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Author Topic: MP2 suddenly noisy after a show  (Read 10354 times)

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SidecarJoeG

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MP2 suddenly noisy after a show
« on: Time Format »

I have an MP2 that has already had the noise mod done to it a while back.  R913 was replaced with a 1/2 watt resistor and all the main electrolytics have been replaced.  This unit used to be virtually quiet.  I played quite a few shows with it this year and was nothing shy of extremely pleased.   :metal:  I run my entire rack through a power conditioner, so I was assuming that big spikes and things wouldn't cook it. 

At any respect, at our last show it started making alot of 120hz hum.  It only does it on tube channels - clean is dead silent.  If I turn off the gate and unplug an input, the unit still hums quite a bit - way more then it used to.  It hums less at home than it did at the show.  Also, it hums really bad (like it did at the show) when the tubes are warming up and then once they are warm it quiets down.  Any thoughts?  I'm right now thinking of replacing the bridge rectifier (I have a spare from a dead MP2 I was trying to revive years ago.... beer is not good for these things). 
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Dante

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Re: MP2 suddenly noisy after a show
« Reply #1 on: Time Format »

SidecarJoeG

First of all, welcome to the Depot. I do apologize about you having to enter a code for your first post, those will disappear after you have posted 5 times. Just keeping the spambots at bay  :thumb-up:


As for your problem, I am very curious to see what you discover, because my MP-2 has been pretty noisy lately. Not terribly, but more so than usual, and like yours, only when playing a crunchy patch...I can hear it behind the noise gate in between the notes. I'm not sure if mine hums without anything plugged in (or if it subsides when the tubes warm up), but I will definitely try this over the weekend.

Dante

MarshallJMP

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Re: MP2 suddenly noisy after a show
« Reply #2 on: Time Format »

start with checking the high voltage,there is a testpoint near R913,you should have around 190Vdc.Let me know.
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SidecarJoeG

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Re: MP2 suddenly noisy after a show
« Reply #3 on: Time Format »

I'm getting 188v between ground and R913.  I get approx 181 across C333.  I checked for AC ripple using my volt meter and it floats around .001v at both R913 and across C333.

Nothing appears to be cooked or damaged obviously.
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SidecarJoeG

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Re: MP2 suddenly noisy after a show
« Reply #4 on: Time Format »

I am in the middle of another project, but once that is completed (hopefully over Thanksgiving break), that will free up my desk and I'll try to get my scope on it and see if I can track the noise down.  Like I said, it does it even when there's nothing plugged into it.  I have eliminated the bridge rectifier as there would be considerably more ripple at R913 / C333, I imagine.

Also, I'm not sure if I said it above but I tried two sets of tubes, one pair which works great in my spare MP1.

As for the verification thing, no problem.  I totally understand.
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MarshallJMP

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Re: MP2 suddenly noisy after a show
« Reply #5 on: Time Format »

maybe a busted cap,try to replace C333 and C334,C911 and C912.
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SidecarJoeG

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Re: MP2 suddenly noisy after a show
« Reply #6 on: Time Format »

I might try that.  The thing that is strange is that everything is really clean.  I got the scope on the power supply and checked everything out.  There's hardly any ripple on anything.  One thing I DO see as weird is on the one side of C303 I'm getting some random noise that's a bunch of frequencies in the audio range (like 400hz - 5k)... which looks like it could be my noise.  It's right on the pin for U301.  What is particularly strange is that noise is not on any other pins of U301.  The other side of C303 is clean.  I have a spare MP2 that is parts only... long story there.. but I'm thinking of swapping out U301 and possibly putting in a new C303.  I'll swap U301 and see if that does anything first.
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SidecarJoeG

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Re: MP2 suddenly noisy after a show
« Reply #7 on: Time Format »

Alright, I take that back.  There is about 15v of this noise on pin 1 of only that tube.  There is some noise on 2 (.5v) and 6(1v).  The earlier gain stage tube is clean.
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MarshallJMP

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Re: MP2 suddenly noisy after a show
« Reply #8 on: Time Format »

Maybe a problem with Q309 and 310,you could try to replce these?These are 2N3904 transistors,very cheap to replace.
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SidecarJoeG

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Re: MP2 suddenly noisy after a show
« Reply #9 on: Time Format »

Well, I bought all the electrolytics to replace the power supply, HV, and some of those 33uF 50v ones floating around for the power supply, post regulator ones.  I also got the 2N3904... got some extras since they're so cheap for general stock, and got new 1N4007s for the 24v supplies, just in case one is popped or funny.  I'll let everyone know how it works out.
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SidecarJoeG

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Re: MP2 suddenly noisy after a show
« Reply #10 on: Time Format »

So my parts came today and I changed out all the electrolytics in the power supply along with C333 / 334, the rectifier diodes for the 24v supply, and also Q311, Q310, and Q309.  After doing this, I checked for noise on my scope (with the gate off) and it was approx 5v of noise on pin 1 of tube 2.  Much better than the 15v I was getting before!  In playing, there is definitely some noise, but its about what it was like before.  I anticipate some noise out of a high gain unit like this, and its comparable to my Rocktron Chameleon black face unit with the gate off.  I also think that I've picked up some gain from all of this as it sounds crunchier than it has in quite a while.  Needless to say, I'm quite happy.  Thanks for your help, yet again, MarshallJMP.

On a side note, does anyone know where I can get cover screws for an MP1 / MP2?  I have 1 MP1 and 1 MP2 ripped apart right now and only enough screws to do 1 cover on any of 'em.  The rest kind of disappeared by the screw troll.
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MarshallJMP

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Re: MP2 suddenly noisy after a show
« Reply #11 on: Time Format »

Ok good you could solve the problem.

I have have some screws if you need them.
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gb

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Re: MP2 suddenly noisy after a show
« Reply #12 on: Time Format »

Hi guys

Sorry to resurrect this thread but im having the same issue.  Having lots of mp2 issues of late. I went back to my mp1 but its just not the same as the mp2 and I love it!!.

Should I just follow the above? Im a bit nervous using a voltmeter inside the unit while its one and near tubes.

Were they all just resistors? And are they common ones from a electronics store?

If I turn the gate off its horribly noisey like horrible!! Clean is dead quiet. The gate has to be all the way up to really mclean it up and its just not usable at all.

I tried different tubes with no change.
Cheers

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MarshallJMP

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Re: MP2 suddenly noisy after a show
« Reply #13 on: Time Format »

There's a schematic here,somewhere in a topic,do a search for it.It's well detailed.
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