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Vox AD120VT

Discussion in 'Electronic Repair' started by N_Cook, Mar 1, 2012.

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  1. N_Cook

    N_Cook Guest

    Where's a hagiogolotre when you need one.
    Do you really need to take the pa apart just to test the valves. Is there a
    Jim Crow for this job, to avoid dismantling? plastic screwdriver to wedge
    between valve and base ? long handled plastic pipe wrench to grab the
    envelope, or some sort of suction cup to pull them out ?

    In case its a safety issue. Owner does not know how it happened but the
    plastic coating to the metal chassis around the IEC inlet is rucked from
    partial melting? well it stretched back, as good as new, with low heat from
    a hot air gun. Hopefully caused by such or an open fire. Unless the amp is
    used with the speakers pointing to the floor then no convected heat from the
    transformer, no sign of overheating of the Tx, and no sign of arcing or over
    temp on leads,tags , IEC fuse , drawer etc- any known weird stock fault
    leading to such internal localised heating.
    No sign of any discolouring / pitting of the pins of the IEC , if it was a
    bad IEC line plug arcing.
     
  2. N_Cook

    N_Cook Guest

    I had to take the other board apart anyway to replace a pot, but wondered
    about more routine checking of valves. Replacement is not blind as there is
    an access gap and maybe finger tips, in there , to guide into place.
    With a radiant fire I expected some sort of discolouration of the Rexine,
    presumably nearer to heat source. Low heat of my heat gun says 320 degree C,
    whether higher or lower from numerous heater element joinings over the years
    , with NiCad stainless steel "crimps", anyone's guess.
     
  3. N_Cook

    N_Cook Guest

    As far as I can tell (present owner not from new) I'm the first to go in
    there from new.
    TOP3 are 2x C5200 but one 2SA1943 and one KTA1943
     
  4. N_Cook

    N_Cook Guest

    Those are straightforward to work on.

    Whats the chance of busting off a SMD device putting all those boards back
    in place, like origami with spaghetti combined with that business of tapping
    your head with one hand while the other hand moving round stomach, getting
    all that stuff lined up again without breaking a valve top pip.
    Just as well I have a little ratchet handle to take otherwise standard
    crosshead hex driver bit for those 2 bright machine screws under the 7
    segment LED section. Stubby screwdriver is not stubby enough.
    Back in place sufficient to test properly, that for tomorrow, quite enough
    cussing for today.
     
  5. N_Cook

    N_Cook Guest

    I've just downloaded the manual as although both PAs are live , only one is
    getting a feed , something wrong with DG211 or the rear power selector or
    something.
    Manual does not include how to dismantle or mantle.
    One tip is to cover the valves with some dense foam a bit longer than the
    valve tip , supported by the too short standoffs , to protect the valves
    when turning all this metalwork over for probing etc
     
  6. N_Cook

    N_Cook Guest

    Anyone happen to know of a designed-in function that would switch out 1
    amplifier of the 2?
    Analogue signal of right levels is getting to the inputs of both amps, which
    both agree quiescent DC fashion but some control line is erroneous, I'm
    assuming.
    Anyone happen to know what switches the A/AB class function , via the 2x
    DG211 analogue switches , I assume the user select, amp type, 16 way encoder
    switch, but which for A and which for AB ? but either way seems to control
    both amps simultaneously, not isolating one amp.
    I thought it would be the 4 way power switch on the rear but that is
    switched attenuators only to both amps, no control function. The phones
    change over function via the MAX333 an. sw. should divert both amps by the
    single ground switch in the phones socket .
     
  7. N_Cook

    N_Cook Guest


    I think I now have a way to prop up a connected live PA board , to probe
    stuff DC and AC, without it falling over or shorting anything. A couple of
    plastic "ring " binder sleeves over lower front edge of the chassis so will
    take the half rotated back edge of the pcb and the top of the heatsink,
    safely over the top panel. Along with the foam protected valves
     
  8. N_Cook

    N_Cook Guest

    Looks related to An Sw failures, FET failures
    No signal on the grids of V1 but signal on Con2 with only a couple of
    long-tail FETs in between.
    Like a stereo amp comparing channels, signals on V2 grids and con1. DC
    around the 2 An Sw are the same, but 0,0.4,220V dc sort of values on the bad
    FETs and about 60,60,140V on the good. How long do you scratch your head
    before you realise that the schematic is wrong. How could there be 0 V on
    one end of a trace at an an sw and 60V on the other at a FET gate of the
    working amp?
    Supplies to these FET are from the valve anode rail
    2002 version
    http://www.valvetronix.net/docs/AD120VT_Service_Manual_Complete.pdf
    about 1/3 down the pdf, left side
    Now , with hindsight , it is bleeding obvious , a shorting connection
    directly across both C51 and C48 is shown in the diagram but of course in
    reality no such short.
    Now to find some 240V,200mA rated FETs , dosn't seem right 240 V on a TO92
    package
     
  9. N_Cook

    N_Cook Guest

    Much more mysterious failure mode , that yellow goo again?
    Removed both FETs and they cold tested ok , changing conductance state, via
    DVM "diode" test, so probably ok . There is that yellow goo in this area and
    one resistor covered with it is 120K 1/3W dropper from the anode rail, so
    difficult to check in situ as between 2 electros, sets the bias. Something
    chemically has gone wrong with it. Small patch of body coating shows
    perfectly normal cream and part of a red and brown band, again no thermal
    discolouration. Under the R the board is perfectly normal green colour. But
    the r is o/c , now replaced with 2x 240K 2/3W raised off the board , JIC a
    heat problem but I doubt it. So as well as encouraging
    deliquescence/hygroscopic or whatever the word is for taking moisture out of
    the air and encouraging corrossion it looks as though high voltage has some
    deliterious effect with the dreaded yellow peril.
     
  10. N_Cook

    N_Cook Guest

    I didn't think to mark the "amp type" knob before removing .
    360 degree hex encoder , splined shaft, have I got this right

    With my mark on one end of the shaft slot at 6 o'clock , (so could be 12
    o/clock ) then A/AB control V at that contact is 5V. From the circuit 5V
    probably relates to lack of negative FB so "class A",
    Then going clockwise 000005555550050
    Anunciators ,
    at 6 o'clock , Boutique Cl then xlockwise
    black 2x12
    tweed 1x12
    tweed 4x10
    ac 15
    15TB
    ac30
    30 TB
    UK blues
    UK 70s
    uk 80s
    uk 90s
    uk modern
    recto
    US higain
    Boutique OD

    I've placed the knob so the block of 6x 5V cover Vox AC15 to UK 70s , right
    or wrong?
     
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