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Vox VT100 DSP amp, 2008

Discussion in 'Electronic Repair' started by N_Cook, Apr 22, 2013.

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  1. N_Cook

    N_Cook Guest

    Loss of bass output, low level compared to mid and treble input test
    signals. Altering bass pot makes little difference in output level with bass
    test signal and marginal max is near the centre. I've not traced the bass
    pot back to a DSP area yet. Bass and other pots are not broken/worn and
    solder seems ok . This is PbF/RoHS and Korg era.
    Anyone know how much circuitry is common to the Korg/Vox AD100VT schematic ?
    or a schematic perhaps for the VT100 . Same o/p STK404-130S and
    "valvetronix" low-voltage LED-lit valve fudge (whats the point of DSP if you
    don't emulate valve function?) but different op-amps types at least.
    This is a DSP amp with dedented pots for rotary function switch action so
    expect future problems there.
  2. N_Cook

    N_Cook Guest

    initial continuity checking from pot wiper to pin 60 of the M3803 seemed to
    show a break, subsequent checking showed ok, x30 viewing looks ok , pin
    sounding sounded ok, but have assumed PbF failure , resoldered the .5mm
    spacing pin with solder paste, viewed for bridges and reassembled and then
    power up with fingers crossed.
    AD100VT schema has the bass pot wiper line going to pin61 so only similar in
    that area
  3. N_Cook

    N_Cook Guest

    What a pig's ear of an amp. There was also PbF failure at the input socket,
    reported by the owner. I don't know what they are called but .1 inch spacing
    IDC "header" for discrete wire "ribbon" soldering to a pcb. Re-work of one
    by the maker, by soldering over the IDC points very messily. Another such
    pcb fixing had failed at an IDC causing intermittent signal dip , I marked
    and cut the wires and soldered individually to the pcb. The nuts , with no
    star washers, were loose , holding the o/p STK404 to the h/s. Usual suspect
    PbF solder points delt with. Now to lay bets on when this amp will bounce
    back with more problems, only 4 years old. My guess would be that the click
    switches are far too sensitive for use on combos and will self-activate from
    vibration before long
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