Electrical Interference

Discussion in 'Tech/Mech General' started by johng, Jul 27, 2009.

  1. johng

    johng Read Only Funster

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    hi all.. I have found the cause of an annoying interference in my van. It is a humming, from the two speakers, which are powered from the stereo in the dash.

    The humming only occurs when I use the line in option...

    the line in comes from a PC half back in the hab area.. (tv, Music, internet etc)

    with me still...

    the hookup into the van is split between a 240V domestic unit, and a yellow step down tranny (for the 110V items)

    its the yellow brick that’s causing the interference all the way down the line... i.e. if it's switched off, all is pleasant...

    I don’t think it's the proximity of the tranny as such, more the fact that it's there in the lineup..

    What easy/cheap things can I try...

    I will try to clarify exactly how the wiring has been done.. when I get home...

    John
     
    Last edited: Jul 27, 2009
  2. Wildman

    Wildman Read Only Funster

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    make sure both items have a common earth, that should cure it.
     
  3. johng

    johng Read Only Funster

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    thanks WM..

    earth is something I really need to pay atttention to, I guess the EHU should earth back into the hook up I plug into.. the van itself is not an earth and none of its chassis should be.,

    whoever wired it for 240V, I really dont think they did a professional job...

    I will try and make a schematic of the wiring as is...

    it seems the tranny is back feeding really.. and the computer is the only think picking this up...
     
  4. Linda and Steph

    Linda and Steph Funster

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    Unplug the PC from the sound system and try headphones with the PC. If there's no hum on the headphones then you should check the screen on the audio cable running from the PC to sound system and use good cables for the speakers.
     
  5. johng

    johng Read Only Funster

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    thanks Steph..

    I have tried that.. its fine with headphones... it was fine when the PC went into a large amp and speakers.. it's only happened since the introduction of the stereo in the front.. and yet the stereo only humms when line-in from PC is used.. any other 'source' or a line-in from something else, its fine..

    it's the most bizarre relationship .... :Doh:

    I have brought 'cores' for all wires from PC and from stereo.. to no avail...

    the only thing that has worked, and worked completely, is shutting the yellow tranny off.. its perfect without that..

    I dont know if it's a proximatey issue or a back feed one... wil experiment more later :BigGrin:
     
    Last edited: Jul 27, 2009
  6. Linda and Steph

    Linda and Steph Funster

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    Something else to try:

    I assume that you're coming out of the PC with a mini stereo jack. If you have anything with a headphone socket that runs on batteries (mp3 player, radio) try using that with the same cable in the same position, that will establish whether the cable is the fault.

    If it isn't the cable then all I can think of is an isolating transformer for the PC:Sad:
     
  7. johng

    johng Read Only Funster

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    ok, have definately proved that the humming noise I get is directly caused by the yellow 240-110 transformer.. if I remove the Output from the tranny, the noise stops, (I guess the transfomer is doing nothing, with only an input) If I move the tranny away from other cables, nothing changes, there is still the noise..


    I think the next thing to try, is a different 'test' output from the tranny (at the moment it feeds the 110 and 12v units of the van) I just want to try a simple device that has a builder's 3 pin plug on, and a light or similar at the other end.... if there is still noise with this output alone, then either the tranny is backfeeding into the original 240 side, or the tranny is old and worn out..


    :Cool:
    Just thinking out loud..
    John
     
    Last edited: Aug 3, 2009
  8. hilldweller

    hilldweller Funster Life Member

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    I find with my laptop, if on charge the noise on line-out is terrible, unplug and noise stops - so it's the charging circuit in my case.
     
  9. Linda and Steph

    Linda and Steph Funster

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    We have a saying in the audio industry to get rid of hums and buzzes on PA systems -

    'If it's earthed unearth it, if it isn't earth it'.

    Only to be done as a last resort and just for testing to assist in tracing any earth loop that might exist. :Wink:
     
  10. johng

    johng Read Only Funster

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    ok have done a quick survery under the bed to see what's going on....

    240 EHU enters the area, into an old pre-used consumer unit. (has stickers that refer to a caravan/other camper, it seems)

    this unit has 4 trips.. the main overall trip, two 10s and a 6.

    the 6 amp is unused..

    both of the 10's are earthed back to a common point, and this is joined by the earth in the EHU

    One 10 does the nearside 240V sockets, and one lone socket on the Offside...

    The other 10 does all offside sockets, via a 4 way multi plug, from this multi also comes the feed to the 110 tranny. The tranny then goes into the origina US consumer unit and from there, the fridge, AC, and 12v for lights water pump etc...

    So, I thought simple, why not wire the tranny into the 10amp RCA , and have the offside sockets using the spare 6amp RCA.. makes sense having one RCA for each of the three different systems.

    Before I did that, I tried plugging the PC only, into the lone socket that is part of the nearside ring.. this would I hope prove that being independant of the 110 tranny would get rid of the noise :) It didn't...
    so I put the bed back together and left it..

    I also tried a decent magnet on the offside 240 and 110 wires.. no difference...

    so I think I have learned the following..

    the Van's 240V sockets are all earthed out through the EHU.

    I have no idea if the 110 tranny, or the systems it powers, are earthed.. and whether this may be the problem.. they are earthed back through the 240V side ? ..

    the 110 tranny is definately the route cause of the interference.

    the 110 tranny can be powered from any part of the Consumer unit, seperate from the ring mains, and it wont stop the noise the PC is picking up. (I suppose it is never really seperate from the other RCA's as they all share Live, Neutral and Earth)

    please note, I am safely finding out bits of this vans wiring, and the noise although annoying, is liveable with.. :Smile:

    J
     
  11. pappajohn

    pappajohn Funster Life Member

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    the yellow 'site' trannies arent designed for domestic use so will probably have no suppression or smoothing circuitry.

    just built for Bob the builder to plug his cement mixer / hammer drill into.

    its a cheap but common way to stepdown to 110v in RV's.
     
  12. johng

    johng Read Only Funster

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    hi PJ yeah know that :)

    its a 3.5kw one too :Eeek:

    I do have an up down tranny in the shed.... only 1000W unfortunately, but may be enough to do some of what I need..


    I cant really fit it in the near future :( so will put up with the silly noise
     
  13. pappajohn

    pappajohn Funster Life Member

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    turn up the volume and see if that drowns the interference.....:Rofl1::Rofl1::Rofl1:

    omly kidding:Wink:
     
  14. johng

    johng Read Only Funster

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    makes it mucho worser :Rofl1:
     
  15. pappajohn

    pappajohn Funster Life Member

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    it needs a ferrite ring.

    loop feed cable to speakers through the ring a few times and that may stop it....

    unless im thinking of something else....getting tired:RollEyes:
     
  16. johng

    johng Read Only Funster

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    tried all that PJ ;)

    I think there are three answers..

    a new tranny - a proper dedicated one meant to be on all the time.. (ie the 1KW one I have in the shed, but 1KW may not be enough, certainly wont do the Aircon)

    or

    running the PC from 110v or even 12V.. (PC's have big trannies in them, with noisy fans, all to get the power down to 12v and 5 v - there is a way to 12V them, I believe..)

    or, buy a laptop ;)



    :Rofl1:
     
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