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Allen 120 Audio issues..

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  • Allen 120 Audio issues..

    Hey all!
    The Allen 120 C is working great with the exception of the pops, white noise, and crackles in one and only one of the amps, I forget which one, but I did isolate it.

    I tried to install the Alesis reverb between the digital to analog sound computer 'thing' but, as I was warned, I need to install the Harrison Lab's Attenuator. I don't know, however if I need the 3db, 6db, or the 12db. I don't have any clue what to order, so any help would be great. I did put it in temporarily. While the sound was too soft to really heard very well, it was darn near miraculous how good this organ sounds, not even considering it was built in probably 1981 or so.

    In regard to the amplifier repair, I assume it is a capacitor, but I'm still confused as to which one. I assumed it was the giant can like ones, but I think I read somewhere on here that it really is some other capacitors somewhere else. Where are they?

    Thanks as always.. After all of this, then I'll start asking you about midification!

  • #2
    It's not really an attenuator, but a impedance matching circuit. You can accomplish the same thing by putting a series 22k resistor in the hot leg of the signal path between the DAC and external reverb device. The Harrison part makes it neat and simple though. As far as I recall, they only sell one model.
    -Admin

    Allen 965
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    • #3
      Yep. Order the Harrison Labs isolator do-dad and it will come with complete instructions. You will need to fiddle with the input and output level controls on your reverb unit in order to get the right amount of sound without creating digital clipping and distortion within the reverb itself. I'm surprised that you are getting a low level audio. Maybe it would work even without the HL do-dads if you turn up the input level a lot on the reverb because the output from the Allen DAC board is quite low and may need some boosting. But keep the output level on the reverb rather low because the Allen amps have a very high gain. (The expression still won't work without the isolators, but you should be able to get sufficient volume.)

      Before attempting to replace that big can-type filter cap, try replacing the several small electrolytics under the amp chassis. If you have T-50 amps, you'll find some 1000 mfd and 500 mfd caps, and you may see that these have oozed a bit of stuff out one end, a definite indication of trouble. The big can filter might in fact fail some day, but when it does you will have the biggest hum you ever heard, not just crackling noises. I almost never have to replace those.

      Get some generic 1000 and 500 caps rated for at least the same voltage printed on the old ones, and it wouldn't hurt for them to be rated a tad higher. (Draw yourself a picture of the cap layout before you begin, and snap a pic or two with your phone so you can be sure to put the new ones in right.) I think the last time I did it, I used 50 volt rated caps to replace the old ones that were 25 or 35 volt rated. The cost is about the same, really just a dollar or two for these little things if you order on the internet. Or get them at Radio Shack. They might cost a little more, but you don't need very many and you won't have to wait or wonder if they are the right thing.

      BTW, filter cap numbers in mfd are not super critical, and generally have a tolerance of 20% or more. So, for example, you can replace a 500 mfd cap with a 470 mfd cap, which is well within the 20% tolerance. For some reason, on-the-dot 500 mfd caps are not as common as 470's. If you have 250's in the amp, you can replace them with 220's if they are what you can find. But always keep the voltage rating at least as high as the original or they will stop working quickly.

      When soldering in new caps, be absolutely sure to put them in correct polarity -- the positive (+) terminal where the old + terminal was, the negative (-) where the old one was. If you put them in backwards they will EXPLODE instantly when powered up! I know this from experience!

      If you are good with a soldering iron, while you are doing caps you can re-flow the solder throughout the amp, especially anywhere a large pin penetrates a circuit board. These are places where cracks often develop and can lead to noises and intermittent operation.

      Even after all this, you may still have amp noise because transistors can also be bad and are not easily replaced. But working Allen amps sometimes come up on ebay. For a MOS organ you have to be sure to get an amp that came out of another MOS organ because Allen amps of later vintage do not have the correct impedance and the expression will not function.
      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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