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Post subject: Any amp gurus here?
Posted: Wed May 15, 2013 10:16 pm
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If you guys remember I just got this amp about two weeks ago. It was sounding great until today. :(



Anyone knows what could be the problem? I need this amp because I'll need this for gigging in a few months. HAAAAAALP!

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Post subject: Re: Any amp gurus here?
Posted: Thu May 16, 2013 9:19 am
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There have been some reports of the Cooling Fan causing some unwanted feedback on startup with this amp (described as 'weird' noises, which go away after the fan is up to speed).

Perhaps this is what you're experiencing.

Cheers!

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Post subject: Re: Any amp gurus here?
Posted: Thu May 16, 2013 9:22 am
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What exactly are you doing when you 'disconnect the left channel' as you put it?

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Post subject: Re: Any amp gurus here?
Posted: Thu May 16, 2013 10:18 am
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Hmmm the cooling fan thing makes sense. The noise is pretty damn loud though.

Ninja, the amp has separate jacks in the back for the speakers. Can't see it on the video because its so dark. So if I disconnect the left speaker and reconnect it after the amp is on, there is no noise.

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Post subject: Re: Any amp gurus here?
Posted: Thu May 16, 2013 4:18 pm
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Don't ever do that again! Even with solid state amplifiers don't power them with no speaker load.

You've probably put one of the opamps or transistors into meltdown. If you have, on the
MG series it's cheaper to replace than repair.

The good news is that because it recovers, it isn't all over. Fire your amp up with every dial on zero and the speaker connected. Slowly bring in the volume, gain and eq. See if it makes a difference. If it does, start your amp up like that from now on.

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Post subject: Re: Any amp gurus here?
Posted: Thu May 16, 2013 5:57 pm
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nikininja wrote:
Even with solid state amplifiers don't power them with no speaker load.


No, that's not true. While tube amps will rapidly suffer damage if run with no speaker, solid state amps can run quite happily without a speaker connected. The only caveat is that the minimum speaker load should be observed (usually 4 ohms).

When Marshall released the 9000 series in the 1990s, the whole range had a single user manual covering both the 9040 solid state power amp and the 9005 tube power amp. In the warning notes it says ...

"DO NOT switch the 9005 on without the loudspeakers connected ..."

The only specific warning about the 9040 was not to obstruct airflow round the heatsinks.

http://www.marshallamps.com/downloads/f ... %20hbk.pdf

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Post subject: Re: Any amp gurus here?
Posted: Thu May 16, 2013 9:06 pm
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GilgaFrank wrote:
nikininja wrote:
Even with solid state amplifiers don't power them with no speaker load.


No, that's not true. While tube amps will rapidly suffer damage if run with no speaker, solid state amps can run quite happily without a speaker connected. The only caveat is that the minimum speaker load should be observed (usually 4 ohms).

When Marshall released the 9000 series in the 1990s, the whole range had a single user manual covering both the 9040 solid state power amp and the 9005 tube power amp. In the warning notes it says ...

"DO NOT switch the 9005 on without the loudspeakers connected ..."

The only specific warning about the 9040 was not to obstruct airflow round the heatsinks.

http://www.marshallamps.com/downloads/f ... %20hbk.pdf


Just because you can do something doesn't mean you should. :wink:

And you definitely don't connect speakers with the amp powered up, tube or solid state, as the OP did.

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Post subject: Re: Any amp gurus here?
Posted: Fri May 17, 2013 12:23 am
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bluesky636 wrote:
GilgaFrank wrote:
nikininja wrote:
Even with solid state amplifiers don't power them with no speaker load.


No, that's not true. While tube amps will rapidly suffer damage if run with no speaker, solid state amps can run quite happily without a speaker connected. The only caveat is that the minimum speaker load should be observed (usually 4 ohms).

When Marshall released the 9000 series in the 1990s, the whole range had a single user manual covering both the 9040 solid state power amp and the 9005 tube power amp. In the warning notes it says ...

"DO NOT switch the 9005 on without the loudspeakers connected ..."

