Ebay Classic organs

Collapse

Announcement

Collapse
No announcement yet.

Rodgers 740 low volume

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • #31
    UPDATE:

    Checked photoresistors and lamp. Lamp works, but there is a weird phenomenon occuring:

    When the expression pedal is fully open, you expect full volume, but what you get is a reduction in volume. At fully closed expression, there is relatively speaking full volume, though this is still very quiet.

    All three photo receptors work, but the one for this channel, responds opposite to light. When you cover it, you lower the volume, and when it gets a lot of light, it increases the volume. This is the opposite of the other two photoresistors, which respond to light with a decrease in volume.

    Basically:

    1. Light = lower volume
    2. Dark = high volume.

    For our broken channel, it's the opposite. This is in addition to the fact that the channel is low volume to begin with.

    Comment


    • #32
      It would be on the output preamp board, where all the voicing adjustments are made.

      Comment


      • Esteban Enrique
        Esteban Enrique commented
        Editing a comment
        I only see an L15, no 16 in there... Hmm...

    • #33
      There were two versions of the 740 and my schematic could be the wrong one. ICs are marked with the letter "U" and a number. My schematic seems to show U15 as the one that amplifies channel 1 just before it goes on out, but I may be looking at it wrong.

      Some years back I had a Rodgers analog with a channel doing what you describe -- gets quieter when expression pedal goes down. The schematic shows that the expression photoresistor is in a FEEDBACK configuration with an op-amp, so when the op-amp is dead, the photoresistor is forced to act as a FEED THROUGH device instead of a FEEDBACK. Thus, the circuit will operate in reverse, as you observe.

      So the op-amp associated with that particular photoresistor is probably the bad one, if you can figure that out from the circuit diagram. Of course it could also be some other part of the expression control circuit, but the op-amp is cheap and easy to replace, so try it first.
      John
      ----------
      *** Please post your questions about technical service or repair matters ON THE FORUM. Do not send your questions to me or another member by private message. Information shared is for the benefit of the entire organ community, but other folks will not be helped by information we exchange in private messages!

      https://www.facebook.com/pages/Birds...97551893588434

      Comment


      • Esteban Enrique
        Esteban Enrique commented
        Editing a comment
        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.

    • #34
      The 740 was built in 2 versions: the 740 with no suffix and the 740B. You can tell the difference in that the B version:

      1. The 740B has only 3 groups of stops in the top row: swell, positiv, generals; the plain version has 4 groups (generals, swell, positiv, and generals);
      2. The 740 B has a 16 ft Pedal Contre Trompette, Swell Contre Trompette 16 and Clairon 4; these are not on the plain 740;
      3. The 740B has a Cymbale II in the swell.

      The output preamp is sure to be different between the two.

      I only have the schematic for the plain version. On it, U14 is the last set of op amps before the headphone mixing; it is a CD4156N quad op amp in a 14-pin package (7 pins on each side). It is probably located near the upper right hand corner of the output preamp. Available from Mouser here: https://www.mouser.com/ProductDetail...DfaCcleg%3D%3D

      If you have the B version you'll need to find someone with the schematic for that version. It seems like JBird's schematic might be for the B version, since it is showing U15 as the op amp in question. Incidentally, all 3 channels go through this IC as it has 4 op amps, though just one of them might be bad.


      Comment


      • #35
        Thank you both for the information. Quick clarification, this is the PLAIN 740, NOT the 740B.



        jbird604 the information you give on the photocell resistor is very important and seems to pinpoint the issue. This mechanism you describe is amazing, and a great way to diagnose the problem!

        toodles could you pm me the schematics for this (740)? I want to make sure I will be changing out the right op amp. I just purchased 4 of this op amp just in case I need more (and to justify the shipping price).

        A few questions as I proceed:

        1. How do I change the op amp? Would this be a regular desoldering job and then resolder the new part?
        1b. Is there a correct orientation to solder it on, or either way would work?
        (EDIT: these are socketed, so I will just use an IC puller)

        I think the channel in question may be channel 2, because the channel 2 adjustment pot is what changes its volume. Again, it is hard to distinguish by sound, since there seems to be so much leak through between the channels - the sound pretty similar (base obviously has the low frequencies). I will confirm which channel is the defective one tomorrow and pinpoint the op amp.

        2. 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.

        If this is almost certainly the solution, I am happy to try it, although a bit fearful of damaging the circuit if there is soldering involved. I will be very careful and perhaps practice on other circuits before attempting.

        Thanks again!

        Peace
        Esteban
        Last edited by Esteban Enrique; 07-02-2020, 04:12 PM. Reason: EDIT: Realized that these are socketed ICs, and no need to de or resolder!

        Comment


        • #36
          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.

          Comment


          • samibe
            samibe commented
            Editing a comment
            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.

          • myorgan
            myorgan commented
            Editing a comment
            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

        • #37
          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

          Comment


          • Esteban Enrique
            Esteban Enrique commented
            Editing a comment
            Thank you friend! I spotted this indentation this morning, good clarification.

        • #38
          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.

          Comment


          • #39
            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.
            John
            ----------
            *** Please post your questions about technical service or repair matters ON THE FORUM. Do not send your questions to me or another member by private message. Information shared is for the benefit of the entire organ community, but other folks will not be helped by information we exchange in private messages!

            https://www.facebook.com/pages/Birds...97551893588434

            Comment


            • jbird604
              jbird604 commented
              Editing a comment
              Just hang on to the extra chips. They may all fail in time.

            • jbird604
              jbird604 commented
              Editing a comment
              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.

            • Esteban Enrique
              Esteban Enrique commented
              Editing a comment
              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!

          • #40
            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!

            Comment


            • myorgan
              myorgan commented
              Editing a comment
              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
              Esteban Enrique commented
              Editing a comment
              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
          Working...
          X