Fender Acoustasonic Junior Chorus effect only works if bias is fiddled with

[SubcutaneousM]

Hi all,

I initially posted this on diystompboxes.com since my account had not yet been verified here, but I received approval minutes after posting there. I know a lot of folks are the same between the forums, but I will copy and paste what I posted there.

You can see the schematic here.

The amp's owner complains that the chorus effect no longer works. After tear down, I found that readjusting the chorus bias (R165) brought the effect back. Unfortunately, every time the amp is shut down, I have to fiddle with the bias by turning it all the way to each side and then leaving it in the correct location. Once that is done, the chorus works for the duration of the amp's on time. I wonder if the MN3007 is latching up for some reason. I don't have enough experience with the chip to know exactly what would cause that.

Anyone have any thoughts based on the schematic? Thanks in advance!

[g1]

Based on your description of something changing in the chorus circuit everytime the power is cycled, I would look at the electrolytic caps in that circuit, maybe just go ahead and replace the 3.  C64 and C77 both connect to that trimpot, C66 connects to the VCC pin of the 2 IC's.