The only specific warning about the 9040 was not to obstruct airflow round the heatsinks.

http://www.marshallamps.com/downloads/f ... %20hbk.pdf


Just because you can do something doesn't mean you should. :wink:

And you definitely don't connect speakers with the amp powered up, tube or solid state, as the OP did.

I think it's definitely related to the fan, I'm gonna remove it and clean it for now. If that doesn't work, I'll take it to a tech. :(

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Post subject: Re: Any amp gurus here?
Posted: Fri May 17, 2013 10:32 am
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The MG is not a 9000 series power amp. A lot different a circuit. Plenty of Marshall transistor amps tell you not to power the amp with no speaker load connected. Why do you think transistor amps would have a minimum load requirement if it were ok to power them up with no speaker load connected? May as well give a minimum requirement as zero if that were the case.

Page 5 "Always use a speaker lead", "Minimum of a 4 ohm load"

http://www.marshallamps.com:3339/downloads/files/MG4%20hbk%202%20Eng.pdf

Page 1
"Don't switch on the amplifier without a speaker connected"

http://www.marshallamps.com:3339/downloads/files/MG100%20hbk%20Eng.pdf

So it blatantly is true.

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Post subject: Re: Any amp gurus here?
Posted: Fri May 17, 2013 9:30 pm
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Update! So after lots of research I found a schematic and opened her up. First thing I noticed was that the power supply was missing a mounting screw and the rest were loose. That explains a rattling noise I would hear when I moved the amp around. Tightened it up, and then I started to take a look at the board. Everything looked good, nothing burnt. I then start to check all connections, and I found a loose ground. Got the ground tightened up and I get a feeling that, that was the problem all along. Put the amp back together and powered her up (with the speakers connected!) and voila! No noise! And also I had this buzz I could hear on the overdrive channel and now that buzz is gone. Needless to say I'm very happy!

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Post subject: Re: Any amp gurus here?
Posted: Sat May 18, 2013 12:12 am
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Just a point about transistors in general. Regardless of circuit topography a transistor doesnt "produce" loads of power that has to go somewhere. Its a switch, a small incoming voltage (and leakage current) is transformed hence "trans" and is allowing the signal to be amplified thru the emitter/collector route.

As for minimum load, open circuit is just that, open.......almost infinate resistance or impedance. Minimum load refers to how low this value can be as lower impedance means more current flow which is what really does for electronics.

Basic rule for me was always "Transistors dont short/valves dont use open circuit".

Still worth sticking to the amp makers recommendations Niki is correct.


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Post subject: Re: Any amp gurus here?
Posted: Sat May 18, 2013 3:02 am
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1neeto wrote:
Update! So after lots of research I found a schematic and opened her up. First thing I noticed was that the power supply was missing a mounting screw and the rest were loose. That explains a rattling noise I would hear when I moved the amp around. Tightened it up, and then I started to take a look at the board. Everything looked good, nothing burnt. I then start to check all connections, and I found a loose ground. Got the ground tightened up and I get a feeling that, that was the problem all along. Put the amp back together and powered her up (with the speakers connected!) and voila! No noise! And also I had this buzz I could hear on the overdrive channel and now that buzz is gone. Needless to say I'm very happy!


A common sense approach to doing things right goes a long way in the amp maintenance/repair arena.

Rawk on!

8)

Arjay

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Post subject: Re: Any amp gurus here?
Posted: Sat May 18, 2013 12:09 pm
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Retroverbial wrote:
1neeto wrote:
Update! So after lots of research I found a schematic and opened her up. First thing I noticed was that the power supply was missing a mounting screw and the rest were loose. That explains a rattling noise I would hear when I moved the amp around. Tightened it up, and then I started to take a look at the board. Everything looked good, nothing burnt. I then start to check all connections, and I found a loose ground. Got the ground tightened up and I get a feeling that, that was the problem all along. Put the amp back together and powered her up (with the speakers connected!) and voila! No noise! And also I had this buzz I could hear on the overdrive channel and now that buzz is gone. Needless to say I'm very happy!


A common sense approach to doing things right goes a long way in the amp maintenance/repair arena.

Rawk on!

8)

Arjay

Thanks! Well I'm a mechanic by trade so I know my way around when troubleshooting. Always go for the easiest things first because most of the times, that's where the problem lies. :)

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