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Allen 120-C Power Supply Repair

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  • Allen 120-C Power Supply Repair

    Hello,

    I have an Allen 120-C model. I believe there is an issue with the voltage settings on the power supply box. The organ "only" produces an either deep hum or a high pitched squeal (when the transposer is at A440) and disables the function of the keyboard and stops. The only way to silence the hum and squeal has been to adjust the voltages, however, it then silences the entire organ including the stops and all keyboard functions. In setting the voltages to the -5, 5, and -27 voltage levels with a multimeter noted on the supply unit, the deep humming persist and all keyboard function does not work. Seeing how the PSU controls have been the only way to either control the hum, at the expense of functionality, it seems that the issue must involve the unit. The fuses in the box are not blown and all the capacitors seem to be normal. The organ worked fine as of a week ago.

    Has anyone experience this before? Will the repairs be inexpensive? I appreciate any help, feedback, or comments.

  • #2
    Sounds like the organ has 60 cycle hum (assuming USA, 50 cycle if Europe) and adjusting the power supply is simply muting ALL of the audio, including that which is unwanted. Metal can-style capacitors, probably just one, are the usual cause. But in short, with or without the hum, this organ does not play? If Allen's usual complement of two or more amplifiers is true here, then you can probably eliminate the amplifiers themselves. However, a shorted output transistor can also produce a hum, as I recall.

    It is not likely to be an adjustment if all was well a week ago. Something has broken which will need to be replaced. A capacitor that looks good can still have failed. I've heard of seeking a bad one by temporarily holding a good one across each in turn, carefully observing polarity. I have not done it and thus do not remember what the recommended test value was. Note that lethal voltages will be present during such a test making the placing of fingers, etc. imperative; perhaps not for one that has never ventured into electrical circuits. At least not for a first solo attempt. Others will chime in soon.
    Roland Atelier AT-90s, AT-80s, AT-70, 30, and 15. Roland VR-760 combo
    Yamaha S-90, Kurzweil PC-3x, Casio Privia PX-330, Roland E-80, G-70, BK-5, Leslie 760, 820
    Moved on:
    Allen 3MT/Hauptwerk, Technics GA1, Yamaha HX1, AR80, numerous Hammonds, including 2 M's, an L, 2 A-100's, XP-2, XM-1/1c, & an XK-3. Roland Atelier AT-30, 60r, 80, & 20r(2 units), and a slew of Leslies (147, 142, 760, 900, 330).
    Korg Triton Le-61, Casio Privia PX-310 & 110, and Kurzweils: PC-2x, SP-88, Pro-III, K1000

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    • #3
      The first thing to do is measure the DC supply voltages with the voltmeter set to AC volts.

      You said the capacitors seemed normal. Which capacitors?
      The modules inside the power supply used paper axial lead electrolytic capacitors which are due to fail if they are original. May only show a little powder substance on the ends. Could be bad with no visible signs.
      We also need to know the part number of the power supply. It will start with 905-.

      td
      Servicing electronic organs since 1969.

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