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SM400 noise problems

Discussion in 'Amps and Cabs [BG]' started by scottlee, Aug 27, 2003.

  1. scottlee


    Feb 17, 2003
    I recently aquired an old (1991?) SWR SM400.

    It has a slight noise problem. There is a low hum as well as a low crakling/hissing (I say crakling because it is not a constant hiss - up and down) coming through my Eden 210xlt speakers.

    I have done searches through previous posts but I have some unique variables.

    - Bass is not plugged in
    - Noise is just coming through speakers/horn, amp itself does not hum when speaker unplugged or plugged in
    - Noise does not increase when gain or master is turned up!! The noise is there when both are on 0 and never changes.
    - The crakling (not the hum) does go through the inbuilt di (only just, but it is the same type of noise as the crackling through the speakers)
    - I changed the tube and both noises where still there - not sure but they may have been lower. It was an old tube I had lieing around so it I cant guarantee it was perfect.

    I have ordered some more tubes which I am going to try

    I have an SWR350 which to compare it against and it is as quite as mouse so I am not just paranoid about the horn hiss. I dont want to pull the tube out of my 350 as I have sold it and don't want to mess around with it before the guy picks it up.

    I can probably live with the hum (rather not though) but don't like the crakling going through the di

    Is it just the fact it is old and I can't do anything about it?

    I actually had it shipped from the US and took it to local amp tech to change the power supply rating (we run on 240 volts). I later rang him after I got it back to ask about the hum/noise - he said he didnt notice it (yeah right)he said there was probably nothing I could do and this could be normal for older SWR's - does this sound right?? He is the best in the area although not experienced with SWR.

    Amps here in Australia are expensive and I've been waiting a long time to get this one so I want to do everything I can to get this one right!!

    Sorry for long post.
    Any thoughts??
  2. tornadobass

    tornadobass Supporting Member

    Nov 20, 2000
    Iowa City, Iowa
    Endorsing Artist: Black Diamond & SuperSensitive strings
    I have an SM-400 and an SM-400S. Both are very quiet. Perhaps there's a capacitor that needs replacing. I sent one of mine to a shop in LA that did the factory work and they replaced a couple of caps.

    A real solid amp. Good luck getting yours going.
  3. brianrost

    brianrost Gold Supporting Member

    Apr 26, 2000
    Boston, Taxachusetts
    SWR SM-400 amps actually can be biased (VERY unusual for solid state). It may be that yours needs to be rebiased.

    I would contact a local SWR service center and have them check this.
  4. scottlee


    Feb 17, 2003
    Thanks guys for the advice. :)

    I will seek out an authorised SWR repairman. I'll repost with the results.
  5. iammr2


    Jun 10, 2002
    Have a tech (not the one you used before) check the solder connections up towards the front of the board. The circuit board is not anchored toward the back side very well, so bouncing around can crack the solder up by the front panel from flexing. Common on this amp. I've got one that's about 14-15 years old and it's very quiet.

    Find a better tech and tell the old one that McDonald's is hiring.
  6. jgsbass


    May 28, 2003
    Floral Park, NY
    The exact thing happened to my almost 1 year old SWR 350X last night. I opened it and found that a large capacitor had come loose from the circuit board ( bad soldering?)When I pushed down on the Capacitor, the hum stopped. While moving the piece around, I got the static-y sound That I got while playing. Vibrations from the sound caused the noise as opposed to an actual electrical overdrivin distortion. The jkicker to this is: when I brought the amp in today for warantee service I was told " opening up the head voids the warantee" After much arguing and chioce logic, they agreed to take in the amp under warantee, after all I had diagnosed the problem for them making their job easy ( solder gun and screwdriver easy)But I think its the end of my relationship with SWR
  7. ihixulu

    ihixulu Supporting Member

    Mar 31, 2000
    South Shore MA
    Check this out:

    My old trusty SM400 (built 1992) was quiet. Then I got in a car accident and it got jostled pretty bad in the trunk (that's how I learned about cases). One channel stopped working so I took it to the shop and the guy fixed it... but now it hummed when I plugged it into the wall...


    only in certain places. In my living room it's awful, not louder than the bass but annoying. At the practice space...silent.

    In researching the problem, I found some places on the net that mentioned transformer winding problmes/damage making the amp susceptible to hum under less than ideal power conditions.

    Is there any truth to that or ultimately is it just a bad filtering cap?

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