3verb reverb - low frequency oscillation and noise

Hallo!

I've just built my first pedal - a 3verb from https://www.musikding.de/3verb-reverb.

It seems to work - but there is quite a bit of 50Hz hum and high frequency hiss, even with the "mix" control down, although it gets stronger when turning it up. Furthermore, there is some loud low-frequency oscillation when I turn the decay up past ~10%.

Is this normal, or have I done something wrong? Is there anything I can do to reduce the noise?

Also - does anyone know what the trimmer pot P1 does?

In the attached pictures I've removed the footswitch and wired the jacks straight to the board to eliminate that as the problem. The output jack ground is disconnected to fix the ground loop because it's already connected through the chassis, but that didn't fix it either.

Danke!

All the voltages seem correct.

I've got:
- 3.7v for Vb, measured on the positive side of C6. This is the middle of the 10k/10k divider.
- 7.4v for V+, measured on the positive side of C7 or across the diode.
- 4.99v between pins 1 and 2 of the Belton module
- 3.7v on pins 3 and 5 of the TL072P (non-inverting inputs), as well as 3.7V on the outputs - pins 1 and 7. Pin 4 is grounded according to the continuity checker on the multimeter.
- 7.4v on pin 8 of the TL027P

Anything else I can check?