Note that the two opamps are "swapped" from the schematic and layout. In the Schematic the low gyrator uses pins 5,6,7, in the layout it's 1,2,3.
The components you should check if the lows gyrator is the problem:
1.5uf electro attached to low 2
330 ohm R between 1.5uf cap and IC pin 1 and 2
68nf cap between 330 ohm and pin 3
100k between pin 3 and VB.
And hey, you haven't posted voltages yet.
did you get the "cut" between that cap and the IC? It is hiding under that 100k R next to it. There are two cuts under that R but that's the treble control side...probably not your problem.
I forgot to mention: I could not find electrolyte 1.5uF so I got a tantalum polarized 1.5uF instead.
By check I assume you mean connectivity, glitch free etc. I feel I had done it during building. And seems none of them are not as they should be.
I had another problem fitting it in the box and the "Inpüt" and "Ütpüt" jacks just ran into some caps. Among them the 68nF you mentioned. So to fix
that problem, I relocated them to the other side of the board. With a few cable plastics as protectors I got 4 of them relocated.
And lo and behold, the shriek is no longer there.
If I max some pots around I do get a shriekish noise but that seems very much more like feedback. It is a powerful circuit as far as boost goes.
So, all fingers and other things crossed, I now consider it solved enough to box it properly. In my experience some noise goes away when you finally put everything in place.
Should the problem pop up again, I have some good ideas where to check.
Many thanks!
