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Loud Hum from Elmo ST-800

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  • Loud Hum from Elmo ST-800

    I've recently got my Elmo ST-800 running and everything works great, except for a loud hum coming from the speaker. I've tried using an external audio source but it's even louder through that. Does anybody know what could be causing that or how to fix it?

  • #2
    Hi Josh,

    I have a couple of ST-800s and in good order their hum performance is pretty decent. It's possible that your electrolytic capacitors have dried out and not smoothing out the power supply ripple any more. These would require replacement. It's also possible your machine has sat so long they aren't completely formed up any more. If this is the case, and they didn't blow up when you plugged it in in the first place, you may be able to reform them by plugging the machine in (volume low) and just let them sit powered up for hours or even days.

    This is my tale of coaxing an ST-800 that never operated from the day it left the factory into running condition, including those capacitors. (In Part 3)

    Elmo Archaeology: Reviving a 40 year old ST-800

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    • #3
      Awesome! I'll plug it in and leave it on for a while and see how that goes. Thanks for the quick response!

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      • #4
        Sure!

        Just note how bad the hum is at a given volume setting, turn it down so you don't annoy the people you live with, give it a few hours, rotate the volume up and listen for improvement. Bad smells, smoke or sounds of battle mean it's time to pull the plug!

        Something to be careful of: this is vintage electronics and inherently hummier than a lot of modern stuff we are now used to. Sometimes it's our own expectations that need to be adjusted to fix the problem. (An old friend of mine actually learned to like the hum!)

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        • #5
          haha yeah I can deal with some hum. Right now it's just louder than the actual sound from the film haha.

          Would it be a good idea to replace the capacitors anyway?

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          • #6
            I thought about that with that one in the article. It's not the first choice: certainly better to try to re-form them up before you go that way.

            (If you did, the two big ones on the large board along the bottom of the machine are a good place to start.)

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            • #7
              cool. I'll try re-forming them first. thanks again!

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              • #8
                haha I fixed one problem and created another. quite a big other problem lol. i found the capacitor that was causing the problem and it just had a weak join on the board so re-soldering that fixed the hum almost completely. but i must have fried something while doing that because now the motor seems to be dead. with the cord plugged in, the speaker fires up and the light turns on, but the motor doesn't start. could that just be a bad wire?

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                • #9
                  Well, that's progress!

                  I never thought of a bad solder joint. That's one I haven't had so far, although other people here have. (-beats replacing the cap!)

                  I could see the motor circuit being done in by a bad connection: it contains a decent amount of wiring. There is a power transistor hanging in space up at the top of the machine under the back cover. There has to be wiring to that. There is a speed sensor on the back of the motor: ditto. There has to be a connection over to two switches operated by the main control knob. It could be that some connector just isn't seated properly.

                  The regulator circuit is on the bottom board. (-where you were working...just saying!).

                  Maybe your vintage 1970s solder job has opened up in another spot.

                  These are great machines, but sometimes at the beginning, you have to earn them!

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                  • #10
                    Josh, I don't know the ST 800 but on the ST 180, there are two fuses. Did you check that ?

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                    • #11
                      Hi Dominique,

                      The ST-800 is single-fused on the AC input.

                      If anything within the machine works, it has to be OK.

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                      • #12
                        I’m guessing that something must be wrong with the regulator circuit on the board. When I put everything back in place after fixing the capacitor, I plugged the power in and there was a tiny pop followed by a little smoke coming from the board. I’m not much of an electrician (just know how to solder haha) if something happened down there would it be fixable? Or could I have potentially fried the whole board?

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                        • #13
                          Smoke is never good!

                          The problem with this one is there is literally zero documentation available1. If I had a set of schematics I could probably fix things like this, but most of the time it comes down to finding a machine even more dead for other reasons and swapping the entire board.

                          (Sorry!)

                          1) I wrote to Elmo USA maybe a year ago: no dice!

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