What PM do you have? I had a similar issue with an old PM2 where the batteries had leaked and damaged the board. I found the repair only lasted a while and it soon broke again. Nearly every issue you will see comes from leaking batteries. I would just buy a new PM as they are better in every way and your old one will go again.
Replacement PM5 are available direct from C2 for about £145 (not sure what supply is like currently). On the older monitors you are limited to what you can connect to it. The PM5 is ant+ and bluetooth compatible. The C2 forums are full of info about this and worth a read.