A puzzler. Buzz with the bottom lid.

[pucho812]

This is a weird one for sure.

A unit  is on my bench that has audible buzz on the line out. It only buzzes when the bottom lid is on. Even more bizarre if I remove the bottom lid it goes away.  Then if I slowly slide the lid back  on to cover the bottom, there passes a point where buzz  starts. The bizarre part is it is no where near the power  transformer  or power supply part of the pcb when the buzz develops. Best is how it is like a switch no buzz then bam, buzz.  It's as if this one spot on the circuit board developed a ground loop or em field with the bottom lid.  The unit is measuring to spec from so tests and nothing out of ordinary is visible.  This is puzzling for sure.

 

[radardoug]

What kind of unit? Steel or ali panels? Where is the power transformer? Just a few of the questions we would ask.

 

[JohnRoberts]

I've seen bottom metal alter the internal magnetic field enough to be audible.

There is a loop or other interface problem that the bottom metal is aggravating.

JR

 

[pucho812]

What kind of unit? Steel or ali panels? Where is the power transformer? Just a few of the questions we would ask.

Steel chassis with Ali face plate. Power transformer in back and soldered to the main pcb via pins.  The mains transformer is well shielded, in it's own can hence the pins.

I've seen bottom metal alter the internal magnetic field enough to be audible.

There is a loop or other interface problem that the bottom metal is aggravating.

JR

Interesting, this is the first time this has ever been an issue.  I would like to think it was a failed component or joint, but alas it's not.

 

[walter]

Is the bottom connected to ground? Clip a lead between ground and panel, does it behave the same? Does it start to buzz if you place the panel on the bench and lower the unit down onto the panel? Maybe move the unit around above the panel to try to locate if there is a specific area to check.

 

[JohnRoberts]

You need to visualize the magnetic flux leaking from the transformer as a 3D volume. The bottom metal may stop it from passing through and may shunt it locally but the field still exists inside the chassis, shifted slightly by the bottom metal.

Lower the water or raise the bridge.

A) is hum coming from internal transformer, most likely?
B) what circuit node is being affected by the magnetic field?

Another consideration I've seen with high power amps, that use the chassis metal as a low impedance ground path, the bottom cover being present or not can alter the impedance and current flow in and around that path.

I'd try to figure where exactly the hum is being decoded (converted either from magnetic flux or ground current, into a signal corrupting voltage).

That may suggest the most productive remedy.

JR

 

[radardoug]

Is it an Eventide?

 

[pucho812]

no...
sorry for being vague on this one guys but was asked to keep the inocent out of the picture. We were able to remove the buzz noise.  As always JR was right on the nose with this. We were aggravating things.  But the unit is still not 100% it has a 20dB louder noise floor then normal.  so instead of the usual -101dBu with the input shorted, we get --81dBu. 

 

[radardoug]

If you are not prepared to give us all the information, then dont expect us to help you. Its a 2 way street.
The particular model may be very significant to some of us, and we might be able to point you to the exact problem.

 

[pucho812]

If you are not prepared to give us all the information, then dont expect us to help you. Its a 2 way street.
The particular model may be very significant to some of us, and we might be able to point you to the exact problem.

I know and I wish I could but  can't this time.