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  • Allen 300C Issues

    Well, I've done it now.

    I've been splicing connectors into the wiring of a 1971 300C to make the top removable, and I made an error that resulted in loss of all stops except for three:

    1) The celeste in the great
    2) The celeste in the swell
    3) The swell to great

    So it seems I still get sound from the analog stops but have lost all the digital ones.

    My error was in the rainbow ribbon cable the connects terminals in the card reader and the alterable voice controls to a board in the lower console. I had spliced a connector into the ribbon, but I accidentally mixed up 3 circuits from the card reader and all circuits in the alterable voice, including one that crossed over into a card reader circuit. These are 28 or 30 gauge wires, so I would be surprised if I fried anything, but even after I straightened it all out, I still have no digital stops.

    I have checked all fuses that I can see, and they all tested good with a continuity tester. I did a visual check of the boards and I do not see any burnt components or printed circuits. Any idea what might have happened here?

    Thanks,
    Toby

  • #2
    Toby,

    You've probably already doubled-checked the power supply voltages, but be sure that the -27 volts is accurately adjusted and free of AC ripple. Also be sure that the wiring you disconnected from the keyboards themselves is properly hooked back up. The analog section (celeste) is very simple, and will work as long as you have 12 volts on the buss. But the digital section is much fussier, and won't work unless all the little white or gray wires are connected exactly as they were.

    Do the pedals work? If they do, that would indicate that the MOS system itself is actually still running, just not responding to the key contacts. If not, then that does point toward the MOS system being dead, especially if everything was working before you connected those wires wrong.

    I believe the card reader connects to the board labeled "Clock and Card Reader Logic" in the main section. That is of course a crucial board, since it provides the 4 megahertz clock signal that runs the rest of the system. So that is probably the board that is bad. As you know, the digital generator cannot operate without that board.

    There's always a chance that the board will come back to life if you simply remove the screws holding it in place, then remove it from the socket and clean/lube the card edge connector before re-inserting. (To clean and lube, rub a bit of Vaseline onto the front and back surfaces of the card edge connector, then wipe most of it off, leaving behind only a very thin film to ease insertion.) It's actually a good idea to do that to each board in the system. In my experience, cleaning and lubing these card edge connectors will cure a multitude of problems.

    If that doesn't do it, you may have zapped something on the board. Try disconnecting the card reader itself from the system to see if it will come up. If it will, you can do without the card reader of course.

    Beyond that, you will probably need a new clock board. Since there are so many MOS organs behind dismantled these days, you might see a clock board offered for very little money somewhere, since it's the simplest and smallest of the MOS system boards. You can get one that is simply a "clock" without the card reader logic, if you see one. The card reader was an option, and the organ will work without it.

    The good news is that the MOS system was way overbuilt, with extremely durable components throughout, so it's unlikely that anything else was damaged. And Allen built many thousands of these organs, so there is a large pool of used parts out there, as well as a deep stock of them at the factory.

    One approach would be to find another used MOS organ and use it as a parts source. All MOS organs built from 1971 to 1980 or so have a nearly identical set of boards inside (there were numerous variations on the MOS board, for customizing the voices, and some variations on the KBA to provide custom features, but for the most part the boards are all interchangeable).
    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

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    • #3
      Thank you, John. How lucky we are to have you on the forum. My electrical troubleshooting experience is all on pinsetting machines, so these organs are all still quite foreign to me.

      I'll take a look at the power supply and check voltages there, making sure to use the system ground instead of an earth ground as you've described in other posts. I did not yet check any voltages, I just traced the black hot wire to the stop rail, and when I engaged the celeste and got relay action, I eliminated loss of power and moved on.

      Currently the pedals do not work. I can engage all stops and still only get sound from the celeste on the great and swell.

      I did not yet touch the white wires from the keyboard assemblies. I made two modifications since the last time the organ functioned properly: first, the connectors I described to card reader/alterable voice, and also connectors in the yellow stranded wires that connect terminals on each of the stop tabs to studs on a pin board. I checked continuity on stops 1 through 19, at which point I couldn't reach far enough to continue, and I had continuity to their respective positions. One thing I noticed, however, is that the 16' stops on the pedal all had continuity to the same three studs (3, 4, 5?), which would normally make me suspect bad diodes, but again I have no literature.

