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Tivo Series 3 - Bad capacitors in power supply

Discussion in 'TiVo Help Center' started by johnsom, Jul 25, 2011.

  1. unitron

    unitron Active Member

    16,389
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    Apr 28, 2006
    semi-coastal NC
    The recently deceased steve614 provided us with an excellent photographic example of how subtle the bulge can be

    [​IMG]

    http://www.tivocommunity.com/tivo-vb/showthread.php?p=8824333#post8824333


    Anything other than an absolutely ruler flat top of the capacitor counts, and it's also possible for a cap to still be flat but going (or gone) bad.


    Even if you don't currently have a power supply problem, as long as you've got the thing opened up, you might as well go ahead and replace all the usual suspects and save having to do it in a few months from now.
     
  2. squint

    squint New Member

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    Jun 15, 2008
    I encountered my first non-capacitor-related power supply failure yesterday. An AcBel power supply from a 652 failed to spin up the HDD but would power the fan and some lights. Swapping out the power supply got it running again. I did a quick check of the power supply and saw no obvious failed capacitors but I did notice that the large heatsink along with the MOSFETs and Schottky rectifier bolted to it had a lot of play. Re-soldering the heatsink and the 3 components bolted to it got the power supply working again.

    I also saw my first 648 power supply without a bulging C701.
     
  3. nooneuknow

    nooneuknow TiVo User Since 2007

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    Feb 5, 2011
    Cox Cable...
    I've seen the heatsink fault as well. Due to the heat-sinking qualities (and other qualities) of a heatsink, the often minimal amount of solder that is there can crack in shipping, during handling, and just fail from thermal cycling (from periods of not having constant heat, from constant operation).
     
  4. Thos19

    Thos19 Member

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    Dec 31, 2002
    Rockland, MA
    Deceased??? :(
     
  5. unitron

    unitron Active Member

    16,389
    2
    Apr 28, 2006
    semi-coastal NC
  6. Lcstyle

    Lcstyle New Member

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    Aug 18, 2014
    Hi All,

    First off new member here, and since this is my first post I thought I'd thank everyone for over 10 pages of awesome information so far! Since I have a problem here I'm trying to fix, every little bit of information helps, so that actually read all 10 pages. Thanks again!

    Unfortunately, my old tivo with the 3y power supply died. I opened it up and noticed a single bulging cap. Well since then I've replaced just about every cap on this thing via the mouser project, except for 1 or 2 of the really small ones on the board.

    The caps I haven't replaced:

    C220 C227 and C306.

    all I hear is a constant clicking or ticking, and I even read one post where some one says that was normal without load, so I plugged the mboard and hdd into the PW supply after I replaced all the caps.

    But still no dice, does anyone have a circuit diagram or any other thoughts as to what component it actually could be?
     
  7. squint

    squint New Member

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    Jun 15, 2008
    I've worked on a good number of power supplies and haven't experienced that issue. I would just check over all the solder joints as it can be hard to melt the solder of the capacitor legs attached to large ground planes. Also double check polarity.
     
  8. nooneuknow

    nooneuknow TiVo User Since 2007

    3,554
    0
    Feb 5, 2011
    Cox Cable...
    It is also possible to damage a power supply, by not attaching the load(s) it is designed to always be connected to, or by not simulating the minimum load required (for the instances you don't want to worry about damaging what would usually provide the load).

    Clicking/cycling is usually a protection mechanism cycling on/off. It can happen when any over/under thresholds are out of range.

    It could very well be the PS protecting itself due to something wrong with the capacitors not yet replaced. I found it odd that this member seems to think the "really small" caps are any less relevant than the larger ones. It's almost always the smallest ones (on a TiVo PS), which are the ones of most importance, when re-capping. If one has an internal short, or is otherwise causing a high-current condition, it could cause what is described (provided the description is accurate).
     
