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harman/kardon hk775 amplifier restoration

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

  1. z-adamson

    z-adamson Active Member

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    An idea that crossed my mind...how about winding 10/12 gauge solid wire around the bolt, then tightening the bolt down to the cap with the wire between the bolt head and the cap? Does this sound viable?
     
  2. jheu02

    jheu02 AK Subscriber Subscriber

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    Seems like you'd have to really crank down tightening the bolt to get a good connection. I haven't used screw terminal caps, so I don't know how much torque they can take. A ring terminal soldered to the wire would probably be better.
     
  3. jheu02

    jheu02 AK Subscriber Subscriber

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    If you look at the service manual page that shows the driver board components, you'll see:

    Q419,425 2SA968(O) or (Y)
    Q422,424 2SC2238(O) or (Y)

    Here is the datasheet for the 2SC2238

    When you look at it you'll notice that there's 3 variants basic, 2238A, and 2238B. These differences are based on the voltage rating of the transistor (160V, 180V, and 200V respectively) Your 775 only needs the basic rating of 160V, however, you can use the other two variants as they are rated for higher voltage. Next, scroll to page 2 of the datasheet. At the bottom of that page you'll see a section that lists hFE classifications. Those are the "beta" groups you're wondering about. Basically the gain of the transistors. When you replace them, you want them from the same grouping so they'll be matched for the circuit. Since the 2SA968 is the complimentary pair of the C2238, if you need to replace them you need transistors from the same group. So, when you look at these Qs in your unit, they should all have an "O" on them, or all have a "Y" on them following the transistor number that's printed on the case.

    If you need to replace them, the recommended modern replacements is MJE15032G for the 2SC2238 and MJE15033G for the 2SA968
     
  4. HarmanKardon

    HarmanKardon Tubes still smell funny Subscriber

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    I would like to see this brand name being spelled "harman kardon". Thank you.
     
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  5. jheu02

    jheu02 AK Subscriber Subscriber

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    I'd rather see it spelled harmin kordon. Or, in the immortal words of Sgt. Hulka..."Lighten up Francis."
     
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  6. z-adamson

    z-adamson Active Member

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    339
    And how does this...

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

    ....apply to the output transistors 2sc2564 and 2sa1094??
     
  7. jheu02

    jheu02 AK Subscriber Subscriber

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    Very same way. Look at the SM, see what it calls for beta group-wise, google the transistor number to find a datasheet to see what hFE range that beta group calls for, and find replacements that have the same or close ratings if you can't find OEM from a reputable source.
     
  8. z-adamson

    z-adamson Active Member

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    I am aware that I typo'd the title. How do I correct that?
     
  9. HarmanKardon

    HarmanKardon Tubes still smell funny Subscriber

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    Well, as a subscriber you could edit your title. It's no biggie - just type it correctly next time... ;)

    :beerchug:
     
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  10. z-adamson

    z-adamson Active Member

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    Removed all the caps from all the boards.

    One thing struck me as odd, two of the aluminum e-caps have three terminals. Pos, neg and one unlabeled terminal. Why is this? What is the 3rd terminal?
     

    Attached Files:

  11. z-adamson

    z-adamson Active Member

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    Word of advice to anyone else that rebuilds one of these: Do not try to remove the filter caps by desoldering the terminals. It will not likely turn out well, the result will not be good. Instead cut the terminals from the caps and get the caps and bracketry out of the way. Then desolder the terminals from the board, one by one. This way the terminals are isolated instead of all joined together and they are easily desolderable with minimal heat. I screwed around trying to desolder with the caps in place and got nowhere and in the process put a bunch of unnecessary heat to the board. Cutting the tabs made it quick, easy and harmless.
     
    Last edited: Jun 10, 2017
  12. jheu02

    jheu02 AK Subscriber Subscriber

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    You'll need to include more information about them...printing on the sleeve, where they are in the circuit like C304... etc. one leg is "probably" just for stabilization, but, it could be a multi cap...two caps in one sleeve sharing a common ground. I see you didn't follow my advice about removing them all at once. Hope you have good photos for the reinstall.
     
