AM21 Repair

Discussion in 'DIRECTV General Discussion' started by hdtvluvr, Jan 16, 2020.

  1. sstv

    sstv Legend

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    Jul 30, 2006
    DALLAS TX
    The duct tape was used to immobilize the LCC . This prevented any wiggling and has worked very good.
     
  2. hdtvluvr

    hdtvluvr Legend

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    Mar 1, 2008
    Now I need some help. I've taken readings using a MESR-100 ESR Meter for the capacitors on the board along with readings of the new capacitors I ordered. For the new capacitors, I averaged the readings of each type. I have instances where the new caps read higher than those on the board and vice versa.

    Note #4 is twice the reading as 5 - 9 which are the same brand/v/uf
    Note #12 is about 7% of 10-11 which are the same brand/v/uf
    Note #15 is about 6 times the values of 13 & 14 which are the same brand/v/uf

    All readings were taken multiple times especially after looking at the comparisons to new capacitors. All decimals are in the correct place.

    Please review the attached. Which do you think need replacing.
     

    Attached Files:

  3. P Smith

    P Smith Mr. FixAnything

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    I would say need change #10, 11 and 16, 18; same time I've doubts in ESR values of new capacitors #13...18, those are looks too high for me.
    Let me dig my box …
     
  4. hdtvluvr

    hdtvluvr Legend

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    Mar 1, 2008
    All new caps were from Mouser Electronics. I could not find any ESR values in the cap specs so I don't have anything to compare them to. It is interesting that # 16 is 0.01 difference from a new cap and 18 is pretty close being within 0.5.

    So do you think the 0.504 on the new cap for #10 & 11 is good? I'm not sure how I'd convince Mouser that all the replacements for caps 13 thru 18 were bad (or potentially 10 thru 12 also).
    All came as sets on manufacturing tape so they would have been bad from the factory. How likely is that? How likely is it that other purchasers would have tested prior to installation?

    Please let me know what you find.
     
  5. P Smith

    P Smith Mr. FixAnything

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    The ESR values should be appropriate for DC rails, depending of noise/ripples spectrum each line; some of them running by DC-DC buck converters with different freq of a switcher … could be 100 KHz or lower/higher ...
    There is a simple rule - less is better !
    Equivalent series resistance - Wikipedia
     
  6. hdtvluvr

    hdtvluvr Legend

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    Mar 1, 2008
    So are you saying the new caps for 10 through 18 should be OK?
     
  7. P Smith

    P Smith Mr. FixAnything

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    I would run o-scope before replacement … perhaps there are [old caps] dumping noise and ripples well.
     
  8. hdtvluvr

    hdtvluvr Legend

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    Mar 1, 2008
    Way beyond my tools and capability.
     
  9. litzdog911

    litzdog911 Well-Known Member

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    Mill Creek, WA
    You're probably over-thinking this. Personally, I would just replace all of those caps and see what happens.
     
  10. P Smith

    P Smith Mr. FixAnything

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    Without getting full picture (measured noise/ripples, freq of the noise) you would spend money, parts and your time for nothing !
     
  11. litzdog911

    litzdog911 Well-Known Member

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    Mill Creek, WA
    He already bought the parts. I would just swap them and see if it's fixed or not. Won't take very long. But maybe that's just me :)
     
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  12. P Smith

    P Smith Mr. FixAnything

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    My point based on _high_ values ESR of new capacitors.
     
  13. hdtvluvr

    hdtvluvr Legend

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    Mar 1, 2008
    Partial success. I replaced 10 & 11 which were way high ESR. The HR44 booted up properly and I had a picture on the TV. However, OTA channels said that signal was too weak so I guess there is also another issue. It should be noted that the solder they used is pretty high temp. My 30w de-solderer wouldn't even begin to melt it. I had to use my soldering iron set to about 70+ and some solder wick. It took a lot of time to get the old capacitors loose/removed. In fact, I burned the board in the process. So I don't know if the failure is due to more caps actually being bad, other components are bad or I damaged some traces during removal.

    I'm giving up on fixing it. The LCC has worked fine so far when viewing OTA sub-channels although it hasn't been a long test. Any recorded shows are done on D* channels and not OTA ones. With the sub-channels on the OTA channels, the D* picture is usually better and never worse - except during bad weather.

    I appreciate everyones' time and assistance.
     
    242424 likes this.
  14. hdtvluvr

    hdtvluvr Legend

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    Mar 1, 2008
    So, does anyone have a spare LCC they're willing to part with? If so, please send me a PM. I'd like to have a spare on hand. Thanks
     
  15. litzdog911

    litzdog911 Well-Known Member

    12,442
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    Progress. Sorry to hear you're giving up on it. It's that crappy lead-free solder that makes it difficult to desolder. Apply a bit of regular solder first, then it's easier to remove.
     
  16. hdtvluvr

    hdtvluvr Legend

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    Mar 1, 2008
    Well, I'm back. I was so frustrated that I left the AM21 in the rack for several days. I finally decided to swap it back out for the LCC and guess what. It seems when I tightened the nut on the coax in connector at the back of the AM21, I must have cracked it or pulled it partially out of the part holding it to the silver box on the board. When I was removing the AM21 from the rack, the antenna cable stayed behind with part of the coax connector still attached. No wonder I had weak signals.

    Any way, I took the AM21 apart again, removed the box cap on the board (about 2 square inch silver box on the board) and saw how easy it was to remove the coax. So I de-soldered it and made a 6 inch coax cable from RG-6 solid copper core. I then soldered the copper core back to where the other one was. I had to enlarge the hole in the chassis so that the coax connector would pass through to the outside. I put everything back together. Unfortunately I did not take any photos.

    Today I was able to put it in the rack for testing. I plugged it in, connected it to the HR44, did a red button reset and the receiver booted up normally. My local channels are back!! So the AM21 is repaired. We'll see how much longer it will survive. I have the LCC now so I now have a backup.

    I want to thank everyone again for their assistance. Replacing the 2 high ESR capacitors seemed to do the trick.

    I do have a question though. It is mentioned above that applying some new solder helps in removing the old. How does that work?
     
    242424 likes this.
  17. litzdog911

    litzdog911 Well-Known Member

    12,442
    152
    Jun 23, 2004
    Mill Creek, WA
    That's great news!

    The flux in the new solder will help melt the old solder, especially if it's the crappy "lead free" used today. You can also buy liquid flux and add a drop on the old solder joint to help it melt easily.
     

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