Low frequency loss thru a deck

[Alex SE]

What can be a reason of low frequency loss thru a deck? I was thought that there is some compatibility issue between a deck and sound card (Creative) but the results are exactly the same even with a new card (Scarlett). The worst thing is that I'm getting the same results with two decks of the same model (RS-B965). The other decks (555, 565 and 727) don't have that problem.
Here is a measurement. (green lines)

Once again, it's not recording, it is signal passing through a deck (monitor).

Anny guessing and advice might help, so, thanks in advance.

 

[J!m]

Are they running signal through Dolby chips even when no NR is selected?

 

[Alex SE]

There are two separate inputs, regular and "CD direct" that is bypassing Dolby IC. Same loss with both inputs. A friend told me that there can be some HF filter but I haven't playing with such a mods. Replaced some opamps, added caps between the rails and replaced some switching ICs. Really nothing special. One of them is recaped but it doesnt makes a difference.

 

[Alex SE]

If this helps...

 

[J!m]

Doesn’t help me… I just know corroded Dolby chip leads can do all sorts of strange things.

 

[vince666]

Are they running signal through Dolby chips even when no NR is selected?

no, the B965 has full dolby bypass, so when dolby is switched off it's totally ruled out of the signal path.

this bass roll-off experienced by AlexSE is making both myself and himself going mad for quite some time now, even because I also own the B965 and the same model/version of Focusrite Scarlett soundcard and I am not experiencing that problem at all... which makes AlexSE's issue even more a mystery, to me.

There must be some reason for that... but which one?