Sound Skulptor forums
Build help and troubleshooting => MU524 Vari-Mu Tube Compressor => Topic started by: ckalinec on March 05, 2025, 03:04:06 AM
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Hey JPK, running in to a good bit of noise on the output of my MU524 and I'm not quite sure what's causing it. It's a hissing/static type of noise. The noise ONLY increases when I turn up the output. The input knob has no impact on the level of noise coming out of the unit. I'm assuming this means it has something to do with the output stage.
Troubleshooting I've done so far:
- Checked an reflowed any solder joints. I couldn't really find any but I reflowed a couple of them that were not as good as others.
- no reversed components
- no component leads touching the back plate
- Nothing seems to be touching the transformer
I read through @egelmett's thread about noise issues as well and tried to see if there was an overlap.
I do have some similarities with his issue and the way he describe the noise being effected by the cover plate. However, even with the cover plate off mine is still giving off a good bit of hiss. just a little bit less.
I tried tapping pieces with a pen and screwdriver and there's only been one thing that has made any difference so far. The Voltage Regulator. The spacers/screws near the voltage regulator were also the ones that made the most difference with noise.
If I tap a screwdriver to the coil on the voltage regulator the hissing completely stops. The unit also still passes audio through when I do this at the same level. I have only been doing this for a couple of seconds at a time as I'm not sure if that could potentially damage anything by doing that.
Does this point to the voltage regulator being an issue or is it something else and this behavior with the voltage regulator is to be expected? Also, just simply moving a screwdriver around the coil without actually touching anything shifts the noise as well.
Thanks in advance!
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Try to change the voltage regulator.
Contact me for a replacement or find one faster here (https://www.mouser.com/ProductDetail/CUI-Inc/P7806B-500?qs=olJun0bQHM84NKv%2BsjuE5A%3D%3D&_gl=1*xvzn47*_ga*MjQ4ODIyNzg4LjE3MjcwNzk0NDM.*_ga_15W4STQT4T*MTc0MTI4MDcxNi4zNi4xLjE3NDEyODA5MjYuMi4wLjA.).
P7806B-500
Also check that C9 is well connected.
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Mouser is actually really close to my house so I’ll just grab that one. Obviously it looks different but I’m assuming the same function as the one provided in the kit? Any differences in sound expected between the too? Being a voltage regulator I assume not.
Im fairly certain last time I had the unit open C9 was fine. I’ll double check it again though when I install the voltage regulator.
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No difference between the 2 regulators except a nice plastic box.
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Mouser is actually really close to my house so I’ll just grab that one. Obviously it looks different but I’m assuming the same function as the one provided in the kit? Any differences in sound expected between the too? Being a voltage regulator I assume not.
Im fairly certain last time I had the unit open C9 was fine. I’ll double check it again though when I install the voltage regulator.
Hi did this help?
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FYI - Still having issues myself.
I have two MU524. The "Left" one is the one with intermittent snaps and pops and noise - it comes and goes. It happens whether in "IN" or "Linked" modes.
The "Right" unit is my control source. It's silent...no noise. It did have a problem with the P1 pot going bad - replaced.
What I did on both was to tape the entire underside (toward the components) of the cover plate leaving only the screw holes and their conductive pads exposed. That worked for the "Right" unit.
But, for the "Left", it's been a long-running process of swapping components between the two devices to try to find the component responsible for the intermittent noise.
Along the process, I did swap the voltage converters, but was not the cause.
Other attempts:
Lightly sanded the board contacts (to the 500 box), lightly added solder to the header pins connecting to the front plate (thinking poor connectivity), tried three different 500 chassis, tried moving positions in Midas L6 (and removing other devices around it), Swap tubes, swap C4 with C7, replace all electrolytic capacitors, replace C17, remove and resolder T1 & T2, swapped U3 & U1 between units, swap tube mount and socket between units, replace C10, C11 & C12, reflow ALL solder joints.
I will still swap U2 & U4, one at a time, to continue the process.
I'm probably forgetting other maneuvers as it's been a year-long process...could it be a bad trace on the PCB? That's where my head is going. It only starts when the device is well warmed...but a reflow of ALL solder hasn't helped...neither has a very tight mount of the support posts to the chassis (thinking ground issues).
I'm about ready to hit JP up for his repair service...I'm so damn frustrated at this point!