*SOLVED* electrolytic cap charges both neg & pos  - Page 1
Forum section: electronics / beginners

author: brcisna
author_rank: Newbie
timestamp: August 22, 2024, 08:46:29 pm
content: Hello list,On a Dell motherboard 'Precision T7810 that doesnt post,will power on fans run ,,,have found an electrolitic capacitor that if i proble BOTH negative and postive solder joimts,in ohms mode in the DMM ,, the cap increments up. Have replaced this twice with new cap.All the other caps in this mode,,,the cap will increment up in ohms mode on ONLY the positive side,,,and when probing negative solder joint get 2-3,,milliamps.This cap is located close to audio jack. There are two 100 microfarods caps side by side. The second one of these caps side by side reads as expected with ohmeterIs there ever a scenario where a electrolytic cap with increment up,,,on both neg & pos with DMM in ohms mode.Looked pretty hard for mobo schematic,,,but could locate one.As an aside,,,when i plug in usb mouse the mouse blinks for one second them light goes out... Just acts like a cap,,,is not pumping up enough.No caps are bulging on the board.TIA

author: CaptDon
author_rank: Super Contributor
timestamp: August 22, 2024, 09:41:35 pm
content: Don't know how you learned to test capacitors "But that ain't it". Out of circuit most electrolytics will charge from a meter in either direction. Your in circuit test is bogus because in reverse direction probably a lot of the clamping diodes begin to conduct when reverse biased and has nothing to do with electrolytics testing funny when probed backwards. As for the audio capacitor it is probably a coupling cap and I would expect it to charge in either direction. You replaced it twice and it still measures the same, that telling you anything? Would the results change if you replaced it four times?

author: brcisna
author_rank: Newbie
timestamp: August 23, 2024, 01:18:21 am
content: @CaptDonOk,Thanks for info . So am curious multilmeter is in ohms measuring mode why does the adjacent cap get totally different results .I realize this isnt acurate... If this is coupling cap,,,would the values not be identical between the two caps...?All the other caps i check on the board (electrolytic) check very suilar regardless of microfarod rating,,,, even the one adjacent to the suspect cap ...which is same microfarode value.You are dealing ,,,witha lowly underling here,,,,

author: CaptDon
author_rank: Super Contributor
timestamp: August 23, 2024, 01:19:58 am
content: If the motherboard won't boot I am guessing you don't get any of the diagnostic beeps? That indicates some fundamental problems to look into. Measure all power supply voltages and processor core voltages. Check for a stuck reset line. Check the master clock oscillators, there may be several. If possible, remove the RAM and try to boot the unit and see if you get a series of beeps. Does the screen show any form of activity?

author: CaptDon
author_rank: Super Contributor
timestamp: August 23, 2024, 01:34:11 am
content: Did you use this computer when it still worked or was it something that you got that wasn't working when you received it? Is the CMOS battery dead or is there any corrosion around the battery causing conduction between traces? Are there any signs of something being spilled into the unit? It sounds like the processor is being held in reset or the master oscillator has failed. It does sound like you have 5VDC at the USB port since the mouse blinks. That would be a normal reaction for a USB mouse. Most sensitive devices today have protective clamping diodes to prevent damage but probing around hap-hazardly is not a good idea particularly causing reversed voltages.

author: brcisna
author_rank: Newbie
timestamp: August 23, 2024, 12:23:48 pm
content: @CaptDonBought this old box 'as is' knowing it would not post.This is a dual processor setup,,,and have taken ram sticks out,,,swapped cpu's etc,,,before diving deeper. The cmos battery was dead,,,dropped a new cmos bat into it. If I take all the ram sticks out of the board it always has the OK white light on the power button. No beep tones whatsoever . The gentleman i bought this from bought it new. He said he paid around $2800 USD for it , 10 years ago !.Update: After lifting the negative leg of the suspect electrolytic cap... if I probe from the negative pad of the cap to ground ,,,no continuity (Never even thought to check this when i replaced this cap X2) . And NO,,,the pad is not messed up,,,,from soldering. If i lift negative leg of adjacent cap that pad does have continuity to ground What could this mean? This is a single layer mobo, just for completeness.With no schematic,,is hard to picture what is going on. FYI,,,Googled high & low,,,but can not find one good site to even buy a schematic PDF,,,from

