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Harman kardon 75+ problems

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  • Harman kardon 75+ problems

    Hi all:
    The problem is that the unit works perfectly except the 2 rear channels which are lower in volume than the front and when turned up past half way, a hard clipping like sound is heard from just the rears, one side more than the other. I have checked the output transistors as good. I chased the signal with a sine wave generator and o-scope back to the 4-gang pot array labeled volume. The signals arriving at the volume controls are all the same gain except one which is much higher. Not sure how this could be as there is not much before that to affect them.
    Later in the signal path I can see the distortion but not sure what is causing it as all the rail voltages are good.
    Anyone care to suggest a remedy?
    Attached Files

  • #2
    There are 4 identical amp boards all with capacitor coupled inputs. I'd inject a signal into each, one at a time, and see if they are all outputting the same thing. That will tell you if it's in the amps or prior.
    "Yeah, well, you know, that's just, like, your opinion, man."

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    • #3
      Injecting a signal into each amp directly:
      The 2 front amps showed a much smaller signal on the speaker outs until I removed the signal lead wire going into that amp.
      Rear amps produced a good speaker out without having to remove the lead. Only difference I could see was the shield (on the signal in wires)was connected on the rears but not the fronts so must be a significant drain with the lead wire connected to the amps with no shield. Shield was trimmed back unterminated and shrink wrapped from the factory going into the front amps.
      There are other strange results when I inject the signal into the normal jacks on the back. For example: Tape Mon in has FR, FL, RR, RL inputs but I can't get a large signal on the rear channels. There are also 4 each Discrete Phono/Aux inputs and 4 each Discrete FM/Aux inputs where Discrete might be taken to mean subtle rear speaker outputs but no such term for the Tape Mon 1 jacks.
      So what to try next? And what about that extra large signal at the volume control pots? The switches and pots have all been carefully cleaned I doubt it has anything to do with it.

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      • #4
        This is not a design problem so no need to go chasing after shielded vs non-shielded cabling, unless you don't think any of this model ever worked right.
        You have the rear channel switch set to 4 ch.? It has been cleaned?
        You have the mode switch set to 4ch discrete? It has been cleaned?
        Which set of inputs are you using?

        If you are not sure about what the different terms mean, or which connections to use, here is a link to the operators manual:
        https://www.manualslib.com/manual/35...75.html#manual
        "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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