Hello,
Little followup of the 75 to 50 ohm migration of my HP 8590A Spectrum Analyser.
First after checking schematics, the differences between the 2 PCB boards are minor. 2 resistors , a calibration connector and also the calibration cable from the board to the front panel.
I manage to found the calibration cable on eBay (1,65€ -$2) with both appropriate connectors on each side.
After some soldering the board is ready with a 50 ohm calibration output. The other cables didn't need to be change. According the given spec the only different cable is the calibration cable from a 75 ohm unit to a 50 ohm unit.
The front panel was sanded. Gaps filled with some putty , sanded again and paint with acrylic paint according to the initial panel color.
After some measurement, the new calibration output was adusted to meet the -20dB specified in the service manual. Some frequency adjustments need to be done too. The signal level measurement accuracy is spot on after running all the calibration menus. But i think the diodes in the first mixer are dead. The datas are not so good at the end of the RL-VENR column of the CAL DATA. As Feedbackloop descrideb in his video. I bought some HSMS 2812 to swap the 2 diodes in the mixer.
That's all for now.
Cheers
Pierre