Need help with a KA-405

Discussion in 'Kenwood-Trio/Kensonic-Accuphase' started by SaltWaterJim, Oct 9, 2017.

  1. SaltWaterJim

    SaltWaterJim New Member

    Messages:
    21
    Location:
    Vinton, IA
    I found this amp at a Goodwill and brought it home. After checking things out I found it has a hum when it is switched to the phono input. I thought it may have a bad cap some where, so I replaced all the electrolytic caps. Power up after cap install and the hum is still there and found another problem. When I touch the volume control (without the knob on) there is a buzz. Is there a small amount of voltage on the volume knob that is grounded when I touch it?

    Any help would be great!
     
  2. rjsalvi

    rjsalvi Active Member

    Messages:
    273
    Location:
    San Diego, CA
    Just for sh*ts and giggles, if it's got a three-prong AC plug, get a cheater plug and lift ground to see if that makes a difference. If it's plugged into a receptacle/power strip/line conditioner in which a cable-connected TV is also plugged in, disconnect the coax cable from the TV and see if that makes a difference. Yeah, I know...basic stuff, but you've started at point E and I'm just verifying whether point A-D ground issues have been addressed. :)
     
  3. SaltWaterJim

    SaltWaterJim New Member

    Messages:
    21
    Location:
    Vinton, IA
    Thanks for the reply.
     
  4. SaltWaterJim

    SaltWaterJim New Member

    Messages:
    21
    Location:
    Vinton, IA
    Found the problem. There is metal plate on the back side of the plastic face plate. It is lose and will fall out when you take the front cover off. For testing I left as much as posable unassembled to save time if things were wrong and that was one of the parts left off. The volume knob is grounded through that metal plate which also is attached to the input selector. Just needed a ground. One thing it did not fix or the recap was the hum on the phono input. I suspect it may be the pair of IC HA1457's.
     

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  5. rjsalvi

    rjsalvi Active Member

    Messages:
    273
    Location:
    San Diego, CA
    :thumbsup: Good catch! RE phono: if it's both channels, I'd stay on your ground-finding mission. Check solder joints, disconnected ground wires coming off the phono input jacks, the (shielded?) interconnects between the TT and amp ... and does the TT have that third ground wire to chassis? The screws that hold the sheet metal cover on the amp...are they original, or aftermarket? Loosely translated, is it possible a mounting screw is touching the pcb anywhere? The input selector switch, is that clean? It's not impossible for those ICs to be faulty, but it doesn't sound probable. If it finally does narrow down to them, you could upgrade them with this adapter board: http://cimarrontechnology.com/ha1457adapter.aspx and use more conventional op amps...keeping an eye on supply voltage of course.
     

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