EzMEQ-5 hum

[Murdock]

Hey folks,

I've build an AML ezMEQ-5 some time ago but never really tested it... 
Today I wanted to do that and noticed a quite loud hum/buzz.
I experimented with the power transformer orientation and some mumetall sheets. The buzz gets worse when I turn up the two potentiometers clostest to the PT. If I put some mumetal between the PT and the inductors it gets better, but still not completely...
So there is some magnetic radiation into the two inductors. 
But I'm quite sure, that there is something else on my part as I'm sure Colin from AML tested his build thoroughly before releasing it. And I didn't read any other threads with this problem. I suspect there is a grounding problem.
I also have the buzz when only connecting the output into my mixer with no input...
Here is a link to the build manual with the schematic at the end.
http://www.audiomaintenance.com/downloads/ezmeq5_colourbook.pdf

Can someone maybe see where I could start checking for the fault?

 

[Rob Flinn]

I suspect there is a grounding problem.

Can someone maybe see where I could start checking for the fault?

You seem to have answered your own question.  Very likely a grounding problem.

 

[Murdock]

You seem to have answered your own question.  Very likely a grounding problem.

Yes, but does anybody have a tip on where I could start to look for the problem?
Is there a "common" solution I could try? 

 

[Rob Flinn]

Normally the AML projects have some of the best documentation ypu can get, so you could just go back through the manual & check that you have done everything correctly.

However if you don't want to do that, normally with this sortof problem I would set my meter to continuity, and check that the ground connections match that of the circuit diagram.

 

[Murdock]

Normally the AML projects have some of the best documentation ypu can get, so you could just go back through the manual & check that you have done everything correctly.

However if you don't want to do that, normally with this sortof problem I would set my meter to continuity, and check that the ground connections match that of the circuit diagram.

Thanks for the tip!
I checked with my multimeter and got 10 ohms between TP16 (Analogue ground) and chassis. So this seems to be ok. I just realized, that even without the power plug and just the output connected to the mixer I get a slight buzz (I think 120Hz). And it gets worse when powering up the unit ... Any clue on what could cause that?
I thought ground loops are are pretty much impossible with transformer balanced gear?

 

[Rob Flinn]

Are you sure it's the unit and not your external wiring ?    Have you tried disconnecting the shields on the connecting cables ?

 

[Murdock]

Finally took some time to get back to this repair project...
Yes, I disconnected the shields on one end to no avail.
Could it be, that the power transformer is somehow saturated and thus radiates more?
The B+ voltages are on the low side. Not by much but maybe this is an indicator?