Digital TV Tuner

Discussion in 'Technical Repair' started by wayneh, Oct 4, 2016.

  1. wayneh

    Thread Starter Expert

    Sep 9, 2010
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    This is the board from a digital to analog TV receiver that is not working. See the empty pad, middle bottom? I pulled a bloated electrolytic off of there. I stupidly forgot to mark the polarity before I removed it.

    With the board powered up, there is a small voltage on the thru-holes and the polarity is in alignment with the shaded area being ground. Maybe 1.5V. The cap is rated 16V.

    I thought the shaded area indicates ground, but I swear the the capacitor was mounted backwards, if that was true. I'm about 90% sure of that. Is it possible it was installed backwards and lasted a year or so anyway?
    FullSizeRender.jpg
     
  2. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    The side marked with the white semi-circle is negative.
     
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  3. MaxHeadRoom

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    You can also observe the practice used on the other electrolytic's.;)
    Max.
     
  4. wayneh

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    It's up and working again!

    I notice that even the replacement capacitor is getting warm. I may need to get a low ESR, high temp rated replacement.
     
  5. wayneh

    Thread Starter Expert

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    For the record, this device is a HomeWorX model HW180STB set top box.

    The offending capacitor was marked:
    DKS® (which I cannot find)
    RoHS
    1000µF, 16V
    -40°C – 105°C
    I replaced it with a Lelon cap from my junk drawer that had the same 1000µF and 16V rating, and even the 105°C max.
     
  6. wayneh

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    Today I'm looking at another of these units that has failed. It's a little different - a red light appears on the front panel and the unit attempts to boot, but then the light goes off and it starts over. And over and over.

    I took it apart hoping the same capacitor might be the problem despite the different behavior. There is no bulging or spewing this time. The DC voltages on the caps and big diode are 5v (along the back edge of the board in this photo). I switched to AC mode and was surprised to find 10V on these components. That seems like a problem?

    IMG_2969.png
     
  7. debe

    AAC Fanatic!

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    I would also suspect the diode, replace it with a Shotkey diode as they handle the switch mode frequency better, & see what happens.
     
  8. wayneh

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    Well, I have some replacements I could try but I'm skeptical it would make much difference.
     
  9. DickCappels

    Moderator

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    Or could it be some peripheral device that when "spoken to" causes the power supply to dip from an excessive load, causing a reset, or something similar?
     
  10. wayneh

    Thread Starter Expert

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    The endless rebooting happens as soon as the device is plugged in, with nothing attached. There is no response to the buttons. Maybe I'll try disconnecting the header for those at the PCB. I'll have to attach a monitor to see if anything happens because the little red power-on LED is on the same PCB with the user buttons.
     
  11. SLK001

    Senior Member

    Nov 29, 2011
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    A capacitor doesn't always bulge when it fails. What does "switched to AC mode" mean?
     
  12. wayneh

    Thread Starter Expert

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    I meant the metering mode of my cheap Can-Tech meter was switched from DC to AC. I agree about the capacitor, but I have no reason to think there's anything wrong with it except the fact that capacitors get old. I suppose if I have a replacement I could just try it.
     
  13. wayneh

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    There are no peripherals but this got me thinking. I hooked it up to my big TV by HDMI and powered it up. The hi-res splash screen comes up immediately but about when you'd expect a menu or a station to come on screen, it goes black and reboots. A couple seconds later it's back to the splash screen. Repeat.

    So I tend to think the power supply is not the problem.

    I looked over the board again and the only thing I see that's odd is a slight darkening on the backside, behind an IC with a heat sink. The heat sink is applied sloppily and is a little off center. Looking closely underneath the heat sink, behind the HDMI port in the foreground, you can see some thermal adhesive has glopped out and may be touching the pins. That's bad, right?

    How do you fix that?

    [edit] It looks like thermal adhesive, other than stuff with silver in it, is supposed to be
    electrically insulating. Hmmm...

    IMG_2975.png
     
    Last edited: Oct 20, 2016
  14. wayneh

    Thread Starter Expert

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    I thought I'd give this one more go this morning.

    Two experiments:
    1) Disconnected the PCB jumper that goes to the user buttons, IR receiver and power-state LED. No effect.
    2) Bypassed the AC power supply and put 5V DC directly onto the capacitor that is missing in the 1st photo. Same symptoms, splash screen followed by reboot.​

    I noticed that the plastic case directly above the aluminum heat sink in the photo above has vent slits that have melted together and closed. This may have caused the passive cooling to degrade and the IC to get still hotter.

    I don't see any path forward. Do you?
     
  15. wayneh

    Thread Starter Expert

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    I cut/tore off that heat sink this morning and scraped the overflow off the pins of the IC underneath. No change.
     
  16. wayneh

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    Well here I am a year later and I just fixed another of these units. A bad cap.

    Can you see it?

    Symptom: Device was stone cold dead. Wouldn't power up. Replaced one cap with one from my junk drawer and full function is restored!

    IMG_4223.jpg
    IMG_4224.jpg
    IMG_4226.jpg
     
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  17. AZImmortal

    New Member

    Dec 9, 2017
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    Hi, I found your thread by searching for DKS capacitors and coincidentally, I also have a dead tuner/DVR box from Homeworx (although I have the HW-150PVR) and the exact same capacitor is bulging in mine. I've actually had multiple HW-150PVRs that will turn off by themselves or freeze, and I have yet to open them up, but I wanted to ask you if you've experienced problems with any other capacitors on the board. I'm pretty much a novice with these kinds of electrical issues so unfortunately my diagnosis skills don't go further than seeing if a cap is bulging/leaking. I guess what I want to know is if I should bother replacing all of the other caps on the board or if this is the only one that seems to be causing problems. Thanks for any insight you can provide.
     
  18. wayneh

    Thread Starter Expert

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    @AZImmortal , I ’m pretty much in the same boat. If it’s not an obvious failure, then it’s just a shotgun approach to replace them all and hope for the best. I’d start with the larger ones, though. I think both the ones I replaced were among the larger ones on the board.

    There is a way to test caps, called an ESR meter. I even built one. But frankly it’s easier to just replace them.
     
  19. AZImmortal

    New Member

    Dec 9, 2017
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    Thanks. Yeah, I believe that these caps are the second largest on the board. Your board's layout is actually almost exactly the same as my board's, and I was thinking about also replacing the cap that's two components to the right because it's also a DKS (It's hard for me to find the manufacturer for most of the rest of the caps). You haven't experienced any other cap failures on your Homeworx boxes beyond that particular one, though?
     
  20. wayneh

    Thread Starter Expert

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    This thread has all my notes in it, so no. I can't recall the details but from what I wrote above, it looks like one of the 3 units I worked on was never repaired. I didn't bother to replace the other caps because I believe my wife was able to get the unit replaced using her credit card's extended warranty feature. I believe we've gone through two or three of these in addition to the two I've repaired. On the upside, they don't even require you to send them back anymore. So if I succeed in a repair, it's gravy.
     
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