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MP-1 Front panel causing 164 Hz buzz, exaggerated when pushing buttons

Started by Purgasound, December 30, 2024, 08:19:58 AM

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Purgasound

Hello everyone.  I'm stoked this forum is still active. I had no idea. Happy to be here!

Here's the skinny...
I've got an MP-1 on the bench initially was in for repair as it didn't retain memory or presets. Easy repair, previous installer put in the battery holder backwards. Upon testing it I noticed it had a lot of noise and buzz present.  Now, I'm no stranger to high gain hiss and ground hum.  This is different, this seems to be originating from the front panel. At low gain settings it's not super noticeable but at mid-level to moderate gain it becomes really obvious.  When you press the Edit button and the LED's start to flash you can hear the noise pulse.  I used a signal generator to find the same tone and it's apparently 164Hz. The buzz gets louder when pressing and holding a button too. Has anyone encountered this before?

I've replaced every electrolytic capacitor except for the 10uF non-polarized caps. I ordered the wrong ones. I have two others and neither one is noisy like this. Another one of my personal ones is in need of serious repair after TSA forcefully removed the front panel which snapped off the input jack and dual-gang pot. Thankfully it appears the ribbon cables are intact... and no, they didn't put the screws back either. I flew with an MP-1 for a European tour a few years back. I instinctively bought a backup before I left although it had the "Marshall mod" done to it.  That's another project, it doesn't sound good so maybe they did the "Awful mod" instead. Either way, that one isn't noisy like this either.

So, don't meant to get off topic.  It's nice to have a working reference model available but not sure if that really does me any good.  I'm not sure whether the noise is being generated in the supply rail for the panel or if it's some sort of parasitic draw from the panel circuitry into the signal path. I haven't isolated the source yet.  Too many other projects so I had to set this one aside for the moment.

Thanks

Harley Hexxe

Hey Purgasound,

Welcome to the Depot!

This is definitely an issue for MJMP to answer. TSA is certainly a pain in the A$$ about music gear. Personally, I think they get their kicks if they get the impression that they screwed up your day.

To contact him, try this link: http://www.marshalljmpmodshop.net
I only have two brain cells left, ...and I'm saving them for the weekend!

rnolan

Hey Purgasound, welcome to the depot :thumb-up: . That noise is a weird one, I've not come across it before.  Hopefully MJMP has some idea what's going on.

I had to look up what TSA is, WTF, that's quite an overreach to pull off the front panel.  It's not as though MP-1s are difficult to look inside.
Studio Rig: Stuff; Live Rig: More Stuff; Guitars: A few

MarshallJMP

So what's TSA?

Some MP-1's have problems with that sort of noise, I think it comming from the led's radiating the noise into the mainboard.

rnolan

Hey MJMP, Transport Security Administration (TSA).  So checking everything going into the US for bad stuff, contraband etc.
Seems stupid (to me) to pull off the front panel but the people they hire for that stuff aren't always the sharpest tools in the shed...
Studio Rig: Stuff; Live Rig: More Stuff; Guitars: A few

MarshallJMP