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Farfisa Compact Deluxe percussion and key volume

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  • Farfisa Compact Deluxe percussion and key volume

    Hi Everyone,
    I just joined this forum. I’m a synth tech for a living, and work on combo organs a few times a year. The latest one is a Farfisa Compact Deluxe, and I have two specific technical questions. First, on the percussion circuit, there is the opto-coupler, and also another NE2 that sits inside that turns on and off opposite the opto coupler. Is this neon bulb really needed for anything? It’s not coupled to a photo resistor. The neon bulb in this Farfisa never comes on, but the percussion seems to work just fine. I measure 100VDC across the lamp, and the voltage drops whenever a key is pressed, as I would expect. I swapped out the bulb, and it has the same problem. Not sure if I have a bad batch of NE2s, or if I’m missing something else? I’m not too worried about it, since the opto-coupler seems okay.

    Second thing I notice is the overall volume seems to drop with the more keys pressed. Especially with all the footages engaged. It’s hard to describe, but individually the notes sound great. But when I start adding notes in a chord, each note seems to drop in volume. The more notes I play at the same time, I can hear each note lower. Then if I let go of some keys, the volume returns. I haven’t measured specific levels, and it’s kind of subtle. Not sure if it’s an inherent trait. I’ve tried different tubes, and still have the problem. It happens with or without the reverb engaged. It’s like the notes are competing for bandwidth in the output signal.

    Thanks for any insight!

    Mike

  • #2
    Hello,

    My notes :
    Percussion: Each key is associated with a double contact: 1 closed at work and 1 closed at rest.
    If one of the 2 is dirty (or if the key is not fully pressed) the percussion does not trigger and / or has a dead time after triggering.
    Note that at the end of the percussion, if you keep the key pressed, the sound disappears completely. For basses with black keys, the operation which uses only one contact is slightly different.

    Other problems encountered:
    Power supply to be completely rewired.
    ECC83 = ?
    Reverb, Piezo cells = KO.
    Cutted wires.
    Wood frame.

    JP

    explanations n french :
    Click image for larger version

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    Last edited by Jyvoipabo; 08-29-2021, 09:35 AM.

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    • lucabox
      lucabox commented
      Editing a comment
      Dove è possibile trovare questo libro? Grazie

  • #3

    Hello, thanks for the reply, JP! The percussion seems to function okay, as far as I can tell. It’s just the extra NE-2 bulb that’s inside the case that should light up opposite of the bulb inside the opto enclosure that’s not lighting. The neon bulb inside of the enclosure that’s coupled with the bass and treble LDRs does light up and seems to do what it should. I just don’t know if that extra, non coupled, neon light is needed for envelope timing or to regulate anything with the other bulb?

    I did some checking on the other potential issue I’m having with the note loudness. I looked at the output signal on the oscilloscope, and I can see the overall amplitude lower with the more keys held down. Not sure if it’s simply a destructive phase type thing, and maybe it’s just normal. I will say I only notice it when I’m using the presets, and I don’t hear the problem with the Tone Booster engaged. Maybe that’s a clue? I’ve seen this before when mixing signals passively in other circuits. Like each addition loads down the output, essentially squashing down the overall signal. I hope that makes sense.

    I think when I work on it today, I’ll record a little snippet.

    Thanks for the help!

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