Hello,
I think, in your case GPIO of front panel MCU controlling AC signal has incorrect state causing the scope is powering on immediately after power cycle. You already confirmed that master CPU on acquisition board can't communicate to slave CPU on front panel via I2C bus. Reason for that is either corrupted program stored in FLASH or faulty MCU. I know it's not convenient and riskily, but you have to go through following:
Probe front panel MCU reset - it should go low after power cycle.
Probe crystal - its frequency will probably be 32.768 KHz.
It doesn't hurt to probe supply voltage rails.
If the crystal is not working and you will not confirm any voltage at its pins (I don't remember, but the level may be around 0.5 Vpp) you have to confirm whether program is corrupted or the MCU is faulty. The type of microcontrollers do not fail just so easy, so I would say there is something wrong with the program.
You can confirm that buy trying to read or debug the program using dedicated hardware and software (PE Micro and CodeWarrior v6.3) or hardware and software of your choice. Any not officially dedicated hardware and software may be not completely compatible causing problems, so you will be using such on your own risk.
You may also just re-program the MCU using hardware and software of your choice.
I attached S-record file extracted from the firmware package for MSO4000B series. The file is called as usually, route66_fp.s19. If a software and programming device you will be using does not accept S-record files you have to convert it. There are apparently dedicated applications for such conversions. I did never found any and did not use any at all. It is possible to manually convert the S-record file to hex of course, but it's time consuming to convert each line of the script.
Note, in case of your model there is also a file called route66b_fe.s19 I have no idea what is the purpose of the file. There may be a second HCS08 MCU (maybe on the acquisition board) in your case for which the program is desired. Or there are just two files and the right one is used depending of an unknown factor at the moment of knowledge I have. One thing is sure, the script lines in both files indicate exactly the same FLASH address range.
Maybe there is someone who has the knowledge. Since I do not own a MSO4000B series scope, I can't investigate it. Maybe you will be the one who figures it out.
I am asking you to upload here the backed up program because I am curious of its content.
Should you have problems obtaining proper hardware and software or at programming of the MCU, I could try to reprogram your front panel board for free if shipping cost to Europe and back would be not a problem for you. I trust only, it will be electrically checked first (checked for shorts, supply voltage rails, XTAL/EXTAL of MCU etc.). Normally there are of course customs charges, but as far I know if is something is sent for repair and you inform customs it will be returned to you let's say within a month of six months (a guess), they should not charge as usually or at all.
It's a bit complicated though and there are additional shipping costs, so I suggest you to try to manage it by yourself first.
I attached route66_fp.s19 and route66b_fe.s19 files extracted from the firmware package. I added *.txt extension to be able to upload the files here. You have to remove them in order to use them.
More information and everything what I went through while investigating issue on my front panel is written under my thread there, which I recommend you to read:
https://www.eevblog.com/forum/repair/tektronix-mso4054-no-boot-not-working-front-panel/msg5626077/#msg5626077If you have any questions, please feel free to ask. My setup is still not disconnected in case you have any questions and you want me to check something for you.
Good luck!