Jack – if you notice on the schematic, diodes D3 & D4 are just an extra clipping diode section and when measuring voltage from ground to specific point one the circuit, one side of the D3\D4 clipping pair should register a small voltage, but the other side should measure zero volts as they are tied to ground. (One side of clipping diodes D3 & D4 ARE ground.) So if you have one probe of your DMM connected to ground at the PCB “GND” connection point, and you are still able to measure voltage on BOTH sides of D3 & D4; then there is something definitely wrong. Measuring across D3 or D4 should register a small voltage – because one side of each those diodes goes to ground – while the other side is basically connected to the output of opamp.
With all due respect, without knowing what level of electronics knowledge\skills you possess, I have to ask: “How confident you are with reading schematics and using a DMM?” They are two crucial skills for troubleshooting.
Also, have you tried using a different opamp like a TL071?