Thank you for the videos, although I wasn’t able to see the second video…it seems to be a dead link.
But from the first video, I can see the problem seems to be with the LED, not the LDR (thank you for testing those resistances).
This is a really weird problem that I’ve never seen before. The issue has to be with either the LED or the driving circuit or with the microcontroller.
So to start on the top of the list, could you please measure the high side optocoupler LED with your diode tester on your multimeter (with the power off)? Just compare it with the low side LED to see if it’s the same. I expect it is.
If it passes that test, lets try to test Q1 (with the power on). Could you please turn the depth knob all the voltage on all the pins of Q1 and compare them with the voltages on the same pins of Q2? Then turn the depth knob all the way up (clockwise) and do the same thing. Keep the LFO rate relatively slow during this test so you can see the voltage changing. The voltage should be changing in relation to the LFO rate, so compare how the changing voltage on the pins of Q1 compare to the changing voltage on the same pins of Q2. Use the DC voltage setting on your meter to measure these. If both the transistors seem to be behaving the same then the problem might be with your microcontroller. But first please ensure that R14 is soldered correctly and is really 10K ohms in value. Then make sure pin 15 of the microcontroller is soldered well and it is seated well in the socket. If all that is good then you may need to re-flash the microcontroller. You’ll have to get a USBTiny programmer for that. We have the software on our website that you can download for free. After you do all these tests, let me know what you find.
-Brach
PS it doesn’t hurt to resolder all the pins on the components that I mentioned here…sometimes bad solder joints can look good.