CORT MIX-10 Weird Chorus Sweep

[ddt]

Hi folks,
I'm working on a vintage multi-effects pedal, which consists of analog delay, chorus, compressor and distortion. It's a cool rare pedal and somehow predated other multi-effect pedals of the future.

Everything works perfectly, apart from the chorus effect, which has a weird kind of LFO sweep. I would call it asymmetrical, irregular and it has direct effect on the guitar signal in a way that chorus effect is building up normally but there's a sudden change in phase and then the cycle repeats. See the video of the LFO and clock signal I took at three different test points and how the guitar sounds. The LFO signal path is depicted in the attached picture.

https://youtu.be/Qjv6dH4lsXQ

But I don't understand, the LFO signal is nice symmetrical triangle, so it doesn't seem it's that.

The chorus part is based on LFO, JFET buffer, MN3102 clock source and MN3207 BBD. There's no schematic for CORT MIX-10 found anywhere and there' only one other post on this forum so far.
But there's also a nice example of a very similar circuit, which is supposed to be a replacement for SAD512 in the MXR Micro Chorus.

https://www.tfrelectronics.com/MicroChorus/

Anybody has an idea what's going on here? Is the problem with the clock driver or the BBD itself?

Thanks a lot for any suggestions.

[dmeek]

Can you post a photo of the foil side? Also are the two electrolytics in the bottom right corner bulged on top? If they aren't perfectly flat, replace them.

[ddt]

Actually the photo I posted is taken from the other post, here's the real one.

[dmeek]

Not sure there is much you can do to improve the sound.
Here is the schematic and parts location. Maybe you can tweak it a little better by adding a bias to the transistor base.
I took a guess at the transistor. What is the number on it?

[ddt]

Thank you. I'll try that and a couple of other things too. Let's see what can be done and has a significant impact. I was thinking of playing with the clock speed of the clock driver. I'll also would like to check the dc bias on the input of the BBD.

[dmeek]

The MN3102 was designed to have a fixed frequency or controlled by a potentiometer. The transistor converts it to a voltage controlled oscillator. The problem will be with transistor circuit. What number is the transistor? Or can you post a photo of it. What are the values of: R156, R158, R160, R162?

[ddt]

Yeah, that was my suspicion too. The transistor is the 2SK30A, N channel FET widely used in other BOSS pedals. As a test I replaced it with a known working one, no change. Perhaps it works in some non-linear part of its characteristics? Wrong overall design maybe?

Here are the values:
R162 4.7k
R160 150k
R158 2.2M (one end goes to pin 7 @ MN3102, the other to R159)
R156 330k (between pins 6 & 7 @ MN3102)
C135 33p (between 5 & 7 @ MN3102)

K30A FET:
DRAIN to pin 6 @ MN3102
GATE to R160, the other end of R160 is connected to R158, on the LFO side
SOURCE to pin 7 @ MN3102

[dmeek]

You could try to shift it with a trimpot and resistor like this.