harman/kardon hk775 amplifier restoration

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

  1. jheu02

    jheu02 AK Subscriber Subscriber

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    Flat enough for what you're doing. Copper is pretty soft.
     
  2. jheu02

    jheu02 AK Subscriber Subscriber

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    Here's a quick attempt using 12 gauge Romex bent around a screw I had lying about and then pounded with a hammer against a concrete slab. 14 gauge may be evn better since it could be pounded a little flatter. I still think soldered to a trimmed ring terminal will give a better connection to the screw terminal, but not sure how much room you need from the terminal to the bend of the lug for soldering.

    IMGP8593.JPG
     
  3. z-adamson

    z-adamson Well-Known Member

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    UPDATE:

    Everything just showed up from mouser and I hope to start putting together the amp that is currently taken apart.

    I opted to go with the copper strip route first, only because I have the means to cut and drill easily and with precision. And I like the idea of starting with something flat and I want it to all be one piece rather than two pieces joined together. Seems ideal.

    The filter caps came out perfect in terms of the size of the cans, but the positioning of the screw terminals is a little off relative to the old filter caps that I am replacing. I hope I can make the new ones work without difficulty. I may have to get creative with the bending of the leads. Speaking of leads, I got a copper electrode made for high school science class experiments. I gave it the scratch test, magnet test and lemon juice test and all indications are that the material is of high purity and I am happy about that.
     
  4. jheu02

    jheu02 AK Subscriber Subscriber

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    Keep us posted and with pics.
     
  5. z-adamson

    z-adamson Well-Known Member

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    Still working on this first of two hk775.

    All caps are in other than the filter caps. That part went pretty straight foreward. The variable resistors are in too and that was less simple. The distance from lead to lead was different in the replacement vr's. On one, I was able to carefully bend the leads and get it to fit nicely. For the other, nope. Not a chance. I had to solder extensions to each lead. This fixed two problems....1, lead to lead distance was corrected and I was able to space the vr away from the pc board a little which was essential because the new vr is bigger and clearance was a problem. But it's in there now. I am not 100% happy how the solders came out on the vr extensions though.
     
  6. z-adamson

    z-adamson Well-Known Member

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    Another thing, the manual specifies that the heat sink compound be silicone based for the output transistors. The stuff I have is silvery, sparkly and I don't think it is silicone based.

    Are you familiar with what I have? Should I seek out silicone compound or use what I have?
     
  7. z-adamson

    z-adamson Well-Known Member

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    Pics...
     

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  8. z-adamson

    z-adamson Well-Known Member

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    Pic of the board the filter caps go to. Note the damage to the foil trace on one of the solder points :( I spent too much time trying to desolder when I could have cut it off like I did the others. Do you see this becoming a problem? It looks salvageable to me.
     

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  9. jheu02

    jheu02 AK Subscriber Subscriber

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    Answering two questions:

    Thermal compound...I would squirt a little test strip on a piece of plastic or cardboard and check to see if it's electrically conductive (continuity function of your multimeter). If it is conductive, I get some non-conductive silicone based product.

    Trace issues... there's really not that much foil missing, and you're bending your manufactured lugs. Just make sure that one is long enough to reach a bit further into the pad.

    One note on the grey trimmer in one of the pics. Looks like those were the ones you needed extensions for. As long as it's electrically sound, there's nothing to worry about. However, now that it's further away from the board how stable is it when trying to adjust? The legs look to be in close proximity to some other resistor legs, so if it moves a little when the screwdriver and some force for adjusting is applied, you may want to desolder them and insulate the legs before resoldering. I'll often just use the insulating sleeve from 14 ga Romex that's stripped off of a piece of scrap wire. Same can be said of insulation stripped from stranded wire, or if you have 1/8" or less shrink tubing.
     
    Last edited: Jun 26, 2017
  10. z-adamson

    z-adamson Well-Known Member

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    As cheap as the stuff is, I think I will just go straight for the silicone.
     
  11. z-adamson

    z-adamson Well-Known Member

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    Thats basically what I was thinking.
     
  12. z-adamson

    z-adamson Well-Known Member

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    Now that is an excellent idea. I had not taken into account the possibility of shorting with the longer legs. I will look very closely at this and for the second amp I will insulate before installing. I may add insulators to the one I already soldered if it looks close.
     
    Last edited: Jun 26, 2017
  13. z-adamson

    z-adamson Well-Known Member

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    Progress..
     

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  14. z-adamson

    z-adamson Well-Known Member

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    The filter caps were a bigger pain in the butt than what I had anticipated. The screw terminals are not located in the same position as the solder terminals of the old filter caps. This complicated things big time. I used 10 gauge stranded wire soldered to ring terminals. I needed a little flexibility, hence the stranded wire. There was no way the bent copper lugs would have worked. Solid wire, maybe, but I needed flexibility because there was no way to solder to the board perfectly in position and the flexible wire left room for error.
     
  15. jheu02

    jheu02 AK Subscriber Subscriber

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    I'm not there to see what you're dealing with, so don't take this the wrong way, but...

    Your initial picture of the original caps appears to show the solder lug terminals (where they attach to the caps) centered across the diameter of the cap. The only offset I see is the lug itself, which apears to descend vertically from the cap at about the edge of the metal terminal of the cap (so just slightly [~ 3mm] offset from the centerline of the cap). If this is the case, I can't see how a metal tab that's screwed into the cap terminal can be bent just like the original to line up with the solder pads. Again, I'm not there, and I can't tell if the terminals are actually offset to the edge of the cap in the originals. I've just never seen caps that only have two terminals, line them up offset from the "centerline" of the cap body. From your latest pics, it appears that you have the new caps installed about 90 degrees off from the originals.
     
  16. z-adamson

    z-adamson Well-Known Member

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    Yes, the filter caps are rotated 90 degrees from the originals. In the new caps, the female threads for the terminals are located at dead center. In the old caps, the terminals are off center as are the solder lugs. The terminals being just above center and the solder lugs being just above center allows clearance for the filter cap bracketry. The board and bracketry index themselves to filter cap centerline and if anything is there, the result is an interference problem. With the screw terminals being centered on the new caps there is an interference problem with the bracketry and the board I am soldering to. Rotating 90 degrees took care of the clearance problems.
     
    Last edited: Jul 2, 2017
  17. z-adamson

    z-adamson Well-Known Member

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    Pic...
     

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  18. z-adamson

    z-adamson Well-Known Member

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    Note that the terminals are off center as are the broken off solder lugs. The bracketry and board line up in dead center and the lugs are up against the board or close to it so that they can be soldered. None of this can be done if the screw terminals are in dead center.
     
  19. jheu02

    jheu02 AK Subscriber Subscriber

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    Necessity is the mother of invention... Anyway, I see the terminals are off center in the new photo, but the snipped lug looks to be nearly on the centerline. Whatever works... It just seemed like the screw terminals would work with home-made tabs.
     
  20. z-adamson

    z-adamson Well-Known Member

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    Got one of the amps all together, hooked it up to the dbt with a 150w bulb. BTW, this is a 130w mono amp that draws 400w at 120v. Anyhow, the bulb shines bright, and stays bright. I never let it run continuously for more than 10 seconds for fear of damaging something. No signal, no speaker hooked up.

    So, the amp is pulling a lot of current. Is this because of a short in the power supply or could this be related to the bias not being set? Is this the right sized bulb?
     

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