  13. z-adamson

    z-adamson Active Member

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    I have two of these amps, they are identical, so a mass desolder followed by a mass solder made the most sence. If any confusion comes up as to what goes where, I can look at the other amp. I made sure to compare first.
     
  14. z-adamson

    z-adamson Active Member

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    C12, C13

    Screening on the board looks ordinary with the usual flat side for negative. More holes for leads though.

    On the cap, NC lines up with the 3rd lead, the stripe lines up with the negative lead, nothing lines up with the positive lead.
     

    Attached Files:

  15. jheu02

    jheu02 AK Subscriber Subscriber

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    I had forgotten you had 2 amps. That's definitely a reason to mass desolder in this case...amp works already and you can check your work.

    As to the cap, looks from the pics you provided that the extra leg is just a blank for stability. The sleeve has NC pointing to that leg. While it could mean North Carolina, it more likely means "not connected" as in no electrical contact. Looking at the component side shot of the board it appears that the solder pads do not connect to anything. Can you confirm from the trace-side? 1000uF/80V, while not huge, is probably a decent-sized cap. Stability pin.
     
  16. z-adamson

    z-adamson Active Member

    Messages:
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    So I got the screw terminal caps on order and I am trying to figure the best way to do this. I liked the idea of drilling a thin copper strip, but I am having a hard time finding strips and sheets of copper that are pure enough to be ideal for this application. How pure does it need to be? Most of the time, sheets and strips of copper are far lower in purity than copper wire intended to be used to carry electricity. Cutting and drilling is easy for me, so if I could find just the right copper for the job, this would be a slam dunk.

    I like the copper wire soldered to a ring terminal, but man it sure does seem like it would be tricky to do with good results. I have had good results with stranded wire because it will crush in the terminal then filling the terminal with solder works very well. But I have never done this with a solid wire but thinking about it, I am skeptical. It wont crush down in the terminal and there is no filling each small strand with solder happening here. Just one big strand.

    Wrapping the wire around the head of the bolt then torquing down the bolt as is common in house wiring would be great if the terminals would handle the torque, but the more I think about it the more skeptical I become.

    Any ideas I should consider? Are there any commercially available products out there made just for this or is DIY the only option here?
     
  17. jheu02

    jheu02 AK Subscriber Subscriber

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    Personally I feel you're overthinking it. How do the component legs(solid wire) get attached to the board where there is NO crimping involved?

    Get the appropriate-sized ring terminal for the guage of solid wire you intend to use. Put a little flux on it (rosin not acid flux). Insert into the crimp hole. Solder.

    Try another one, but this time after putting the wire in it, pound it flat and then solder. See which works best.

    Pics are of a snap-in cap (10mm spacing) I modified to be used with 22mm spacing in a Sony STR-7046.

    IMG_0772.PNG IMG_0773.PNG
     
    Last edited: Jun 14, 2017
  18. jheu02

    jheu02 AK Subscriber Subscriber

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    Or, here's another thought...

    Take the bare wire and bend it into a loop slightly larger tha for the screw to fit in, and then pound the whole thing flat. Enlarge if necessary with a dril bit.
     
  19. z-adamson

    z-adamson Active Member

    Messages:
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    So I have a "Frey Scientific Copper Electrode Strip, 5" Length x 3/4" Width x 3/64" Thick" coming. Bought it on the bay. Cutting and drilling is cake so I will explore this and see how it goes. I will also try out the soldering to a ring terminal and use whichever comes out best.

    I am a little worried about the purity of the copper strip.
     
  20. z-adamson

    z-adamson Active Member

    Messages:
    339
    Thats a good idea too. Not sure how flat I would be able to get it though. Might be able to get it as flat as I can with a hammer then use a surface grinder to level it out further.
     
    Last edited: Jun 14, 2017

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