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VOX AC100cp

Discussion in 'Electronic Repair' started by Phil Allison, Dec 14, 2011.

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  1. Phil Allison

    Phil Allison Guest

    ** Hi,

    I have a VOX AC100cp ( Chines made) on the bench - it is about 6 years old.
    It has all its original parts but is in a very bad way. The known faults

    1. One (Ruby) EL34 has lost vacuum.

    2. Two scorched 5W screen resistors, one detached from the PCB.

    3. One blown HT fuse, a 1 amp slow blow - which may well not be the only

    4. Output tranny OPEN on one side of the primary and exhibiting symptoms of
    shorted turns.

    5. Bridge rectifier only conducting on one half cycle - giving 50Hz

    The scorched 5W resistors are on the side with the open tranny winding.

    The EL34 with no vacuum is on the other side.

    Wanna have a go at the sequence of failures ?

    .... Phil
  2. Phil Allison

    Phil Allison Guest

    ** Never said any bridges had failed.

    There is 50Hz ripple on the HT, of course.

    Since my post I discovered that R107 ( 22 ohms 7W) is open - this is is
    series with the high voltage secondary.

    All other supplies are OK.

    .... Phil
  3. Phil Allison

    Phil Allison Guest

    "Gareth Magennis"

    ** The HT fuse prevents huge DC through the output transformer primary.

    I suggest you read the list of failures again.

    The thing is to work out what part failed FIRST and how that led to each of
    the others.

    .... Phil
  4. Phil Allison

    Phil Allison Guest

  5. Phil Allison

    Phil Allison Guest

    "Gareth Magennis"
    ** You had better sit and watch this one then.

    ** Hint - how did one half of the OT primary go open ?

    ... Phil
  6. Phil Allison

    Phil Allison Guest

    "Gareth Magennis"

    ** Not tricks here - I have detailed all the faults and there is no sign
    of any such damage.

    .... Phil
  7. Phil Allison

    Phil Allison Guest

    "Gareth Magennis"

    ** When a power tube loses vacuum, it usually conducts very heavily
    regardless of grid bias setting. The *low pressure* gas inside ionises with
    applied DC and it is lights up like a neon tube.

    ** This is a large transformer and it would normally have almost no temp
    rise in operation - the primary resistance is 20 ohms per side. Even under
    fault conditions, the 1 amp HT fuse protects it from damage.
    ** With any large OT - this is a very unlikely scenario.

    ** This is correct.

    ** Read the thread again. There are no faulty bridges.

    ** I figure more than one HT fuse has blown, but the one in the holder
    looked like the original.

    ** Now you are getting warm...
    ** No way is that the case here.

    .... Phil
  8. Phil Allison

    Phil Allison Guest

    "Gareth Magennis"

    ** The OT failed first due to internal arcing in the primary winding. This
    is induced by overdriving the output stage until "spiking" occurs - ie
    spike voltages on each rising edge of the squared off wave. Spiking only
    occurs with loudspeaker loads ( or simulations ) and is produced by the
    energy stored in the inductance of the voice coils. The voltage spikes are
    of the order of +/-4kV. The problem is mainly with EL34 valves and
    Marshall type output stages.

    Arcing damage will produce shorted turns and sometimes open windings - both
    happened here.

    Once half the primary went open, the two EL34 screen resistors on that side
    began to severely overheat, the screens would have been glowing white.

    The two EL34s on the other side began operating under great stress as the
    tranny now had shorted turns and a much lower impedance than normal.
    Dissipation goes well beyond the max 25 watts, the plates glow red and one
    EL34 quickly became gassy and out of control of the grid.

    Then the 1 amp fuse blew, another maybe larger fuse was tried and that
    resulted in the 22 ohm resistor failing.

    .... Phil
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