harman/kardon hk775 amplifier restoration

Discussion in 'DIY' started by z-adamson, May 7, 2017.

  1. z-adamson

    z-adamson Well-Known Member

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    Top adjust on both, I will get pics

    Havent diagnosed the dual resistors as bad, just want to have them in case. They are quick and easy to change so I figured why not, as long as good replacements are available. I seem to recall from those who use HK citation 16 that multiturn VRs for bias are a must so I figured they would serve well in this amp too.
     
  2. jheu02

    jheu02 AK Subscriber Subscriber

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    When you say Top Adjust, the boards are horizontally mounted in the case and you look down on the top of the trimmers? I ask because it looked like the amp board plugged into something and was oriented vertically, with the trimmer sticking out of the side. So, even though from overhead it looks "top adjust", the trimmer would ACTUALLY be a side adjust part.
     
  3. z-adamson

    z-adamson Well-Known Member

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    The boards that the VRs install to are perpendicular to the ground and the adjustment is made from the top of the amplifier chassis which would be the side of the VRs.
     
  4. jheu02

    jheu02 AK Subscriber Subscriber

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    That's what I thought. So they're side adjust. Is the middle pin above the other two or below? SM makes it look like they're above on both VRs.
     
  5. z-adamson

    z-adamson Well-Known Member

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    As viewed from a birds-eye-view with the amp right side up?
     
  6. jheu02

    jheu02 AK Subscriber Subscriber

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    If the amp is sitting on a horizontal surface...say a table...and you have the top cover off and are looking down into the amp...you can see the adjustment slots in the trimmers, but the circuit boards themselves look like thin slices with the components sitting horizontally to you looking down on it, yes?

    Here's a pic looking down into a Pioneer SA-8500II I rebuilt. The amp boards are mounted vertically on the heatsinks (as referencing the ground plane) but you can see the tops of the trimmers circled in yellow. These are "side adjust" trimmers, because the mounting pins go sideways into the board as referenced from above.
    trimmers.JPG
     
    Last edited: Jun 7, 2017
  7. z-adamson

    z-adamson Well-Known Member

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    Yes, I follow 100%, we are on the same page. Thats what I meant with birds-eye-view, right side up. I will check that when I get home.
     
  8. jheu02

    jheu02 AK Subscriber Subscriber

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    I edited my post to include a pic. please refer to it so we make sure you order the correct trimmers. There's MANY variations and you have to order the correct part number.
     
  9. z-adamson

    z-adamson Well-Known Member

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    574
    Middle pin is up as per the point of view we discussed.
     

    Attached Files:

    Last edited: Jun 7, 2017
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  10. z-adamson

    z-adamson Well-Known Member

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    Viewed this way they would be considered side adjust.
     
  11. jheu02

    jheu02 AK Subscriber Subscriber

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    OK. One other question: are you comfortable with using breadboard to turn a "top adjust" into a "side adjust"?

    This would be a perfect replacement for VR402, but it's top adjust. If you place it in breadboard and run jumpers sideways it will work.

    Otherwise, you'll need to go to 200ohms to get side adjust.

    Here is a single turn that should work well. 200ohm

    And a multiturn if you insist. 200ohm

    For VR401 1K ohm

    These are on order.

    Also on order some Bourns

    Finally, some that are in stock
     
  12. jheu02

    jheu02 AK Subscriber Subscriber

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    I haven't found any double resistors yet, but you can use two singles if necessary. Just make sure you give them some space from each other for heat dissipation. You'll twist one leg of each together to make a "V" out of the resistors. Solder the double leg in the one hole (may need to enlarge it a tad with an X-acto blade) and the single leads in their respective holes.

    These would work.
     
  13. z-adamson

    z-adamson Well-Known Member

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    Thanks for the list :)


    This last one says 6 weeks lead time. Any other alternatives?
     
  14. jheu02

    jheu02 AK Subscriber Subscriber

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    Numerous fuse resistors:

    100 ohm 1/4W - R24
    1.5K ohm 1/4W - R403, 404, 409, 410
    56 ohm 1/4W - R431, 432
    39 ohm 1/4W - R473, 474

    47 ohm 1/2W - R438, 439
    27 ohm 1/2W - R443

    I like the following:

    Vishay/Dale RN60C or RN60D (don't let the 1/8W rating scare you off...it's underrated for Mil-Spec, the same resistor for industrial use is CMF and it has a 1/2W rating) VERY low noise resistors
    KOA Speer MF 1% tollerance (500V rating vs MFS 250V rating)

    Others are fine also, but just stick with 1% vs looking for 2%...better resistor.
     
  15. jheu02

    jheu02 AK Subscriber Subscriber

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    Do you need more than the 725 that can ship immediately??? :D
     
  16. z-adamson

    z-adamson Well-Known Member

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    Oops, the on order, 6 week lead time threw me off.
     
  17. z-adamson

    z-adamson Well-Known Member

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    Thanks, I did plan on replacing every fuse resister considering both of them in the hk725 were way out of spec.
     
  18. deltalight

    deltalight AK Subscriber Subscriber

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    15,000uF 100v 51mm x 80mm x 22mm(screw): 647-LNR2A153MSE
     
  19. z-adamson

    z-adamson Well-Known Member

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    Thanks! :)
     
  20. z-adamson

    z-adamson Well-Known Member

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    One thing that the manual says is...

    Replacement for output and driver transistors, if necessary, must be made from the same beta group as the original type.

    ....what exactly does this mean and how do I make sure that the replacements, if I decide to replace, are appropriate??
     

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