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Rodgers 740 low volume

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  • Esteban Enrique
    commented on 's reply
    I'm pretty sure the organ was off... We literally moved it a foot and a half or so. This is issue is almost more frustrating than the last because we caused it! The channel issue came with the donation

  • myorgan
    commented on 's reply
    Congratulations on finding your issue! I understand your exuberance when an issue is FINALLY fixed.

    Stupid question, but was the organ on when you moved it? If so, if you jarred the console in the move, that could have caused the issue.

    Michael

  • Esteban Enrique
    replied
    Hallelujah!!

    I replaced chip U14 and the organ sounds like new! Channel 2 in full operation !!!

    Thank you guys so much! I would have never fixed this without this forum.
    ​​​​​​
    I still have to fix the lights for the stand and the pedal board, but this is nothing compared to the problem we just fixed.

    I checked the fuses, it seems like there is continuity in each of the three, but the multimeter sometimes goes back and forth, like it's not 100 percent.

    Other than the fuses, what else could I be checking? I'm almost sure the wires are not the problem because the lights were working perfectly before until we moved the organ a few inches literally.

    Are broken fuses common enough that I should just go ahead and replace them to see if it fixes it?

    Thanks again!

    Leave a comment:


  • Esteban Enrique
    commented on 's reply
    I overlooked the error, as I understood as soon as I looked at the schematic. Makes sense though, quite elegant when you look at it!

  • jbird604
    commented on 's reply
    EE -- note that I made an error in the post above, which I have now corrected. I meant to say that the PHOTORESISTOR (or LDR) appears in the schematic as a resistor inside a circle with the Greek letter lambda underneath it.

  • jbird604
    commented on 's reply
    Just hang on to the extra chips. They may all fail in time.

  • Esteban Enrique
    commented on 's reply
    Great, that makes a lot of sense. Ill start with U14 which on the top right of the board. The schematics are really helpful. I ordered 4 of the chips which was probably excessive.

  • jbird604
    replied
    To answer some of your questions..

    It does appear that U14 is the chip that is defective (I'm looking at the schematic for the 740A, so if you have the 740B, your chip numbers could be different). But in my schematic, U14 is the one that is in the expression portion of the pre-amp board.

    An op-amp IC can have one, two, three, or four separate op-amp circuits in the same chip, and yes, one of the circuits can be dead while the rest are working. That would explain why the other two channels express normally while just one of them does not.

    The photo-resistor (or "light dependent resistor") appears in the schematic as a CIRCLE with a resistor going through the center of it and the Greek letter "lambda" inscribed under the resistor. Also the letters "LDR" appear beside the symbol. If you look at drawing # 5008-305, at the right-hand edge of the drawing, you will see the three LDR cells depicted. Note that each LDR is in a feed-back circuit with its associated op-amp -- the more the LDR conducts, the more signal it feeds back to the negative input of the op-amp, thus the more it decreases the volume of that channel. The resistors and capacitors in the same feedback circuit serve to make the expression most effective at high frequencies, letting the bass roll off more slowly, similar to the way pipe organ shades operate.

    The chips are cheap, and it won't hurt a bit to change them one by one until you find the bad one. Just be sure to maintain the proper orientation and don't bend any legs or make them curl up underneath the chip as you insert them.
    Last edited by jbird604; 07-04-2020, 03:25 PM.

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  • Esteban Enrique
    replied
    While I wait for the IC and subwoofer foam to get here, I had a question about the schematics:

    1. Where are the symbols for the photocells on this? I found on page four the output schematics which show the U15 and U16 op amps but I can't recognize where the photocells are in the circuit?
    2. I see U15 amplifies all three channels. From my observations, it seems U16 amplifies for the headphones?
    3. If U15, again, amplifies the three channels, is it actually possible for it to be defective for one of the channels but not the other two? Wouldn't it prevent all three channels from sounding?

    I forgot to mention, I plugged in the headphones again yesterday and both ears worked this time! I think the problem with the headphones before was a faulty 1/4" adapter which can be iffy depending on how you twist it in.

    EDIT: Basically, since I bought several of these op amps, I can actually just change both the U15 and U14 op amps to cover both bases. They both use the same type of op amp.

    1. If the problem is heard in the headphones, U14 is the one to change because it is further back
    2. If the problem does not exist in the headphones, U15 is the one to change.

    I will change both, one at a time, and see which does it, as I can't tell from the headphones if I can hear channel 2.
    Last edited by Esteban Enrique; 07-04-2020, 05:52 AM.

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  • myorgan
    commented on 's reply
    Sam is giving excellent advice. I always take extensive photos of the interior of a new organ before I delve into it. It helps me return things to original should I take an extremely wrong turn.

    Michael

  • samibe
    commented on 's reply
    I find it helpful to take lots of pictures before changing anything. That way I have a reference when I start putting things back together.

  • Esteban Enrique
    commented on 's reply
    Thank you friend! I spotted this indentation this morning, good clarification.

  • you795a
    replied
    Here is a photo of an IC. You will notice the "U" shaped indention at the top if the IC as toodles explained. The IC is not the Op Amp but all ICs have a marking similar indicating pin 1 just as toodles said. I thought a photo would help.
    Attached Files

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  • toodles
    replied
    Here is the schematic for the 740 Output Preamp: Output Preamp 740.pdf

    Q1. How do I change the op amp? Would this be a regular desoldering job and then resolder the new part?

    As you noticed, the IC is in a socket. You pull the old IC out and insert the new one. Usually you have to slightly bend the new IC's pins inward. I use a flat surface like a table top, and bend all the pins on one side simultaneously. Be cautions and don't apply too much force. Then do the other side.

    IC's are always identified at one end: sometimes it is a depressed dot, and sometimes it is a U shaped indentation or a rectangular indentation. In all cases, when that end is up, pin 1 is the upper left hand pin when facing the labelled surface of the IC. IC sockets are usually marked similarly. In any case, note the orientation of the IC before removing it, and replace the new IC in the same orientation.

    It doesn't matter which channel it is since all 3 channels are handled by the same IC.


    Q2. Given the defective channel is based on the TOP photoresistor (the three are: left, top, right), which channel does this indicate on the schematics? It is a little confusing because I think the channel indications on the bottom of the organ where the speaker cables are may be wrong.

    LIke I just said--it doesn't matter. You will be changing all 3 at the same time.

    No soldering should be needed.

    Leave a comment:


  • Esteban Enrique
    commented on 's reply
    Thanks, I will try this first. That's a very interesting mechanism and helps to diagnose the problem. A schematics diagram would help me make sense of all this, but I trust you are right.
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