I've been looking at this and came up with a possible solution. Could you please indicate whether this process is acceptable.
https://forum.tek.com/viewtopic.php?t=112620
As no USB is detectable I was planning on the following.
1. Remove RS232 shield from back of device.
2. Connect serial to PC. (I'm using a usb to serial converter cable)
3. From OFF, press and hold shift and power button.
4. From upgrade software, menu press enter to upgrade mainframe.
5. Run update.exe and upload V1.28 to Keithley power supply
Reason I believe this is the correct path. Link attached and when I downloaded the firmware package and unpacked, it resulted in 4 files under the 2200-CPU-V1.28. Running the update.exe shows a pull-down menu with a title RS232 Port.
Is this a good path?