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Allen ADC 3500 repair advice

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    Allen ADC 3500 repair advice

    I recently acquired an Allen ADC 3500. It wasn't free, but it wasn't terribly expensive either. It's a joy to play, but there are a couple of items that need repair, and I'm hoping to get some advice, as I think the nearest authorized repair is going to be too far away to be practical, even if I was willing to pay for the repairs.

    1.) In the pedals, the top B and C do not sound (is that C4?). I've moved the pedal assembly, and used a magnet to activate the various notes, and those two still do not sound, but all the other notes do using the magnet. I then took the cover off and examined the reed switches, and I couldn't see any visible difference with the B and C from the other reed switches. Is there always a visible difference if a reed switch has gone bad? I've read in these forums about replacing the reed switch, and I'm sure I can handle that. Can I use alligator clips to jumper the contacts to see if the note will sound that way? (therefore testing the theory it is a bad reed switch, just with no visible sign of the problem). Any other thoughts as to how to approach this problem or what might be the issue? (And I don't know if this is relevant, but for the first few days I had the organ, the C# didn't sound either, but it came on and has been fine since).

    2. The bass drum tab on the pedal division is not activating the bass drum (however the bass drum does sound when used by the automated rhythm section, blech!). Is there a way to jumper and test the stop mechanism? Is it difficult to swap the stop mechanism? I have a reverb tab that is not in use (there is a Nanoverb hard-wired under the hood). I wonder if I could swap the tabs and stop mechanism easily.

    3. The Nanoverb often just stops adding reverb, and passes the signal right through. I have to power-off the whole organ, and back on, and the reverb will be back. Length of time the system has been powered up doesn't seem to affect it when it stops, it just does. I don't have to fix this, but I assume, like many electronics, it's not necessarily healthy for the device to use cycling the power as a remedy.

    4.) Not a repair issue, just a question: is there any ill-effect of leaving stop tabs down when finished playing and powering down the organ?

    I get that the Allen organs are built like tanks, when I looked under the hood, everything looked fairly simple to get to and to work on. Moving the organ away from the wall to work at the back of the organ is another story though!

    Maybe too many questions in one post? Should I break it down into individual isses?


    #2
    1. You could physically short around the pedal reed switches with a jumper wire to see if the reed switch is the problem--if the notes play with the jumper, then the reed switch is defective. A defective reed switch might look fine. You have to look very closely at the reeds with them perpendicular to your sight to see them move, so there's a good chance you won't a problem.

    2. The bass drum tab on the pedal division is not activating the bass drum (however the bass drum does sound when used by the automated rhythm section, blech!). Is there a way to jumper and test the stop mechanism? Is it difficult to swap the stop mechanism? I have a reverb tab that is not in use (there is a Nanoverb hard-wired under the hood). I wonder if I could swap the tabs and stop mechanism easily.

    2. If the bass drum sounds with the automatic rhythm, then the sound generator is working. The stop tab could be suspect. I don't know if the ADC 3500 has reed switches on the tabs or used the older leaf contacts. If it's leaf, you could check to see if they make contact. In either case, jumper around the switch contact to see if that turns on the bass drum. Potentially, it could be the wiring from the stop tab to the multiplexer. Changing out a tab mechanism is not terribly difficult, but getting them adjusted for aesthetics takes some fussy work.

    3. You could try the reverb outside the organ to see if the problem is in the reverb unit or the drive signal to it.


    4. There are no ill effects to leaving tabs down when finished playing the organ.


    Comment


    • Admin
      Admin commented
      Editing a comment
      Not sure about this model, but on my MOS-2 theatre organ, there were two sound generators. One was controlled by the stop tabs with other controlled by the rhythm generator. So, it might not follow that a working rhythm section voice translates to a working stop tab voice.

    #3
    Excellent answers! Thank you so much!

    Comment


      #4
      Originally posted by Vebo View Post
      I recently acquired an Allen ADC 3500. [snip] It's a joy to play, but there are a couple of items that need repair, and I'm hoping to get some advice, as I think the nearest authorized repair is going to be too far away to be practical, even if I was willing to pay for the repairs.
      Vebo,

      Welcome to the Forum!

      You are only approximately 2 hours from Atlanta, and I'm sure there is an authorized Allen service person near Atlanta. You can search for a technician here: http://www.mitatechs.org/service-locator/.

