How to Beat an Input Hum in a Soundcraftsmen 300x4???

Discussion in 'Solid State' started by toddalin, Dec 26, 2018.

  1. toddalin

    toddalin Super Member

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    I recently picked up a Soundcraftsmen 300x4 and when I connect it to my electronic crossover, I pick up a hum. It doesn't matter if the crossover is on or not or even plugged in. And, it seems that the more input plugs I insert into the amp (up to 4 possible), it picks up more hum with each plug.

    I've changed cords, tried cheater plugs, and even ground the chassis together all to no avail. It is not a bad hum (about like a 60's tube Fender), and you don't notice it when music is playing. But the other amps connected to the crossover are dead quiet and you certainly hear it (Altec 604s) when the music stops.

    Any ideas?

    [​IMG]
     

     

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  2. ryuuoh

    ryuuoh FFXIV Summoner Subscriber

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    I'd pop it open and start looking at solder joints.
    Nice condition, BTW.
     
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  3. toddalin

    toddalin Super Member

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    One of the LEDs (C) was out. You can see that it is broken in the picture. I tried four different green LEDs trying to get the right color (semi-frosted), fit, and brightness. Fourth time time was a charm.

    Anyway, when I put it back together and tried it out with the first couple of LED, there is a voltage regulator that hangs in the way (and made it a real pain getting the rack/face off). Anywho, the leads on this regulator kept getting tweaked, and I could feel that it was about to break. (The regulators/heat sinks sit higher than the top of the case and are bent over to put the top on.) Before, it did break, I fired up the amp to check the LED..., looks good, and NO HUM! It did hum with the first couple LEDs I tried.

    Of course the leads did break, and I now need to replace the regulator. (Maybe one was already broken.) But, if this replacement fixes the hum problem, we'll just call it a lucky break. :p

    Luckily, they are only $1.50 each (ordered three), and subsequently found one in my parts bin.
     
    Last edited: Jan 7, 2019
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  4. ryuuoh

    ryuuoh FFXIV Summoner Subscriber

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    Let's hear it for the parts bin, :beerchug:
     
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  5. toddalin

    toddalin Super Member

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    Still audible.
     
  6. toddalin

    toddalin Super Member

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    Seems to emanate from the "input" board and each of the four inputs produce a certain amount of 60 Hz hum all varing a bit. Input board uses XR084 chips.

    The a/c and d/c reading on the speaker terminals all show less than 5 mV, unloaded.

    https://datasheetspdf.com/pdf-file/1257423/Exar/XR-084/1
     

     

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  7. Racingh11

    Racingh11 Active Member

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    It seems like a ground issue. The outside ring of each input goes to ground and so does a resistor from the negative input of each opamp on each channel. I would upload the schematic, but apparently a 1.3MB pdf file is too big. NJM2059 could be used instead of the XR-084 if bipolar junction transistor input is preferred to a JFET. Does the amp hum without anything hooked to it? I usually have to put my ear inches from the speaker on good Soundcraftsmen amps to hear hiss or hum at idle.
     
  8. toddalin

    toddalin Super Member

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    The amp only hums when something is plugged into the inputs, and the more plugs, the more hum. If the four plugs/cables have no components on the input end, there is no hum. I've tried all the usual remedies to no avail and will take it to the shop this/next week to be looked at.
     
  9. Racingh11

    Racingh11 Active Member

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    I'd be interested in hearing what they find.
     
  10. toddalin

    toddalin Super Member

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    I received a call from the repairman today. He said that he did not hear the hum.

    I went over and took the crossover and my cords and we hooked it up. His shop is an open garage just off a semi-busy street with a pair of smallish speakers hanging from the rafters. When I got there he turned on a source and it played nicely. He said, there was no problem.

    So I hooked up the crossover, and asked for a step stool to get close to the speakers. Sure enough, there was a hum. He got on the stool and acknowledged its faint existance and said this was nothing he could fix. He said that if there is no hum with nothing plugged in, it must be the source. I acknowleged that that makes sense, but went over the list of what had been tried and the source(s) makes no noise through other amps. He thought there was nothing wrong with it, and I told him he needed to hear it in a quiet room over Altec 604s and it would be obvious.

