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Hum from DBS7200 Marshal head normal?

Discussion in 'Amps and Cabs [BG]' started by chuffedkiwibass, Jan 10, 2017.

  1. Heya all, does anyone have a DBS7200 (or 7400), and if so does it hum fairly loudly? Just trying to figure out what's normal with this amp, and whether it even needs a repair. Its not noticeable when playing it but when it's just sitting there is pretty bad. The level of hum is constant, and you can hear it coming out of the speakers. My peavey amp is very quiet in comparison.

    I've tried all the obvious things like moving the amp to another plug, trying different leads, guitars. I even replaced the fan. And I replaced the tube (these are a hybrid solid state/tube preamp head). My quad sounds fine with the other amp, so I know it's no that...so I'm now thinking that perhaps it's just the way it's supposed to be!

    Thanks bass peeps :)
  2. Hi Matt, welcome to TalkBass. Please fill out your profile so we know more about you.

    If the amp is new return it to the store for a replacement. If not talk to the seller.

    Does the amp make the same level of noise with nothing plugged into it except the speaker cabinet?
  3. I would have thought you can safely say it's the amp making the noise, since he swapped amps and it goes away. So I am a bit confused. Maybe you missed that titbit or can one amp really reject input hum better than another?
  4. Thanks Paul, I've filled out my profile. I guess I'm a long time listener, first time poster!

    Hmm yes I got the amp pretty cheaply mostly as it came with an excellent 410 cabinet, and presumed it was the fan that was at fault which is an easy fix. The fan's bearing were shot so it was VERY noisy, so I purchased it this way almost 2 years ago. After replacing the fan I realised there was the other another problem of this hum.

    Yes it hums the same when nothing is plugged in. The amp is at least 20 years old, and I really like its tone/vibe. Its quality UK built gear, but in saying that it's probably not worth spending a whole lot of money on it. I could take it to the local amp repair man, but just trying to see if anyone here has experience with this amp first.
  5. One has to start somewhere in trying to diagnose a problem. As the OP has replied then yes it is the amp. now we can take the next steps in trying to find out where within the amp the problems lies. :)
  6. When you rotate the volume control from zero to maximum does the noise change.

    What sockets are on the back of the amp? A photo would help.
  7. basscooker

    basscooker Commercial User

    Apr 11, 2010
    cincy ky
    Owner, ChopShopAmps
    Is there any pitting or corrosion happening on any metal on the outside of the amp? For that matter, since you have had it opened, was there any shielding around the power supply? I was thinking if the amp were ever stored for a while in a humid basement or garage, maybe some shielding has developed some "leaks". Double check that the ground is making good contact with bare metal, as well as running a plug into and out of all connections a couple times.
  8. Thanks Paul, sorry for the silence been on summer holidays...if it can be called summer this year.

    When the volume knob or any other knobs are turned there is no effect on the buzz. It stays constant.

    Uploaded a photo of the sockets on the back for you to see. When the speaker cable is inserted into either of the speaker outputs the exact same buzz occurs.

    Attached Files:

  9. Wow awesome thanks for your suggestions. I've done all, and even sanded the ground connections to be sure. There isn't any corrosion on the unit that I can see. As far as shielding goes see the photo above, it just seems to have some steel around the transformer and thats all. Probably needs an expert with a multimeter I'm thinking. I hope its not the transformer itself.
  10. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    It sounds like the amp should be examined by a tech, there are many possibilities. More than likely, this is not a transformer issue. There could be a bad ground somewhere in the amp.

    Try an instrument cable patched between the effect send and return. Cycle the plug into each jack ten times or so in quick sucession first. This can help if there is oxidation on the contacts.

    Failing that, try plugging your instrument into the effect return and see if this affects the level of the hum.
  11. Thanks for your suggestions beans-on-toast, all tried with no luck unfortunately. Off to the tech it goes then.
  12. Tech said it was a 'dry joined filter cap'. Like that from the factory and came loose at some point. He found it really quickly. Enjoying the amp properly for the first time :) :bassist:
    Downunderwonder and GregC like this.

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