Problem with 41C

[heliocrono]

I recently purchased a 41C (No Modules) calculator. It turns on and I can enter numbers with the keys. But when I press ENTER the calculator turns off. After a few minutes it appears on. The blackout occurs the same if I do sin, cos, tan or even LOG, gold key, etc. operations. It seems to turn off only with the top keys. It seems like it can't perform stack operations. The ALPHA, PRGM and USER keys work.

The calculator is perfect, very well maintained with no corrosion. The batteries are new, and the module bay is immaculate. What can I do?

I have done a MASTER RESET (<- and ON), and MEMORY LOST appears, but then the error persists. I have followed the steps in the book "Extend Your HP41" by W Mier-Jedrzejowicz (section 4.1). I am asking for ideas, or alternatives.