1) Where is the guy who was fixing my computer?
He was doing this for some period of time in the past and he is no longer doing it. It's over.
He was fixing it yesterday, last week, earlier this morning. The action of fixing was occurring or was ongoing yesterday, last week, earlier this morning. It is no longer occurring. It is not that it was once off. That would be: that fixed my computer.
It is that it continued for a certain period of time, specified or not.
2) Where is the guy who has been fixing my computer?
Up until now when I mention it. That is to say, he started at some point in the past and until now as I speak at this point in time, he has been doing it.
To sum it up, in 1) something continued to happen during some period in the past; it was ongoing during that time.
In 2), the action continues up to or continues to be true up until the time of speaking (the speaker's present time). It started in the past and has been going on up until the speaker's present time. I have been explaining this for the past five minutes. [in relationship to "now" for me here in this forumn to you, the OP.]
In English, it is extremely important to have a timeline in your head. When you speak, it is the present time on that line. Everything else that occurs will be in reference to that single point of present time for you.
past____________time of speaking [now, present] __________future
The verb was fixing in Sentence 1) above occurs at some point before the time of speaking.
The verb has been fixing Sentence 2) above occurs coincidentally with it.
The verb fixed would come at a point on the line before the time of speaking.
Past continuous and present perfect continuous are not interchangeable at all. I explain why above: the action in a past continuous is over at the time of speaking; the action of a PP or PPC verb is true or not true up until the time of speaking.