I have a Pen EED and it did not like LR44's (which you will find recommended from time to time) as they are 1.5V and the original PX625 merc batteries are 1.35V. However if you use a 386 battery (1.55V) in a MR9 converter, this will reduce the voltage down to 1.35V and px625 size. - works perfectly.
wich battery do u use?
I have a Pen EED and it did not like LR44's (which you will find recommended from time to time) as they are 1.5V and the original PX625 merc batteries are 1.35V. However if you use a 386 battery (1.55V) in a MR9 converter, this will reduce the voltage down to 1.35V and px625 size. - works perfectly.