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Thread: jr20

  1. #21
    ppp Pianississmo hank's Avatar
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    Quote Originally Posted by lcid View Post
    The finish looks beautiful to me! Looks like Howard was an easy fix and I'd enjoy seeing more photos of your work.
    Thank you LLoyd, this is all new to us, hope we are doing the organ well.

    BTW, Wife and I used to live in Berlin Center, OH. before we moved to Sacramento, CA.

    I want to dig into the pre and power amplifiers, a general cleaning and maybe some capacitor replacements. Electronics is my main deal now after retirement, and we both work part time for an amplifier manufacturer, although not instrument amps. (High end audiophile amplifiers) Everything appears to work electronics wise on the organ, except for the delay line, due to a missing 6sc7 tube. I'm hoping the delay line itself is ok, though I have heard that frequently, that is not the case. We'll see.

    hank

  2. #22
    fff Fortississimo David Anderson's Avatar
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    Quote Originally Posted by hank View Post
    I'm hoping the delay line itself is ok, though I have heard that frequently, that is not the case.
    You can do some tests on the spring reverb unit on its own. Usually what fails on them is the crystal pickup; because it is a Rochelle salt unit, humidity and other environmental factors cause them to deteriorate. Crystal pickups have a fairly high output, so it's not too hard to verify some output from the pickup just by, for example, shaking the unit or driving the input transducer with a signal from another amplifier.
    I'm David. 'Dave' is someone else's name.

  3. #23
    ppp Pianississmo hank's Avatar
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    I had already decided to connect my scope up to the output, and shake the springs a little. 100 yrs ago when I was a kid, everyone was buying those delay lines and using them on their car audio systems. I remember hitting pot holes and hearing the loud crash.....Then I discovered guitar amplifiers and continuous loop tape recorders........more fun!

  4. #24
    fff Fortississimo David Anderson's Avatar
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    If the spring unit doesn't work, you can convert a later spring tank, the type standard in guitar amps, for use in the cabinet with the addition of an extra gain stage between the tank and the recovery stage. Hammond published a service bulletin with a circuit sketched on it using a single transistor.
    I'm David. 'Dave' is someone else's name.

  5. #25
    ppp Pianississmo hank's Avatar
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    Thanx David, I did a little research on the net and I did find the TSB (in PDF format) from Hammond describing a model 4 reverb unit, with wiring and the little 1 transistor pre-amp. Looking around the internet, I see these model 4's available for 25 to 50 dollars. Might be just the thing if my reverb is toast. Will post the results of my testing. If it's OK, then I will look for a 6SC7 tube. I also discovered on the AO-15-C amp three switches for the reverb. Bass, Treble, and a 2 position "bass correction" switch. The bass correction switch does not appear on the copy of the schematic that I have, but that may be for an earlier AO-15 model. More digging required, but I love it all anyway!
    BTW, I really like the tube numbers and switch descriptions that are engraved on the chassis, a very nice touch for anyone trying to get this thing to work, 62 years after it's made!

    hank

  6. #26
    ppp Pianississmo hank's Avatar
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    Guess I'll reply to myself, as I discovered a label on the inside of the jr-20 cabinet describing what all the switches do - the 2 position switch is a 4db cut for the bass, and is based on room size.

    hank

  7. #27
    pp Pianissimo lcid's Avatar
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    Quote Originally Posted by hank View Post
    Thank you LLoyd, this is all new to us, hope we are doing the organ well.

    BTW, Wife and I used to live in Berlin Center, OH. before we moved to Sacramento, CA.

    I want to dig into the pre and power amplifiers, a general cleaning and maybe some capacitor replacements. Electronics is my main deal now after retirement, and we both work part time for an amplifier manufacturer, although not instrument amps. (High end audiophile amplifiers) Everything appears to work electronics wise on the organ, except for the delay line, due to a missing 6sc7 tube. I'm hoping the delay line itself is ok, though I have heard that frequently, that is not the case. We'll see.

    hank
    Hank, you lived very close to me! Only about an hour away. Do you miss these terribly bad winters? Glad you are receiving so much helpful information, having fun discovering and working on your Hammond.
    Lloyd

    Happily retired organist/pianist from the Church of the Brethren...Allen ADC-4300-DK.
    Primary occupation structural and civil engineering.
    Home...Wurlitzer (ES) Orgatron Series 20 Serial #11608 (retrofitted with MIDI and VPO-Hauptwerk) with Leslie 44.
    Hammond BC Serial #5070 with Leslie 31A tallboy tone cabinet
    A.L. Swan antique pump organ (C.1852) Cherry Valley NY
    Member of the Lutheran Church (LCMS): traditional worship.

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