The HDD is connected using a traditional IDE tape. The FDD cable is also regular, the drive A: needs to be connected after the “tape twist”. The power connector on the motherboard side is not like in more modern PC’s. In lack of the original power connector, you may use individual pin connectors removed from the AT motherboard power connector and insulate them individually.
Reverse-engineered power supply connector pinout.
When the power supply has no load, it periodically resets itself, which can be seen in the drop of the voltages and restoring them again. Following picture shows the difference between pin 30 +5V power ready signal (purple) and all other +5V pins used to deliver power (yellow).
Analog Video and Tube
Removal of the individual keys from the keyboard has to be done extremely gently. The keys are very likely to break in that process on that keyboard. You can put them together by using any type of cyanoacrylate “minute” glue.
In order to make Compaq Portable II boot you will need an original setup program to configure it. It does not have a BIOS which you can enter at the boot time.
One of the very important parameters is HDD type. Without setting it correctly, you will get HDD not visible at all or return write/read errors. Here you can find a table describing disk types.
There is a special DOS release from Compaq for this machine. However, I would recommend installing DOS 6.22 instead. Unfortunately, the small amount of memory (640K) will prevent you from running any of the nice programs like Norton Commander.
This page contains the images of the setup program, Compaq DOS and maintenance/service manual.