I am using KornShell (ksh) on Solaris and currently my PS1 env var is:
PS1=\"${HOSTNAME}:\\${PWD} \\$ \"
And the prompt displays: hostname:/fu
hostname:/fu
Try this:
PS1="\H:\W"
More information on: How to: Change / Setup bash custom prompt, I know you said ksh, but I am pretty sure it will work.