author: perieanuo
author_rank: Frequent Contributor
timestamp: August 23, 2024, 01:37:39 pm
content: the cap can be for -12v rail

author: perieanuo
author_rank: Frequent Contributor
timestamp: August 23, 2024, 01:41:15 pm
content: btw, put the ram back, a mobo needs processor and ram to start, then check all voltages on connectors from power supply. if they are all fine, dig into mobo power rails,regulators, check every power rail for short first with diode tester and power connectors disconnected and tell us rail by rail the result

author: brcisna
author_rank: Newbie
timestamp: August 23, 2024, 02:06:41 pm
content: Quote from: perieanuo on August 23, 2024, 01:37:39 pmOk so if this cap is associated to negative 12 rail i will NOT get continuity from ground pad of this cap to ground pad of motherboard.?I have probed the board till im cross eyed with the multimeter in diode mode,,ohms mode,,,cant not see any shorts voltages appear correct if i thorw the boad back into the case. Dell of course has propritery power plugs 24 + 10 + 10,,,,so i have to set back into case,,,,cant hang on a desk,,, to conventional PSU.Also,,failed to mention,,if i do a self power test of the PSU,,,,it will run the CPU fans full blast so i think,,the PSU is copacitecI dont live and breathe this stuff like many people do,,,so takes me a while to get the jist of this stuff.Should just buy a used board Ebay,,,but like to figure this one out.TIA

author: CatalinaWOW
author_rank: Super Contributor
timestamp: August 23, 2024, 02:53:12 pm
content: If the cap is for the -12 v rail the + terminal of the capacitor will be connected to ground.The ohms test of a dmm puts a fixed current through the device being tested and measured the resulting voltage. If the device is a capacitor the current causes a linear (with time) voltage increase until one of a couple of things happens. The dmm times out, it reaches an internally set voltage limit or the capacitor reaches its breakdown voltage. When charged in reverse an electrolytic capacitors breakdown voltage is not specified and is always much lower than the specified forward voltage. I've never heard of any relationship between the reverse breakdown of an electrolytic cap and it's suitability for use. Such a connection may exist but it isn't well documented.

author: brcisna
author_rank: Newbie
timestamp: August 23, 2024, 03:12:18 pm
content: Quote from: CatalinaWOW on August 23, 2024, 02:53:12 pmOK Thanks for feedback,I think i dont explain what I am seeing very well.OK,,I have the suspect bad cap location the capcitor is NOT on the motherboard ,,If i set my multimeter to ohms...if i touch the ground probe of the multimeter , to the screw pad of the motherboard ,,,and the red probe of the mulitimeter to the ground side pad of the cap location,,i get 0L,,,NO continuity.It's that simple.How can this be,,?,,from what someone else mentioned "This could be -12 volt side capacitor,,regardless,, i cannot picture what this has to do with no continuity(with no capacitor in place on motherboard from the ground pad of the cap on the motherboard to the ground pad to the screw mount pad of the motherboard,Adding: I can see the negative trace from the cap goes over to a small ceramic cap,,,then goes to a diode,,,from there im lost... IFrom what i can tell these two compnents ohming them,,,appears to not be 'open"...Without a schematic,,,it is very tough,,,,and im not that smart in the first place.I know much of this is hard to picture with no pictures..TIA

author: CatalinaWOW
author_rank: Super Contributor
timestamp: August 23, 2024, 06:38:42 pm
content: What do you mean by the ground pad of the cap on the motherboard? Often capacitor pads have shape identifiers to show where the positive lead of the capacitor is to be connected. Often square for positive and a circle for negative. If this capacitor is serving the -11V bus, the square pad would be connected to ground and the round to -11V. There would be high resistance between the circular pad and ground.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: August 23, 2024, 07:10:00 pm
content: There should be some type of debugging header. Probe it with a scope for activity.

