I did some more test to understand the problem, the amplifier is still supplied with 12V by a stabilized power supply.
- without any input or output connected element, I measure 5V on the output terminals
- after connecting the probes directly to the input terminals, I measure 94mv on output 1 and 99mv on output 2 (probes at room temperature, measure directly on the terminals)
- the inversion of the 2 probes gives me exactly the same result (94mv on output 1 and 99mv on output 2)
- when I dip the probes in boiling water, I measure up to 404mv on output 1 and always 99mv on output 2
- the inversion of the 2 probes gives me exactly the same result (up to 404mv on output 1 and 99mv on output 2)
My probes cannot be questioned since both work very well on channel 1 and give consistent values for boiling water (404mv = 101°C)
If there’s a short circuit, it’s in the amplifier.
But I couldn’t highlight it with the multimeter