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KA-8100 rebuild

Discussion in 'Kenwood-Trio/Kensonic-Accuphase' started by Overundr1, Sep 2, 2018.

  1. Overundr1

    Overundr1 AK Subscriber Subscriber

    Messages:
    2,515
    Location:
    Mississippi
    First Kenwood on the bench in ten years for me, was given this one for free as it sort was in a basket during a visit to a fellow AK'r residence last week. Anyways, the back story was his friend tried to repair it, was un-successful and thats how the journey started. Hmmm, dual mono block 75wpc integrated for free, lets get her running and compare it to my 717.
    The left side rectifier diodes were both missing which led me to eye the non nos output transistors, both tested dead shorted so replaced both sides with Sanken 2sa747 and 2sc1116's. Worked backwards towards the beginning of the amplifier stage installing new caps, any 2sc2089's I came across with ksc1845's and the 10 and 14v zeners. Replaced those missing power supply diodes with a couple of 1n4007's I had along with a new transistor and relay just for good measure.
    On the dbt she went with the pre main jumpers pulled, no fireworks and a nice relay click after three seconds, both channel bias levels were under 10mv (on a 60w bulb) so went after the offset. Those dual FET pairs do wander around a bit but got them both to settle down +/-10 mv or so. Brought the unit up on mains and set the bias to 14mv and went after the pre-amp.
    Now, that board is nice to work on after you take the whole front apart. Spent a lot of time getting the three filter buttons to pass a resistance test, dang fussy things, was tempted to take them all the way apart but D5 finally cleaned them up. The rest was straightforward r&r and buttoned everything back up again.
    Replaced the covers after some tidying up, hooked it up to a set of HPM 100's I finished repairing recently and gave a listen. Very responsive tone controls, I like the multi level loudness settings too. Fun little project, easy to work on and a nice piece to add to the collection.



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  2. hopjohn

    hopjohn Silver Face Subscriber

    Messages:
    1,092
    Location:
    Dallas, TX
    Nice work. Happy to hear you saved it.
    I'd recommend replacing the timing relay cap and upping the capacitance a bit to slow things down. It should help protect your speakers against transients a little better. Also, the relay driver takes a beating staying in a saturated state so something a little hardier is a good preventative measure. I typically use a KSC2690 there.
     
  3. Overundr1

    Overundr1 AK Subscriber Subscriber

    Messages:
    2,515
    Location:
    Mississippi
    Thanks for the tip. Already replaced the timing cap with a 100uf 35v and the transistor with a ksc1845, going back and comparing specs your ksc2690 does have more current handling, the relay slowing with a 220uf cap also is a most excellent idea, have plenty of both so will make the change before putting the unit away till its next turn at rotation comes around.
    -Lee
     
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  4. inductor

    inductor Super Member

    Messages:
    1,572
    Location:
    uk
    used 1n4007 for the main rectifiers ? i hope not
     
  5. Overundr1

    Overundr1 AK Subscriber Subscriber

    Messages:
    2,515
    Location:
    Mississippi
    1n5407 diodes
     

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