Thank you to the guys at HEGE supporting Badcaps [ HEGE ] [ HEGE DEX Chart ]

Announcement

Collapse
No announcement yet.

Can a normal appearing capacitor still be bad?

Collapse
X
Collapse
 
  • Page of 2
  • Filter
  • Time
  • Show
Clear All
new posts
Previous 1 2 template Next
    #1

    Can a normal appearing capacitor still be bad?

    I acquired a Dell 171fp LCD monitor that will not turn on at all. Even the power light will not come on. After researching a bit I found that likely it is a dead power supply. Since this monitor has its power supply inside the case rather then some with external adapters a cracked it open to take a look. I read a bit about the common capacitors that go on it and for 9 bucks even ordered the power supply repair kit for the monitor which consists of 4 capacitors that are common causes. Sorta a ripoff but oh well. Anyway I looked at the capacitors on the power supply that normally fail. None of them are bulged or leaking. Is it still possible they can be bad?

    Thanks in advance,
    Adam
    Tags: None
    #2
    Re: Can a normal appearing capacitor still be bad?

    Yes, it is very common for caps to fail without any visual signs.

    But this could also point to other problems, not just the caps.

    Also, what brand/series caps came with this kit?

    Comment

      #3
      Re: Can a normal appearing capacitor still be bad?

      The capacitors in the kit are labeled Xicon.

      Oh and I tested the on/off button for continuity when pressed and it is working properly so I don't think its the button.

      Comment

        #4
        Re: Can a normal appearing capacitor still be bad?

        Wow.

        Xicon are known crap, 9 bucks was a major rip off.

        Some good panasonics (matsushita) or united chemi-cons are good choices, and can be had for much cheaper than that.

        Comment

          #5
          Re: Can a normal appearing capacitor still be bad?

          sounds like a scam to keep you buying kits over and over and over and over...
          sigpic

          (Insert witty quote here)

          Comment

            #6
            Re: Can a normal appearing capacitor still be bad?

            I realized I got ripped after the fact but now I just want to get this working so I want to narrow down the problem. If necessary I would eat the 9 bucks and get em elsewhere. Are capacitors sold by Radio Shack any good?

            Comment

              #7
              Re: Can a normal appearing capacitor still be bad?

              Radio shack dosen't sell caps that are suited for this application, or that are any good.

              One of the more experienced members here can suggest a good replacement for capacitors if you post the original capacitors makes/series here.

              Comment

                #8
                Re: Can a normal appearing capacitor still be bad?

                Xicon are not crap, they just don't make low ESR.

                Yes, that is/was a rip-off!

                Radio Shack sells mostly Xicon and they only have the 85c flavor.

                Usually Panasonic FC or FM from Digikey are suitable.
                If you'll give the make and series of the old caps I [or someone] can check specs.

                If the monitor is stone dead it may need more than caps.

                .
                Mann-Made Global Warming.
                - We should be more concerned about the Intellectual Climate.
                -
                Be who you are and say what you feel, because those who mind don't matter and those who matter don't mind.
                - Dr Seuss
                -
                You can teach a man to fish and feed him for life, but if he can't handle sushi you must also teach him to cook.
                -

                Comment

                  #9
                  Re: Can a normal appearing capacitor still be bad?

                  I changed the 3 specified caps that came in the "kit" I bought. Figured since I bought em may as well try em. Of course as i suspected it still won't power on.

                  Just as another point of reference. The person that gave me the monitor said it worked fine. No issues whatsoever. She said normally she never turns it off at the button, she just lets it go to sleep. But this time she turned it off at the button. The next day she went to turn it on and nothing happened. It was dead. Not sure if this may give someone more to go on but figured I'd mention it.

                  Comment

                    #10
                    Re: Can a normal appearing capacitor still be bad?

                    The other thing you might want to look at is the bridge rectifier. That is also a common failure with LCD's.
                    www.bcrelectronics.ca

                    Comment

                      #11
                      Re: Can a normal appearing capacitor still be bad?

                      Can you tell me where on this power supply the rectifier is located. Pic of power board is below but if you need a shot of the video board or the board that the LCD panel connects (I forget what its called) let me know. Also when the monitor is stone dead, no light on the button or anything can the rectifier still be the cause?

                      Comment

                        #12
                        Re: Can a normal appearing capacitor still be bad?

                        Well I figured out that the bridge recitifier is the black rectangular component with the 4 pins just to the right of the big capacitor in the picture. I ran a test on it with a multimeter using some test instructions I found online. When I test from the AC pin to the negative pin I get a 2.5 ohm reading. When I reverse the probes I get no reading. The same thing occurs from the AC pin to the positive. The probes show a 2.5 ohm reading but when I reverse the probes I get no reading. According to the test instructions if reversing the probes still showed an active ohms reading regardless of which way the probes were positioned that means that the rectifier is bad? So in my case it seems the recitifer is good. Am I right?

                        Comment

                          #13
                          Re: Can a normal appearing capacitor still be bad?

                          i am not 100% sure but you might be right. it could be failed in another way.

                          maybe the problem is the transformer. i have an elec. eng. friend that had a ps fail because somthing wen t awry there.
                          sigpic

                          (Insert witty quote here)

                          Comment

                            #14
                            Re: Can a normal appearing capacitor still be bad?

                            Hi amn70

                            Yes, I'd say that you're probably right.

                            If a monitor is "stone dead" it likely isn't the caps, or at least not the secondary caps that often fail. Those usually cause a "momentary on" condition where the monitor shuts down in a second or two and generally cause the light to blink.