      I'll start by cleaning all connectors. Would you recommend dielectric grease as an alternative to Vaseline?

      I would not be surprised if I damaged the clock board. Is there an established method to bench test components on there to check functionality? Or do you just swap boards until you isolate the problem to one of them? (I have more time than I have money.)

      I really appreciate this!

      Toby
      Last edited by TSchwartzman; 08-27-2018, 01:09 PM.

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      • #4
        Toby,

        There is in the floor of the console a large metal chassis with a cage around it, and it is labeled "MOS Power Supply" (I think that's the nomenclature). It has a number of terminals across the front, and you will see that there are several voltages it supplies. With the organ powered on, you put your black voltmeter lead on the "ground" terminal set (far right, I think), then use the red probe to look for the other voltages, as labeled on the face of the supply. You should have -27 volts (possibly one that is always on and one that is switched on by the power switch), as well as +5 and -5. There is a 12 volt terminal, but it is actually an unregulated voltage that comes from a separate supply in the organ floor. It can run anywhere from 12 volts to 16 volts, depending on line voltage and load. There is also a card reader lamp voltage, but all you need to do at this time is verify that it reads around 8 volts. All the voltages except 12 volts are adjustable via small potentiometers on the face of the supply.

        Verify that the voltages are within a tenth of a volt or so of correct, and then put your meter on the AC range and check to see how much AC (ripple) rides on each DC voltage. It shouldn't be more than a few hundredths of a volt on any of them.

        I use Vaseline to clean card edge connectors because I know it is completely inert and non-conductive, but it doesn't cling to the metal or leave a residue other than the slightest bit of lubrication to ease insertion. I don't know about using anything else. You mustn't use anything that does conduct electricity though, such as silver-bearing grease.

        It occurs to me that the celeste will play if none of the voltages are present other than the 12 volts. The -27 and the +/-5 are all used by the various MOS boards, but the celeste is keyed off the 12 volt supply, and even the relays in the celeste circuit use that 12 volt supply. The voltage that un-mutes the audio system also comes from that supply. So your entire MOS supply may be down, though I doubt it. You'll know once you check it for voltages.

        BTW, the black wire that is screwed to the stop rail is just a ground wire. In the MOS system, all switching on the stop rail is done by GROUNDING various circuits. Each wire that runs from the Keyboard Array (KBA) board to the stop rail turns on its corresponding stop when it gets grounded, and the celeste relay operates the same way. It has 12 volts sitting on one side of the relay coil, and when you flip the celeste tab it provides a path to ground and the relay trips.

        I only point that out because the fact that the celeste relay trips does not prove anything at all except that the 12 volt supply is running.

        Unfortunately, schematics to most of the boards in the MOS system are not available. Not sure about the clock board, as it is relatively simple and could have been published. If you aren't able to resolve the matter by checking the power supply I'll see if I can dig up a clock board schematic for you.

        The usual manner of working with Allen digital organs of any age is to swap boards. The defective boards can be sent to Allen for factory repair, though some of them carry a pretty high price tag. I have been able many times to repair a defective KBA or SBA or DAC board, as the early ones were built with discrete components. but I haven't had to tackle a clock board.

        I'm hope you might be able to boot up the organ with the card reader disconnected. It's possible that the card reader itself is what was damaged by the reversed cables, and if so, it could be shorting out something and keeping the organ from coming up.
        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

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        • #5
          Thank you, John. The organ is now working, but I still have a mystery to solve.

          I followed your procedure and tested the terminals on the power supply. Initially, everything tested good except the -5V and +5V, which read -.680 DC and -.386 DC respectively. They were totally unresponsive to the adjustment knobs. While scooting around on the floor, the organ powered off. I scooted around more and it powered back on. I realized that by jostling the cables on the line side of the power supply, I was turning the organ on and off. After getting it powered up again, I re-checked voltages on the power supply and found the -5V and +5V reading about -3.8 and +4.0, and this time they responded to adjustment. Bingo, all stops are working and I have full functionality.

          So my wiring mistake turned out to be a red herring. Instead I have a connectivity issue in the cables carrying the line voltage. Any thoughts on that, or just replace?

          Thank you, sir.

