radial capacitor question

[brcisna]

Hello All,

Mr Newbie here,,,with firesuit,,,being put on,,,
Purchased an Dell Precision T7810 'as is',,,knowing the unit did not post. I purchased the box,,,from gentleman that bought the unit new.


Done the usual ,, remove /move ram,, remove one/swap cpu locattions,,,dual cpu,,, Put in new cmos battery.... soo,,,
The unit powers on,, the status light on power button is always white...no amber blinks, no beep codes . Never gets a display. The PSU,test will run the cpu fans full blast,,so i think the PSU is kosher.

After much wranging,,,probing,,yada,,yada,,, i thought it had a faulty electrolitic capacitor,,,, desoldered,,it,,,and after going back and forth,,come to find out,,,
If i probe the negative pad of the suspect cap location ,,,with ohmmeter,,,,and probe to ground pad on the mobo,,there is no continuity... I deslodered a cap sitting right next to this cap,,and that negative pad to a mobo ground pad has continuity.
Can anyone explain,,,what may be going on here.... Keep in mind,,,both caps are clear off of the motherboard,,,only probing the mobo,,,pads themself.

If i plug in a mouse the mouse blinks for a second then goes out. My usb voltage /amps checker shows 5 volts at usb.

As an aside,,,Have probed the mobo,,out of the case,,,,in diode mode,,,continuity mode,,and checking rail voltages,,, and cannot really detect anything funky other than this no continuity on the ground pad debacle.

Any ideas?

TIA

 

[ty_ger]

I can only presume that the ground pad has a bad/corroded/burnt up via where it links to the ground plane. Or, the ground could go through a FET that turns the ground on and off some times for some reason. Or, the "ground" side of that capacitor could go to the positive side of another capacitor for a series connection that is more fault tolerant. Or the "ground" side of that capacitor could go to a diode or resistor.

Who knows. Just guesses.
Have some pictures?

 

[eidairaman1]

Northridgefix might be able to help

 

[brcisna]

OK, Attach three photos,

The circled cap is the one that has no continuity from gound side pad WHITE side to the ground pad of the mother board. This is with neither of the two caps in picture on the board! I temparily dumped caps back on to try and check voltages earlier Black probe of DMM,,,to ground pad of cap WHITE TO red Probe of DMM stuck on the ground pad of the motherboard. 0L no continutiy
If i check the adjacent cap,,,negative pad of the cap,,,this negative pad to mobo ground has continuity.
I hope i am explaining this to be understandable.

 

[eidairaman1]

Well no continuity means you have an open somewhere, which could be a cracked/damaged trace or a damaged IC.

 

[Carillon]

If you want a chance at fixing that, you'll need to find the board schematics, which might cost you more than buying a whole new PC.
As ty_ger said above, the open line might mean anything.

 

[ShrimpBrime]

OK, Attach three photos,

The circled cap is the one that has no continuity from gound side pad WHITE side to the ground pad of the mother board. This is with neither of the two caps in picture on the board! I temparily dumped caps back on to try and check voltages earlier Black probe of DMM,,,to ground pad of cap WHITE TO red Probe of DMM stuck on the ground pad of the motherboard. 0L no continutiy
If i check the adjacent cap,,,negative pad of the cap,,,this negative pad to mobo ground has continuity.
I hope i am explaining this to be understandable.

So just jump a ground to the cap from any other ground point.

 

[Shrek]

When you power the board (as far as it will go) what are the voltages on each capacitor?

 

[brcisna]

@ShrimpBrime

That sounded like a good idea. I did the wire from neg cap pad to the mobo mounting hole. Now when i probe the two side by side caps,,the positive side reading with ohmeter is always open?
I can see the trace from the negative pad pf cap goes over to a gang of tiny resister clump,,,with caps. That is the only spot i see for a trace from the neg pad.. The trace is good over to the resistor it connects to. Almost certain,,this is like a two channel audio stack,,that the resistor is connected to. I just dont do this stuff,,,to get acclimated with what i should be seeing for multimeter readings.
I couldnt come close to finding a schematic.
Wound up just buying a used board for cheap on Ebay.. I would still like to figure this one out,just to learn something.
Without a schematic,,im not smat enough to narrow stuff down.

 

[Harlow]

When you identified the two electrolytics for replacement, were either of them "bulging", i.e. domed or split top, brown goo leaking out of top or bottom?

This tends to be my criterium for further investigation on a motherboard or PSU. If the top of the capacitor looks flat, I leave well alone. The risk of damaging the motherboard unnecessarily is too high when extracting the legs of the electrolytic.

A cheap ESR (Equivalent Series Resistance) capacitor test meter is probably a good investment, to check the electrolytics when you've desoldered them from the motherboard.

I have a growing heap of faulty/dead motherboards which I've abandoned as unrepairable (by me). If I didn't obtain the boards brand new myself, but bought them second hand on eBay, there's a good chance the previous owner failed to observe good ESD (anti-static) handling procedures and zapped the components with static.

 

[lilhasselhoffer]

Hello All,

Mr Newbie here,,,with firesuit,,,being put on,,,
Purchased an Dell Precision T7810 'as is',,,knowing the unit did not post. I purchased the box,,,from gentleman that bought the unit new.


Done the usual ,, remove /move ram,, remove one/swap cpu locattions,,,dual cpu,,, Put in new cmos battery.... soo,,,
The unit powers on,, the status light on power button is always white...no amber blinks, no beep codes . Never gets a display. The PSU,test will run the cpu fans full blast,,so i think the PSU is kosher.

After much wranging,,,probing,,yada,,yada,,, i thought it had a faulty electrolitic capacitor,,,, desoldered,,it,,,and after going back and forth,,come to find out,,,
If i probe the negative pad of the suspect cap location ,,,with ohmmeter,,,,and probe to ground pad on the mobo,,there is no continuity... I deslodered a cap sitting right next to this cap,,and that negative pad to a mobo ground pad has continuity.
Can anyone explain,,,what may be going on here.... Keep in mind,,,both caps are clear off of the motherboard,,,only probing the mobo,,,pads themself.

If i plug in a mouse the mouse blinks for a second then goes out. My usb voltage /amps checker shows 5 volts at usb.

As an aside,,,Have probed the mobo,,out of the case,,,,in diode mode,,,continuity mode,,and checking rail voltages,,, and cannot really detect anything funky other than this no continuity on the ground pad debacle.

Any ideas?

TIA

So...the simplest answer to this is that there are an infinite number of answers which we could provide you...and unless you've got a circuit diagram none of those infinite answers can be proven right or wrong. There are ICs that require a temporary ground...which a capacitor connected to ground would provide. The capacitor itself is relatively easy to replace...so if you're fishing for an easy fix it'd not be unreasonable to simply replace the capacitor and be good. Probing around with a multimeter is great...but it doesn't tell you what the intent of a circuit is.

I...have to ask though, what's the endgame? These computers seem to be something relatively cheap...given they're in the $300 range. Unless you're buying these at a hundred bucks or less you're going to spend more time reconditioning and repairing than these things are going to be worth.



If you ever have the itch to actually learn...there's a decent 101 understanding here: Capacitors in circuits

 

[brcisna]

*SOLVED*

The fix for this board wound up being an open 25.000 mhz crystal oscillator.
After replacing this oscillator the machine has run fine for 3 days now.

Thanks for all the suugestions.

 

[Shrek]

Impressive work