WELCOME! to the forum.!
Hang in there for more responses to your post and best of luck.
I would be surprised if the main PCA is meant to be a field-serviceable unit. You may have to take it to a Philips Respironics repair depot.
Sadly, my guess is that you will spend less money and grief by simply replacing the entire device with a modern unit. For that, if you are going to stay with PRS1 machines, get either the Pro (for fixed-pressure treatment) or the Auto if you can manage the small extra cost. Both of these are data-capable machines, so you can keep track of your treatment.
I also find the Auto more comfortable at the higher pressures, since it will automatically adjust to the pressure I need, rather than the one I have configured for it.