I have rebuilt my Mini 1 - a 4 or so year old machine. I replaced the extruder thermistor fixed a broken wire in the lower drag chain - one of the purple ones for the bed heater. I got the machine working again but lousy prints with pillowing on top an horizontal ridges on side walls. I have determined that most of the issues were my Z drive. The leadscrews were not fully aligned and I have done that now with a digital caliper, and the Z offset also needed adjustment. I also played with print settings but have gone back to default settings for ABS. I always end up there because that is where my best prints in the past which were generally excellent were.
After much experimenting I am getting first 1/4" height is excellent then I get separation of all the layers above.