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    Wurlitzer 950 quit working

    I turned on my Wurlitzer 950 and it ciphered a few notes for about 4 seconds and quit working. The power light lit up but no sound, leslie motor did not run, just audio hiss from the speakers. I checked the voltages at the amp/power supplies and found that the -15 and +12 volts were low. I disconnected the molex connectors from the chassis, both of them, and checked the voltage. The +12 was fine but the -15 was 0. I removed the chassis and found that the 723 was bad. I replaced it and the chassis showed -15 volts. I connected the molex connectors and the organ pulled down the +12 volts to about 2.75 volts. Does anyone have any ideas where to look next. Any help would be appreciated. I do have the service manual. Thank you.

    #2
    Off hand it appears you have a short somewhere. Perhaps a part that shares both voltages? I would put an ohms meter on the +12v line (separated from the supply) and see what the resistance is. It will likely be low, (below 200 ohms is low for that voltage) and then you just have to track down where the problem is. You can follow the schematic to determine the flow of the +12v from power supply to all boards that use it. By watching the meter and separating various boards from the line. Tedious but effective...

    Geo

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      #3
      Thank you. I will check and get back to the forum with the results.

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        #4
        Pin 7 on the +12v , pins 5 & 8 on the +20 and pins 13and 14 on the -15 have low resistance reading. I will take your advice and trace the lines and try to remove them from the boards to see if and when the resistances read high.

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          #5
          Hmm, that sounds like the two supplies have got tied together by the fault, as they are otherwise independent being on different amp chassis. If the fault is an IC it may have many pins shorted together, so once you've traced the supply lines that have a suspiciously low resistance to ground, as per Geo find things they feed that use both voltages and suspect any LSI where they meet. If one supply has won out and pulled the other the wrong side of ground, other damage may have been done. Otherwise, I'd suggest re-energising the appropriate board and checking for hot ICs, but there is a possibility of further knock-on effects.

          Alternatively, I wonder whether the 723 failed first and drove the -15V rail all the way to unregulated volts, and that killed something which is now decking the +12. It was only a few weeks ago I was saying in another thread that the 723s in LSI Wurlis are dropping like flies.

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            #6
            I have started the process. It is slow going since the voltage wires are daisy-chained to the boards and the wires are in the harness. I have been searching where the wires go. It looks like the +12 volt in question ties to the Orbit board and other boards in that area up to and including the piston board. I hope it only affected one board and not multiple boards. I was thinking along the same lines that the 723 failed first. Luckily, I had one in my junk pile that I could replace it. I have also installed a DIP socket for the 723. If I get this thing up and running, I will be ordering several 723s and also replace the other one as well. I will let you know what I find either way.

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              #7
              I need to correct my previous post. I meant the voltages were low on those pins not resistance. I traced and removed the voltage wires of the 12 volts on one of the wires in question and no change. Here are volt readings 12 volts reads 2.75, +20 reads 19, -15 reads 0. The resistance readings are as follows with the plugs disconnected from the power supply and one probe connected to ground on the organ and not power supply: 12v legs, pin 7 - 1.77K, pin 8 - 1.6K, pin 9 - 4.42K, pin 10 - 4.54K; -5volt legs, pin 3 - 9.7K, pin 13 - 5.75K, pin 14 - 2.3K; 20 volt legs, pin 5 - 7.14K, pin 8 - 19.3K. I was using the 20K setting on the meter, if I go lower is just flashes the 1. I hope this makes sense. Thanks.

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                #8
                I made some progress today. It appears to be one of the console keyer boards located behind the lower manual, the ones that are double stacked. Is it worth trying to fix and if so how do I work on them without breaking any of the small wires?

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                  #9
                  I think you can unscrew the subassembly from the console carcase and, possibly by unplugging some cables, swing the whole thing out. If you are lucky, the fault will be on the lower manual PCB which has no ribbon interconnects. You can slide the boards up the pins maintaining the same relative positions, then unfold them on the flat, to minimise flexing of the ribbons. Encourage the ribbons to flex in the middle rather than at the terminations and they will be OK.

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                    #10
                    Thanks, Lucien for the information. I may try it. Just a thought here. I believe that there are other problems since the -15 volt line went. If I would disconnect the power wires to the individual boards and connect a +12 and -12 volt supply to the boards and check voltages on a board by board basis. I know some of the boards have +20 volts but the +20 volts seem OK. Do you think that would work and be viable?

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                      #11
                      I got back to looking at the organ. It appears that the -15 volts is missing on all four of the Console Keyer Boards (the ones that are double stacked). I checked each board separately. I contacted Morelock Music but I am afraid the replacement parts or boards will be expensive. Does anyone else have any other ideas? I'm just wondering what else might be damaged.

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                        #12
                        Just an update. I was checking the different boards in the organ and found that it seems that most of the boards that have -15 volts to them are defective. I was checking each board separately as I was told this would not damage anything. This includes the Voicing and Signal Processing board and the Symphonic Presence Modulator boards. The only board that doesn't seem bad so far is the Orbit board.

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                          #13
                          I have another problem. I was working on the organ again. I was checking the voltages and I found the -15 volts on the Orbit Auxiliary board was reading -9 volts. I was going to try to repair the board myself. There are only a few components that utilize the -15 volts on that board. I removed and replaced the 4016 and I rechecked the voltage and the -15 still read -9 volts. I then removed the 4016 from the board and rechecked the voltage again and it read the same(-9 volts). I then replaced the capacitor (C15) and the voltage still read the same. The diode in the circuit checks fine. I was wondering if there is still something wrong with the -15 volt power supply that it can't take a load. What would be the best way to test the supply outside of the organ under load, and how much current does the supply put out? It seems like the other voltages are fine. Any help would be greatly appreciated. Thank you.

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                            #14
                            Usually what I do is disconnect the power supply from a board rather than replacing the board parts. If it changes when a specific board is disconnected, then that board is suspect. These voltages run all throughout the organ so working on a board level should be faster. Also, isn’t there a -9Vdc in that organ? If the source is a three legged IC regulator then the amps should be 1 to 1 1/2 amps.

                            geo

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                              #15
                              There is no -9 volts in the organ. The one power supply/amp has the +12volt, and -30 volt and the other power supply/amp has +20 volt and -15 volts. The voltage regulator is a UC723CN type IC with 2 transistors with it. The -15 volts is low on almost every board that utilizes the -15 volts. I thought I'd work on this board first since it is seems like a simple circuit The organ uses transistors and LSI ICs.

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