author: brcisna
author_rank: Newbie
timestamp: August 23, 2024, 07:32:53 pm
content: Quote from: CatalinaWOW on August 23, 2024, 06:38:42 pmOK 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 continutiyIf i check the adjacent negative pad of the cap,(to the right) ,,this negative pad to mobo ground has continuity.I hope i am explaining this to be understandable.TIA

author: CatalinaWOW
author_rank: Super Contributor
timestamp: August 23, 2024, 11:17:14 pm
content: Did you check continuity from ground to the other pad?

author: brcisna
author_rank: Newbie
timestamp: August 24, 2024, 03:51:21 am
content: Quote from: CatalinaWOW on August 23, 2024, 11:17:14 pmThank You for the idea...No I did not check continuity between each of the two caps negative pads. I just now checked,,and NO continuity....Opps i misunderstood your question,,,,,no,,,on the "positive" pad,,,,to ground no continuity to ground,,,sorry...What would this indicate?... I am out to lunch on this for ideas... ..Thanks again

author: CaptDon
author_rank: Super Contributor
timestamp: August 24, 2024, 01:18:19 pm
content: Why do you assume every capacitor must have one of its leads connected to ground??? Audio coupling capacitors would almost never show connection to ground. The mobo failure to boot is about 99% unlikely to be a capacitor issue. Start looking for other problems. Testing capacitors in the ohms mode is pointless unless checking tantalum capacitors for shorts or low ohmic leakage and testing most capacitors in circuit is also pointless. My comment about removing the RAM was dis'd by another poster so let me tell you why I made that comment. There is a possibilty that the RAM had failed with a hung up data or address bus. On most FUNCTIONING motherboards if you remove the RAM and power up the mobo you will get a series of beeps that indicates the mobo IS FUNCTIONAL and has encountered a RAM failure, which would be obvious since the RAM is missing! But at least it shows the mobo HAS LIFE!!! His comment "put the ram back in because the board can't run without it", well that's a no shitter dick tracy. My comment about removing the ram was to help get through a POST self test and let you know if the board had any form of life in it. With the serious lack of knowledge and lack of equipment to properly test this mobo you may get more enjoyment by using it as target practice.

author: brcisna
author_rank: Newbie
timestamp: August 25, 2024, 02:02:54 pm
content: @JustMeHere,I do see the debugger header,,,but what should i see if i start probing the various pins on the debugger header with multimeter .I did in fact try and find something out on this,,but see no info in regards to dell precision,,, diagnostics matrix.Wound up buying another mobo on Ebay for cheap,,,but would like to figure out what problem is with this board just to try and learn something is all.TY

author: tooki
author_rank: Super Contributor
timestamp: August 25, 2024, 09:41:38 pm
content: Quote from: brcisna on August 23, 2024, 12:23:48 pmThat’s not even possible. What would make you think this?!?

author: perieanuo
author_rank: Frequent Contributor
timestamp: August 26, 2024, 11:50:07 am
content: Quote from: tooki on August 25, 2024, 09:41:38 pmop assumed some things, we pointed out some errors in his assumption, now it's his part to understand how/where the cap is connected and so on

author: brcisna
author_rank: Newbie
timestamp: August 30, 2024, 12:33:24 pm
content: *SOLVED*The fix for this board in the end wound up being an open 25.000 mhz crystal oscillatorAfter replacing this oscillator the machine has run fine for 3 days now.Thanks for all the suggestions.

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: August 31, 2024, 12:40:26 am
content: Interesting, but it would be more so if, how you determined the crystal was at fault and what led you in that direction?Jeff