                            First thing to check are any fuses. A blow fuse usually indicates other problems. Some fuses don't look like a fuse. For a clue, the board will usually carry a marking "F" as in F1, F2, F3, etc.

                            Second thing to look at is that large primary cap. Specifically, look for a loose connection at the solder joints. I've seen a couple of these where the small dab of glue gives way allowing that big heavy cap to hang on the leads and break the solder. Discharge it first!

                            Next, for a "stone dead" condition, check out the small cap(s) involved with the 5v standby circuit. If 5vsb fails, nothing else happens because that's what usually runs the Power switch. You could also check to see if there's any voltage at the power switch.... there might be +5v present.

                            You might want to look over the "Displays" section... there's a lot of useful information over there about LCD monitors

                            Good luck,
                            Keri
                            The More You Learn The Less You Know!

                            Comment

                              #15
                              Re: Can a normal appearing capacitor still be bad?

                              Also because of the way these are soldered in mass production the transformers often have bad solder joints.
                              With a casual inspection it looks like they are okay but upon close inspection there is often a crack in the solder all the way around the lead.

                              Someone poke eguevarae and Wizard in the ribs and tell them coffee's up.
                              They [and some others] do a lot more monitors than I do.
                              .
                              Mann-Made Global Warming.
                              - We should be more concerned about the Intellectual Climate.
                              -
                              Be who you are and say what you feel, because those who mind don't matter and those who matter don't mind.
                              - Dr Seuss
                              -
                              You can teach a man to fish and feed him for life, but if he can't handle sushi you must also teach him to cook.
                              -

                              Comment

                                #16
                                Re: Can a normal appearing capacitor still be bad?

                                Rat, if its the transformer whats the best way to test it?

                                Keri, I checked the two fuses, which on this board are marked FH1 and FH2 and I have continuity across both of them so they seem fine. The large cap seems fine. The glue is holding that thing solid. I can't move it at all. Solder joints look fine though do you think I should resolder them just to be sure? And by the small caps do you mean the ones on the video board that appear as tiny aluminum colored cylinders with a black crescent stripe on their tops and rest on a tiny black plastic base with two small solder points on each side of the base. There are quite of few of those on the video board were the switches connector also resides. I tested the switch connector and there is definitely 5v power going to some of pins. I had tested the switch for continuity earlier on and got continuity when depressed so it seems like it was working.

                                PCBonz, The transformer solder joints seem fine. Should I resolder them? Going back to what I asked Rat, would testing the transformer rule out bad solder joints?

                                Comment

                                  #17
                                  Re: Can a normal appearing capacitor still be bad?

                                  I may have found the problem. I probably should have tested this earlier.
                                  I ran an test with the power cord plugged. I started the test from the pin the feeds AC power to the board and followed the path with my tester. It appears I have power along the circuit until I get a certain point. The red line indicates successful AC power along the circuit. You can see along the circuit there is one fuse marked FH1 and is allowing AC to pass thru it and which earlier had tested OK for continuity. Where the red line ends is where AC power stops running. Subsequently the point where it stops are two blue shiny disc shaped capacitors, which I then tested for continuity. Both failed. Do you think I found my culprits?

                                  Nearby I noted another fuse FH2 which is outside my AC test but tested fine for continuity as well. Figured I'd mention it anyway.

                                  Last edited by amn70; 07-12-2009, 10:53 AM.

                                  Comment

                                    #18
                                    Re: Can a normal appearing capacitor still be bad?

                                    Hi Amn70

                                    I can't imagine a SMD cap on the video board causing a "stone dead" condition. Actually, most of the stuff on the video board could fail and you'd probably at least get a power light even if only for a second or so.

                                    I see that you have traced ONE of the two necessary 120v circuit traces.
                                    What about the other one?
                                    The one that appears to go through F12 and TH1?

                                    The power should stop at the caps C3 and C4 because those appear to go to the ground trace.

                                    I wonder if that TH1 thing didn't go open?
                                    Maybe some kind of current inrush limiter?

                                    Looks like there should be 120v across terminals 1 & 2 of L1? Or for that matter 3 & 4?

                                    Can't say that I would recommend testing 120v side when live,
                                    (except maybe sometimes)

                                    Play Safe,
                                    Keri

                                    PS. I'm learning this stuff too....
                                    Last edited by KeriJane; 07-12-2009, 02:09 PM.
                                    The More You Learn The Less You Know!

                                    Comment

                                      #19
                                      Re: Can a normal appearing capacitor still be bad?

                                      Well everyone. Problem solved. I should done this in the first place but somehow doubted it was involved. It was the little aluminum cap (I forget what type its called) supplied in the ripoff kit that came with the 3 other electrolytic caps that I did change. Pretty stupid of me to ignore this one. Monitor works like charm now.

                                      I appreciate everyones help on here. This was a learning experience for me. I am a self employed computer tech but no nothing about actually repairing circuit boards. This is my first attempt. Maybe now I will take a wack at repairing motherboards in the future.

                                      Below are the instructions supplied with the kit. The black arrow points to the cap I didn't bother to change until now.

                                      Comment

                                        #20
                                        Re: Can a normal appearing capacitor still be bad?

                                        Congratulations!

                                        It sounds like whoever made that kit is familiar with that monitor series.

                                        Usually the little aluminum ones are called "Polymer Aluminum Electrolyte" caps or just Polymer for short.

                                        It wold be interesting to know what that particular cap actually does.

                                        Have Fun,
                                        Keri
                                        The More You Learn The Less You Know!

                                        Comment

                                        Previous 1 2 template Next
                                        Working...
                                        X