      Reed switches can fail with no visible signs of damage. I had one I needed to replace on my ADC-4300, and there was no visible sign of cracking, etc. I have also replaced a reed switch in the pedal, where someone helping me didn't know what (s)he was doing and shoved the bench into the plastic cover over the reed switches. I also didn't see visible signs of damage in that case, but knew what the issue was.

      Personally, I would replace the C# reed switch as well. If it was intermittent, I would replace it anyway. You never know when it could return. And be careful when you replace them. If you place them too high or too low, the pedal note could be activated at either extreme without warning.

      Hope that helps. Thanks for being part of the Forum.

      Michael
      Way too many organs to list, but I do have 5 Allens:
      • MOS-2 Model 505-B / ADC-4300-DK / ADC-5400 / ADC-6000 (Symphony) / ADC-8000DKC
      • Lowrey Heritage (DSO-1)
      • 9 Pump Organs, 1 Pipe Organ & 4 Pianos

      Comment


        #5
        If the reedswitches are the culprit, Allen uses Meder brand reedswitches. In the older organs they used the glass reedswitches and in the newer organs they use the SMT reedswitches. In the pedalstrip, where the reedswitches are soldered to the pins, there should be a groove with a deeper groove. The edge of the glass part of the reedswitch should be at the edge of the deeper groove, That was the rule of thumb we used at the factory. Sometimes they had to be moved up or down but not too often. Be careful with the reedswitches because they do break easily. I'd recommend ordering a few extras just in case. Hope that helps.

        Comment


          #6
          The Allen 3500 uses reed switches in the stop mechanism. They do fail on occasion without warning. They are sensitive reed switches and need to be replaced with the same kind. You talked about 2 dead pedals. I would bet bad reed switches there also. Not really common to see them bad in stop action or in the pedals, but anything is possible. They are sealed to the air, normally. If they loose the seal, the contact surfaces get oxidized and stop working. Replacement is the only fix. Same problem with the bass drum, I would assume... bad reed switch.
          Be advised the reed switch in the pedal and on the stop tab are not the same type. Pedal ones are not as sensitive as stop type.

          Now the Nano Verb unit. These suffer from a couple of things. I own 4 of them and have been servicing them for years. Problem #1 is the controls inside really get oxidized badly and no longer do what they are supposed to do. Hence.. dead sounds. You have to disassemble the unit and clean with a good no-residue contact cleaner, the controls within the unit. With the 4 I have, as well as my Micro Verb unit, this is a major cause of trouble. The other thing, which sounds more like you problem, is there are several filter capacitors in the power supply section of the unit. The wall-wart unit is 9VAC, not DC. You cannot use a DC power supply for this unit. The capacitors fail and the internal microprocessor cannot operate with any AC in the filtered DC part of the unit. VERY common problem. It is fixable, if either of these are the problem. Having to shut the organ off, indicates a possible failure of filter caps. My only question is this... Does this one actually have a Nano Verb in it? Usually Allen has their own ADR-4 system installed there. It could have been changed by the previous owner.

          Hope this helps you out somewhat. The Allen is a great organ. And still All-American made!

          Digichip

          Comment


          • myorgan
            myorgan commented
            Editing a comment
            Digichip,

            Great advice and analysis of the situation–thank you for being part of the Forum!

            Michael

          #7
          Originally posted by you795a View Post
          If the reedswitches are the culprit, Allen uses Meder brand reedswitches. In the older organs they used the glass reedswitches .... I'd recommend ordering a few extras just in case. Hope that helps.
          How do I know which reed switch to order? Obviously I can measure the physical size of it, but apparently they carry different current loads...?

          https://standexelectronics.com/products/reed-switches/


          Originally posted by D1gichip View Post
          My only question is this... Does this one actually have a Nano Verb in it? Usually Allen has their own ADR-4 system installed there. It could have been changed by the previous owner.
          Yup. pop the hood & there it sits (literally just sitting, turned up on it's side) on the main shelf that is under the ACCOMP manual. Last night I played for 2-3 hours and never lost reverb. But I could get home today and sit down and start playing and the reverb effect disappear before the end of the first song. It's just kind of random.
          Last edited by myorgan; 09-12-2019, 06:04 PM.

          Comment


          • you795a
            you795a commented
            Editing a comment
            They would be low voltage and would be SPST. The length and diameter of the reedswitch would be the most important. They used several different types over the years. Some were shorter in length and larger in diameter and some were longer in length and thinner in diameter. Therefore, I would measure the size of them and see which ones come close. You can try digikey, the PR126 looks similar to what they were using prior to switching over the SMT.
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