Finally I found it!
based on the diagram here: https://raw.githubusercontent.com/dc42/OrmerodSensorBoard/master/SchematicAndPCB/Mini-differential-IR/V1.2/schematic.png
I saw that the value of the R5 resistor was +10M ohms.
In fact the resistor was poorly soldered and is not possible to see it with naked eye!
Now the probe read correctly 465 and 535.
Luckly i'm not bad with electronics. someone else would just have tossed it in the trash! ...