The relay coil/driver and flyback diode orientation in the schematic are logically correct now: D2 cathode is on +5V, D2 anode is on RELAY_COIL_NEG, Q2 collector is on RELAY_COIL_NEG, and Q2 emitter is on ground.

The most important schematic issue I found is the relay contact mapping: the Flux symbol for K2 labels COM1 as pin 3 and NC1 as pin 4, but the Omron G6S-2F datasheet pinout shows COM pins are 4 and 9, NC pins are 3 and 10, and NO pins are 5 and 8 for the SMD G6S-2F top view. That means the schematic symbol/footprint pin-function mapping is likely wrong or mirrored for at least the COM/NC contacts. This can absolutely produce the symptom: relay clicks, but the effect path is not actually routed where the schematic appears to say it is.

A second likely issue is the anti-pop photorelay U4: it is wired from EFFECT_OUT to GND, so if PA6 drives U4 on during normal effect operation, it will short/mute the effect output.

For Omron G6S-2F-DC5 / G6S-2F-Y-DC5, the contact pinout found from the Omron datasheet/web result is:

Important: Omron shows the G6S-2F SMD terminal arrangement as top view. The through-hole version uses a bottom-view drawing. If a footprint/symbol was created from the wrong view, the contact functions can be mirrored/swapped.

The K2 schematic symbol currently exposes these pins:

This conflicts with the Omron datasheet mapping above:

Based on the schematic labels and adjacent bias resistors, the intended audio relay routing appears to be:

If the relay symbol were correct, the desired truth table would be:

Because the schematic symbol appears to assign COM/NC incorrectly, the physical relay may behave differently from the schematic view.

Using the Omron physical mapping:

That means a board built from this footprint could have the physical commons on the nets intended to be NC/bypass, while the nets intended to be commons are actually NC terminals. The NO contacts may still go to the effect input/output, but the relay common may not be on JACK_IN_TIP / JACK_OUT_TIP. This is the top suspect for “relay clicks but no effect signal.”

Schematic connections:

This is the correct flyback orientation for a low-side relay driver.

Correct installed orientation:

If D2 is reversed, it becomes forward-biased whenever Q2 turns on, effectively shorting +5V through D2 and Q2 to ground. Symptoms can include regulator collapse, high current, Q2 overheating, diode damage, or the relay not energizing properly. Temporarily removing D2 allows the relay to click, but it removes inductive kick protection and can stress Q2/U2 over time.

Schematic connections match the expected low-side NPN driver:

Expected voltages when relay is ON:

This part of the schematic is consistent with the relay clicking.

U4 is a Toshiba TLP3431 photorelay. In this schematic it is wired as an output mute:

Therefore:

Answers:

A quick test is to force PA6 LOW or temporarily disconnect U4/R3 and check whether the effect path returns.

The op-amp supply net appears correct:

Expected measurement:

VREF is generated by R9/R10:

Expected VREF:

VREF has decoupling/filtering around the analog section; if VREF is missing or near 0 V/9 V, the op-amps will bias incorrectly and the effect path can be silent or badly distorted.

The schematic uses portals named DGND, but the actual power portal appears to tie into the global GND net. Analog and digital grounds appear connected as one net, which is acceptable for this single-supply pedal if layout return currents are handled carefully.

The intended signal flow appears to be:

Potential schematic problems found:

The potentiometers are represented as separate terminal components, so the schematic relies on correct external wiring/pin numbering. Verify the physical pot wiring against the intended nets:

The single-pin ERC warnings at R21:P2 and C20:2 strongly suggest checking the volume/output section first.

D3 and D4 are connected anti-parallel between the gain-stage nodes:

That is consistent with symmetrical clipping in the op-amp feedback/gain stage. They do not appear to be accidentally shorting directly to ground. They are unlikely to be the primary cause of total silence unless one diode is physically shorted or the surrounding gain feedback path is open.

Perform these with the pedal on the bench, using circuit ground as the black probe reference unless stated otherwise.

The first point where signal disappears identifies the failing block.

Most likely causes, in priority order:

Before changing firmware, verify K2 physical relay contact continuity with a meter directly at the relay pins. If the physical relay follows the Omron pinout, update/fix the K2 part symbol/footprint mapping so pins 4 and 9 are COM, pins 3 and 10 are NC, and pins 5 and 8 are NO. Also force PA6 LOW during steady-state effect ON or temporarily disable U4 to confirm it is not muting EFFECT_OUT.