  9. unitron

    unitron Active Member

    16,389
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    Apr 28, 2006
    semi-coastal NC

    The way I figure it, the caps that handle the most current and the high switching frequencies are the ones being stressed the most and the ones most likely to go bad because of that, although we now have one report of the 200V cap that comes right after the bridge being leaky on the bottom.
     
  10. Lcstyle

    Lcstyle New Member

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    Aug 18, 2014
    Hey thanks for the response there.

    So after reading 10 pages of this thread, I came to the conclusion based on some member's opinions that the primary caps that go bad are the 2200uf ones, C501, C701, C702, etc. That's why I didn't bother replacing the 3 on the far side of the board.

    In any event, this morning I refluxed the joints I had done for all the caps, and I replaced the the final 3 caps with equivalents.

    Still no dice, I get a fast tick tick tick (same as before). This is why I asked for a circuit diagram so that I could figure out how or what to test. I've also taken a few high resolution pics. here : tiny url slash l224dt3

    Showing Polarity on 2200uf
    [​IMG]
     
  11. Lcstyle

    Lcstyle New Member

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    Aug 18, 2014
    I replaced 2 or 3 of the caps that were on the far side of the board, (again these were caps not in the mouser project) with the same capacitance but lower voltage rating temporarily to test. For example, the 47uf 50V cap I replaced with 47uf 35V.

    I think that's OK as far as functionality, but could lead to shorter life span? Is that a valid assumption or will it not work properly even temporarily for testing purposes?

    [​IMG]
     
  12. Lcstyle

    Lcstyle New Member

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    Aug 18, 2014
    as you can see from the different design on the heads of the caps, I've replaced all of them.

    [​IMG]
     
  13. Lcstyle

    Lcstyle New Member

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    Aug 18, 2014
    I also checked the fuse on the far side of the board by removing the plastic coating, I doubt I would hear any ticking if it was bad, but just wanted to make sure it wasn't half blown.

    It seems to be OK

    [​IMG]
     
  14. Lcstyle

    Lcstyle New Member

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    Aug 18, 2014
    I even replaced that one:

    [​IMG]
     
  15. Lcstyle

    Lcstyle New Member

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    Aug 18, 2014
    One Question I had was whether in the following pic this was simply epoxy from the factory and or fabrication, or whether this thing overheated?

    [​IMG]
     
  16. squint

    squint New Member

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    Jun 15, 2008
    The orientation of C227 is weird and looks like its polarity may be reversed.

    My LT7202 looks the same as yours with the two "melted" looking spots.
     
  17. Lcstyle

    Lcstyle New Member

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    Aug 18, 2014
    OK thanks for letting me know about the LT7202.

    As far as C227, I'll double check it, but I think it is OK. It's a little hard to see but its not the regular design cap with two legs coming out the bottom, I found a replacement where the legs each come out opposite side. If you notice the diagram indicating polarity underneath and the c227 label as well as the arrow pointing to the right on the cap itself I think it is OK.
     
  18. SCSIRAID

    SCSIRAID Active Member

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    Feb 2, 2003
    Vero Beach, FL
    Those apear to be ferrite beads. They add inductance and suppress high frequency noise.

    Nothing looks melted to me.
     
  19. Lcstyle

    Lcstyle New Member

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    Aug 18, 2014
    Couldn't this also be a cap rapidly charging discharging? If I didn't know any better I would say the ticking noise like I am hearing is mechanical in nature. However there doesn't seem to be any relays in this board. It almost sounds like one of the cap suddenly discharges, or arcing.
     
  20. unitron

    unitron Active Member

    16,389
    2
    Apr 28, 2006
    semi-coastal NC

    The caps on a TiVo power supply board, at least all the ones I've seen so far, the ones with both leads sticking out the bottom, are called "radial" as far as the packaging is concerned.

    The cap that looks like a lead goes in one end and comes out the other is called an "axial" cap.

    In the old vacuum tube point to point wiring days, axials were probably the more commonly used design.

    Are all the replacement caps you installed Low ESR and 105 degree (C) rated?



    And is this supply out of a 648 or a 652/658?
     

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