I had to have a look at the circuit to see what the problem was, with something like a mixer such as this I was expecting to see the standard circuitry. But I discovered something totally unusual, analysing the circuit told me the circuit could not ever work properly. All the parts are there, correct ICs but the circuit configuration was wrong. These mixers will clip and distort by design.
Hence my decision to nominate this item as a do not buy, ever, however attractive they may seem, they can never work correctly, nor can be recovered by simple modification.