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Elmo ST-600 loud hum.

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  • Elmo ST-600 loud hum.

    Hi I have not used this projector for some time. Now I have very loud hum on the sound . Any help would be appreciated.

  • #2
    Hi Barrie,

    It could be that your electrolytic capacitors have sat powered down for a very long time. They need to have voltage applied in order to keep their capacitance, in order to filter the hum out of the DC voltages in the sound circuit, so the hum doesn't end up in the speaker(s).

    I have an ST-800 that sat dead in a camera shop brand-new until 2017 when I got a hold of it. I gradually applied higher and higher AC voltages to the plug until it got high enough without blowing anything up that I became brave and just plugged it into the wall.

    -of course had the thing blown sky-high at that point, "brave" would have changed too "foolish" and I probably never would have mentioned it here. (-have to have SOME kind of dignity!)

    You've already plugged it into the wall and not had an Electrolytic Eruption. Maybe if you leave it on with the sound turned on for a while the caps will form up again and the hum will go down. (I'm sure yours haven't sat dead since the 1970s like mine.)

    Of course, when I did this, it was something like "June" and the first maybe 8 hours plugged in were out on the patio, where if a cap did blow up, it wouldn't have gassed the household. This is not a great "January" thing, so I'd say if you decide to do it before warm weather you stay with the machine and be prepared to pull the plug if things go south all of a sudden.


    • #3
      Thank you, I will give it a try. I thought it could be a capacitor. Just knowing which one. None of them show any signs of swelling up.


      • #4

        They just tend to get less and less capacitive with time. There is a point of no return where they can't be reformed, but based on my own experience, I have hope for you. My caps were probably in dead storage before 1980 and stayed dead something like 37 years: they came back with some gentle persuasion and 6 years later, they are still OK.

        What I noticed was that even after the caps had taken voltage for a while, the stability of the amplifier was off. What I mean by this is below a certain volume, the hum was pretty normal, but then as I cranked the volume up further there came a point where it broke into a low frequency tone: "ooooWowwwwwwww....". If I backed off the volume this went away, and I could start it and stop it by rotating the knob back and forth. I allowed the machine to sit plugged-in with the volume below this initiation point. (It just seemed unhealthy to let it stay oscillating like that, and besides: the rest of my family just might find this sound more than a little annoying!)

        What I found is as the hours on the caps piled up, the volume setting needed to break into the instability kept rising, until eventually I could crank it full and there was no instability.

        You have inspired me: I have a Eumig machine that has been on the shelf about a year now: maybe it's time to take it out for exercise!


        • #5


          • #6
            -Could be!

            I've seen record switch oxides cause all sorts of trouble in these machines. The most common is the loss of audio playback. Once or twice I've also seen this cause a high pitched whistle. I don't think I've heard it either worsen hum or cause a low frequency instability, but can't rule it out either.

            It's always worth working the switches: if nothing else it costs nothing to try it out.


            • #7
              Hello all, I have tried everything suggested but still no fix. The capacitors have been changed, switched tried but still just a VERY LOUD hum which gets worse as the volume is increased. I can just about hear some sound with the volume well down but it's drowned out by the dreaded hum. Really don't know what to check next. Could it be the audio amplifier transistor? It checks OK on a meter though.


              • #8
                There is another possibility. These machines have a number of integrated circuits on the sound board which eventually will fail. I had a pre-amp IC fail on my ST-1200HD which put that channel into a high-pitched squeal. I tried all sorts of things before I replaced that IC and it finally recovered.

                Maybe you have an IC failure with a low-frequency instability as a result: not hum in the usual way we think of it, but an amplifier feeding it's own output back to its input and breaking out in an oscillation,

                I have ST-600 schematic scans somewhere. If I find them, I'll send them to you.


                • #9
                  Hello again, my machine is model ST-600M. Not sure if the M makes a difference. Here are some photos. The two large capacitors have been replaced with new equivalents.


                  • #10
                    Hi Barrie,

                    I don't think either of those boards is your sound board. The one with the pots looks to be motor control and the other is AC input.

                    Is there a third one lurking somewhere? -maybe on the right side of the chassis?

                    (I'm not very fluent in "ST600": I know its cousin the ST800 much better!)


                    • #11
                      Hi Steve, thank you for your reply. I have looked hard into the inwards of the projector and can see nothing. The only possible space would be behind the main function switch and volume control. The trouble is, I can't find out how to gain access. I presume the left hand assemble comes apart with some of the film guides and rollers. I really do appreciate your help but I am conscious of taking up your time. I just wish we were closer and could look at it together. Take care and enjoy the rest of your day.


                      • #12
                        Thank you Steve for your help. I did send a thankyou but you may not have got it. In the end, it was switch contacts on the record switch. I now have to get the speed right as one of the pots got altered. All good fun.


                        • #13
                          Hi Barrie,

                          Glad to be of help, even in only a little in this case!