          Toby

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          • #6
            If it's that simple, just replace any suspect wiring. Not surprised that you could have problems with that, as the organ is pushing 50 years old!
            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

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            • #7
              Good deal. I really appreciate the information. I've been reading your posts on delay modules as well, since I get a pretty good pop when I turn the console on and need to make sure my stereo receiver is off. Both amps are gone from this console. The previous owner put 1/4" mono plugs on the truncated cables, which we plug into a small mixer and output to RCA plugs. We don't get much volume out of our setup, but it's just a practice instrument for my wife.

              Again, thanks. Hopefully smooth sailing for a while from here.

              Toby

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              • #8
                Yes, lacking the original amps you would not have muting on the audio, thus the loud pops upon turn-on and turn-off. The only way to handle that is to turn the volume all the way down on your stereo system before powering the organ up or down.

                The low volume level is due to the fact that the Allen DAC outputs a very low level signal, far lower than a standard "line-level" device so common today. So, your receiver's aux input is expected the signal to run around one volt or so, but the DAC only outputs a few hundredths of a volt at best.

                The cure is to use a pre-amp to raise the signal level before it goes to the aux inputs on your stereo. A microphone pre-amp usually works well enough. But you may also get no expression (the volume won't change when the "loudness" pedal is pumped on the organ) when connecting to a non-Allen amp system. This is because not only is the signal level weird, the "impedance" is too. The DAC is expecting an amplifier with a very high input impedance, around 50K to 100K ohms. This problem can be eliminated by using expression "isolators" from a company called Harrison Labs.

                All this quirky adapting required to make an old MOS organ work with a modern amp is quite disconcerting, so one of these days you might try to find a pair of the original T-50 amps and set it up properly, providing the original wiring is all still in place in the console.
                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


                • #9
                  Ultimately I would like to get the organ back to rights with proper amplifiers and speakers. We currently live in a small space and don't really need much volume, but if indeed the crescendo pedals don't work (I'll have to test those), then I suppose we'll want a set of T-50 amps. I already received a private message indicating where I could acquire some.

                  Another thing I know nothing about is organ speakers. Are there varying sizes? Can I use a single speaker, or do the amps output two or more channels? The previous owner demonstrated it to us using a large guitar amp!

                  Toby

                  Comment


                  • #10
                    If your seller demoed it using an ordinary guitar amp, he may have internally connected the two channels together. But the original configuration would've been two channels, as all Allen MOS organs have two channels coming out of the DAC board.

                    If you ever set it up again with T-50 amps, you should get two of them if you can, then restore the separation of the channels. It's not essential, but the organ was designed to be heard that way.

                    There are two distinctly different ways a MOS organ of that era could handle amplifiers. If your organ never had amps in the console, it probably has the expression already tied into the audio signal in a junction box in the floor of the console. But if it originally had amps in the console, the expression would have been wired to the amps. Depending on how the organ was originally, configured, you will need either code 10 or code 5 amps. But we'll cross that bridge when we come to it!
                    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

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                    • #11
                      After a little more splicing on the organ, I can verify that the volume does adjust with the expression pedal, even though the original amps are missing. The seller did not combine the two channels, he just took the two cables and put mono plugs on the end. We plugged those into a mixer and have a single channel running to our stereo receiver.

                      One question about this organ. When I have all of the stops off, I still get a single voice on both manuals and pedals. My impression is that I should hear silence. Is this normal, or do I have an issue on my hands?

                      Thanks,
                      Toby

                      Comment


                      • #12
                        Maybe the crescendo pedal is not fully closed, or maybe it needs adjusting. Look at the crescendo roller from the rear and be sure that none of the "whiskers" are contacting the rotary bar.
                        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

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                        • #13
                          I'm in business. You were correct, John, the crescendo pedal was floored and I didn't know enough to realize what that pedal even does. I finished the wiring on this console on Saturday and we moved it up into the house on Sunday. At some point I'll open it up and take some pictures. It isn't gorgeous, but I put connectors into every wiring loom. For the 30-gauge wires coming off the keyboard, I cut each wire and fed the ends into "breakout boards" -- printed circuit boards with screw terminals -- which I connected with those big old D-sub cables. Everything is working except the card reader, so I'll have to look into that.

                          Toby

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