Let's Get Technical > Troubleshooting Problems

MP2 suddenly noisy after a show

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SidecarJoeG:
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). 

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

SidecarJoeG:
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.

SidecarJoeG:
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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