Meanwhile, I replaced the mixer diode with a B-E junction from a Ge transistor. Measured on the DVM diode indicates 150mV. So it means 1mA per junction is 150mV drop voltage . This junction mounted in place of the original 1N76C diode receives bias as follows:
10Mhz-1,8Ghz ........... 175mV / 0.9mA
2,5-5Ghz ..................... 112mV / 0,108mA
5-9Ghz ........................ 174mV / 0.880mA
9-12Ghz ....................... 249mV / 4,3mA
12-18Ghz ...................... 242mA / 3,8mA
Obviously, during this time OL is in operation.
The bias source is a UA741 in repeater configuration and there is no limitation resistance on the output. Maybe at most an RF shock inside the mixer block but does not have measurable resistance.
This UA741 which has a differential supply of -15V and -18V according to the diagram and in fact flows out with no load output between 0.12V and 4.2V depending on the frequency range. Each bias can thus be adjusted between 1V-4.2V with no load . Calculations according to values from the schematic diagram check this range. I can not calculate what is happening with the diode connected since the UA741 resistance is several tens of Ohm (under 75) and the diode is nonlinear I / U.