 13th, 2012 07:00
Opened the psu this afternoon and blew out all the dust and fluff - not too much really. Everything looked nice and clean with no sign of overheating, but there was one cap. right next to the main heatsink which was domed, this is probably the cause of the time delay switch on problem. I shall repair it at my leisure and put it back into service, I think. It looks very good for a 7 year old!
picturevalve
9 Posts
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Posted November 14th, 2012 05:00
No, it's quite a common problem with modern caps, and the domed top is a symptom of high esr with the cap getting warm due to the increased dissipation caused by the higher internal resistance. The cap in question is right up against the heatsink which has probably caused it to dry out over the years. The x mark on top of the caps is so that with the expansion inside the cap the top gives way, this stops any electrolyte leaking out of the bottom over the board ( well, in theory). A few people on here who have had the delayed switch on problem have probably got something similar in their psus.
osprey4
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34230 Posts
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Posted November 5th, 2012 16:00
Hi Picturevalve,
Do you see any indication of a power problem on the diagnostic lights?