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

    Distinguished Member

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

    Expert

    Jul 18, 2013
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    You can also observe the practice used on the other electrolytic's.;)
    Max.
     
  4. wayneh

    Thread Starter Expert

    Sep 9, 2010
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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

    Sep 9, 2010
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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

    Thread Starter Expert

    Sep 9, 2010
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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

    Well-Known Member

    Sep 21, 2010
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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

    Thread Starter Expert

    Sep 9, 2010
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    Well, I have some replacements I could try but I'm skeptical it would make much difference.
     
  9. DickCappels

    Moderator

    Aug 21, 2008
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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

    Sep 9, 2010
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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

    Well-Known 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

    Sep 9, 2010
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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

    Thread Starter Expert

    Sep 9, 2010
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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

    Sep 9, 2010
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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

    Sep 9, 2010
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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.
     
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