Results 1 to 3 of 3

Thread: B3 percussion troubleshooting

  1. #1
    Senior Member jimmywilliams's Avatar
    Join Date
    Jun 2005
    Location
    Newton NJ
    Posts
    2,613


    B3 percussion troubleshooting



    Last year I restored a B3/22H setup(I restored the Leslie amp; did not work on the AO-28 preamp in the organ itself). I was overat the location again todayforour parish picnic and of coursedropped by the chapel to "check up" on the organ. I examined it a bit more closely this time and I can't get the percussion working at all. I didn't have tools, etc. but was just playing it. Is there a standard troubleshooting technique for the percussion. I know there is a "percussion cutoff" pot somewhere on the pre-amp and I know you have to use the "B" preset button on the top keyboard to use the percussion effect. I tried second and third harmonic. Neither seems to work. Remember I know little about these organs, although I have been trying to read up on them. Do I start with the percussion cutoff adjustment and work from there? Are there any common points of failure in the percussion circuit? I don't know if they know or care that the percussion is not working but I just want to learn anyway. Thanks for any advice.</P>


    -jim</P>
    Jimmy Williams
    Hobbyist (organist/technician)
    Gulbransen Model D with Leslie 204
    Farfisa Compact / Leslie 860 and Combo Preamp III / Hammond Porta-B

  2. #2
    Senior Member geoelectro's Avatar
    Join Date
    Aug 2008
    Location
    Houston Texas
    Posts
    3,246


    Re: B3 percussion troubleshooting




    There is info I have provided on my site at Bentonelectronics.com Service Info </p>


    Look under servicing the AO28 preamp.</p>


    George
    </p>

  3. #3
    Member bluetantra's Avatar
    Join Date
    Sep 2007
    Location
    Pineville, Louisiana
    Posts
    450


    Re: B3 percussion troubleshooting



    Hey, Jim-</P>


    Here's what the B-3 service manual says:</P>


    The "Touch Response" percussion feature is controlled by four tilting tablets. It is available only on the upper manual and only when the "B" adjust key is depressed. Percussion tones are produced by borrowing the second or third harmonic (depending on position of the "Percussion Harmonic Selector" tablet) from the corresponding drawbar of the upper manual "B" adjust key group, amplifying it, returning part of it to the same drawbar, and conducting the balance through push-pull control tubes which, when keyed, cause the signal to fade away at a pre-determined rate.</P>


    With the percussion tablet on, "B" adjust key pressed, and an upper manual playing key pressed, the second or third harmonic signal goes to percussion input terminal H on the preamplifier chassis and is amplified by T4 and T5. The percussion input transformer T5 not only provides push-pull signal for the control tube V7but also has a third winding which feeds signal back to the 2nd or 3rd harmonic drawbar through equivalent key circuit resistor R50 and terminal "J".</P>


    When a key is depressed, the note first sounds loudly, after passing through the control tube V7, transformer T6, a high pass filter, and terminal D to the grid of V4. Immediately, capacitor C31 in the control tube grid circuit begins to discharge, causing the signal to fade away.</P>


    The circuit works as follows: Terminal K (Approximately -25 volts) is connected to the 8th harmonic "B" adjust key drawbar wire, which is connected through the adjust key contact to the manual bus bar. Pressing any upper manual key connects this bus bar to a tone generator terminal and virtually grounds terminal K through the tone generator filters. This virtually grounds the plate of V5, stops conduction, isolates the cathode of V5, and thus isolates the grid circuit of control tube V7. The grid then drifts from about -25 volts to about -15 volts, at a rate determined by the time required for C31 to dischargte through R57 and R58. At the completion of this sequence, the percussion signal is blocked so that it is no longer audible.</P>


    No further percussion signal can be heard until all keys of the upper manual are released so that the control tube V7 grids can again drop to -25 volts (the rate of this drop is fixed by the time required to charge C31 to -15 volts through R55 and R56). Thus the percussion effect is heard only when keys are played in a detached manner; that is, when all keys are released before pressing the next one.</P>


    I serviced an A-105 a few weeks ago that had no percussion, among other problems. The tube tester showed that both tubes in the percussion section of the pre-amp were gone and going. I'd check tubes first, a 6C4 and a 12AU7. Hope this helps!</P>


    Best,</P>


    John</P>

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •