https://groups.io/g/HP-Agilent-Keysight-equipment/topic/hp8563a/24792332

Hi, 
In order to expand the frequency to 26GHz the connector won't do anything really ... You must modify the U506 memory to add 
the option 026, then you must recalibrated it to reach the 26GHz properly. The YTF and YIG must be adjust to reach this band 
properly. If you don't calibrate it you'll be able to tune on 26GHz but with a lot a loss.

The options are usually list next to the spectrum model in clear text in the memory. for example if you extract the data 
from U506 you'll find some clear text aroung address 0x300 with the device model and the options next like this : HP8563A,H01

The model in the example have option H01 if we replace the HP8563A,H01,026 then you'll see the 026 options add in the options
 menu of the spectrum analyzer and it will boot with span to fit 26GHz bandwidth as a default BW.  The TL866II Plus is an 
amazing tool to backup these old school device and fairly cheap. 

I recommend you make a backup of this memory before you attemp to modify it. Also you may do the same thing with the firmware
 of the spectrum U306-U307-U308-U309 if anything goes wrong with the memory you'll be covered. Also if you have the revision 
920504 of the firmware I'll be very interested !! 

Good luck with the upgrade ! 

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I've forgot to mention earlier, if you stick with the service manual in order to upgrade it all the way to 26.5GHz you may 
also consider modifying some HW that are optimized to work all the way to 26.5GHZ. 

- Attenuator the 22GHz version is model 5086-7783 vs 5086-7796 (26.5GHZ)
- YTF 0955-02777 vs 5086-7850 (26.5GHZ)
- RF Switch 3106-0022 vs 3106-0032 (26.5GHZ)

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But if you only want to see some signal at 26.5GHz with some decent performance the stock 22GHz will do the job. 

I've forgot to mention if you add options via the memory U506 after the options you wanted to add you need to write the byte 
to 0x00 or else the device won't be working properly when you'll try the ID? command via GPIB the device will reboot instead 
of responding properly. 

For example HP8563A,H01 after the 0x31 "1" you write 0x00 in order for the program to respond correctly.