    He gave me back my $30 deposit. :(
     
  11. Binkman

    Binkman AK Subscriber Subscriber

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    Might be inherent of a poor amp design perhaps a PA design or as said Fender amp.. Marshalls have a similar idle hum. Over you're speakers? lol... you could probably hear a fart on a bad recording. :D
    So why not make a guitar amp out of it..?
    bink
     

     

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  12. Racingh11

    Racingh11 Active Member

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    Could be a lot of things, but it is not normal operation for that amp. It could be a ground issue. Soundcraftsmen amps have a resistor going from the circuit board grounds to chassis ground. They are usually 220 or 10 ohm depending on the amp. If something goes wrong and they burn open, they can have symptoms like induced noise. Having bad capacitors can do similar things. If one cap is leaky, it can pull ground off center and ad noise. It can also be an issue with the input differential and current source transistors. They are usually degraded over time and create all sorts of issues from noise to DC voltage, etc. Could be loose solder joints too. Is the problem on both channels? Another thing I have come across lately is loose cables on Souncraftsmen equipment. It's like the plating is a little thinner on the RCA jacks and the cables are not tight. I had all sorts of noise and hum problems on my Soundcraftsmen preamp and found several different brands of cables felt tight, but were loose on the ground ring. I had to use a pliers on the cable to get it to fit tight and then everything was good. It actually looked good and felt tight before, but was just the center pin I was feeling. So it's possible for an RCA cable to fit on one piece of equipment, but have a poor connection on the next.
     
  13. toddalin

    toddalin Super Member

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    Mine uses the 220 ohm resistor and this is good.

    My next move will be to open it up and spray the connectors and their pins with contact cleaner. But the inputs are 1/4" phone jacks and all internal connectors (and there are lots of them) are "pins." I'll also try to run my MP3 player directly into it. This does not connect to an A/C line so...

    Problem is in all 4 channels and the more inputs plugged in, the worse it gets. "Open" input cords do not create the hum, only those that are connected.
     
  14. Racingh11

    Racingh11 Active Member

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    I wonder if you put a meter or an oscilloscope from the ground on the amp input, to the ground on the crossover output with no cables hooked up. It would be interesting to see if there is any AC or DC voltage between the two. Also would be interesting to measure resistance on the different amp inputs to see if they have a similar input impedance.
     
  15. Racingh11

    Racingh11 Active Member

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    I looked at the schematics. The 300x4 is not a balanced amp. Were the others you tried balanced? It would not take much effort to convert it to balanced if you wanted. It already has an input board with op-amps, so the basics are already there. I would also try some deoxit on the stereo/bridge switch.
     
  16. toddalin

    toddalin Super Member

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    Interesting thing is that it hums regardless of whether the crossover is plugged into the wall or not, just so long as the cables are in place. It does the same with the Yamaha integrated from the preamp output.
     

     

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  17. toddalin

    toddalin Super Member

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    No, everything is unbalanced.
     
  18. Racingh11

    Racingh11 Active Member

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    That's a very interesting problem. What kind of hum, 60hz power hum or something else? Makes me think those sources are acting as antennas for the hum frequency and the Soundcraftsmen picks it up for some reason the others do not. If you hook up a cable one at a time or try one on each channel only; does the hum come out of only the corresponding speaker or all speaker outputs equally? Also take your crossover and move it around near power outlets and house wiring to see if it changes in strength.

    And I would use a meter if you have one and check input resistance on all of the amps and compare. The soundcraftsmen should be about 47.5K ohm from input pin to ground ring. It may be possible with the combinations of resistors and input capacitor of 100pF may create a tank circuit with the source equipment and resonate or allow hum. Those parts could easily be changed with other acceptable values. Soundcraftsmen used different values depending on usage of amp being pro or home, etc. It may be a mis-match with your equipment.
     
  19. toddalin

    toddalin Super Member

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    RTA with mic next to the speaker shows it to be a 60 Hz hum.

    Thanx for the suggestions.
     
  20. Racingh11

    Racingh11 Active Member

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    You could also check output resistance of your sources for reference. It should be far less than the input of the next piece. Soundcraftsmen pre-amps are 180 ohm.

    The input-board can also be bypassed and input feed directly to the amp-board. That would lose the bridging ability, but eliminate the input board. The wires at the amp-board input would have to be disconnected to keep the input-board from sinking the input current at the amp-board source.
     

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