AML ezMEQ-5 HF loss

[Murdock]

Hey folks,

A few years ago I got an AML ezMEQ-5 DIY kit and build it right away. But never really used it. Partly because there also was a little hum issue but also a HF loss when not in bypass...
Now I got some time and wanted to solve this issue.
I probed around the unit while injecting a HF tone and found out, that the loss starts just after the HF boost potentiometer (10k Lin). Frequency wise it already starts quite early at I think around 1k. But would have to check again.
The HF loss is not there when the unit is in Bypass (which just bypasses the filter section, but not the makeup amp).
Whats also weird, is the fact, that the accompanied inductor (IN3 in the schematic) is the only inductor that "reacts" to touching... I get buzz sound when touching it with one hand. It gets louder the more fingers I use. One finger is quite quiet all five fingers it is loud. It stops, when I also touch ground somewhere. Can also be the scews from the rack...
The other two indcutors don't do that.

Another weird thing is, when using the "DIP" potentiometer, I also get buzz... But some certain frequencies seem to  exhibit that more (200, 300, 500, 1.5k). At the other frequencies it's also there but really quiet...
Using the right "PEAK" poti also amplifies the buzz. The left "PEAK" function is dead silent. Even when boosting 1k.

I also checked every resistors color codes and they are all in the correct place...
Does anybody got a clue on what that could be?  ???

 

[radardoug]

Your inductor should be shielded. If it has a metal case it should be grounded.

 

[Murdock]

Your inductor should be shielded. If it has a metal case it should be grounded.

Thanks for the tip. But the inductors are not shielded. And I don't think they have to be. Collin from AML also offers these kits build and tested without shielding and I never saw anyone with the same problem...

 

[radardoug]

What level are your hums and buzzes? I have designed and built eq's like this, and shielding the inductors makes a difference. But hey, you dont have to listen to me.

 

[Murdock]

It's not that loud. When I turn the "DIP" and "PEAK" pots at 1.5khz fully clockwise to 10 the hum is around -70dB. That's the loudest "combination".

And it's not about me not wanting to listen to you...
It's just that I know that AML kits are highly regarded and normally pretty foolproof  :
So I dont think it's a design fault but something else.

And you are right. Shielding makes a difference. But to get the hum levels down to an accectable level I would have to put a 1mm Mumetall sheet between the whole filter board and amp board...

I suspected the power transformer somehow drawing too much current and therefor radiating more. But it doesn't even get warm...
I also noticed that my voltages are not on point with the voltages in the schematic... I know that tubes you have some kind of variance but how much is too much?
I have 277VAC (should be 270VAC) coming out of the PT. After the rectifier bridge it's 366VDC (should be 390VDC). At the output transformer center tap it's 277VDC (should be 290VDC).
Is that within the normal variation?

What I also really don't understand is the fact that When I use the left "PEAK" pot at 1Khz it doesnt boost the hum. But when I turn the right "PEAK" pot it amplifies the hum...

 

[radardoug]

That coil that you touch, and it buzzes? Its not grounded. Look for that.
Also, make sure its in the right way. And check the circuit points it connects to.

 

[Murdock]

That coil that you touch, and it buzzes? Its not grounded. Look for that.
Also, make sure its in the right way. And check the circuit points it connects to.

How can I check that it is grounded? There is no pin for the ground connection ( I assume you mean the core). Just three pins for the different inductor taps...

 

[Murdock]

Nobody?