HP-42S battery drain

[GreyUser]

The dim display issue is evident from the lcd voltage. Easy to measure. If the voltage is correct, the transistor(s) are fine. A bad lcd is a common cause of a dim display.

You’ve said nothing about the external 8 KB ram... have you isolated that as a possible current consumer? The unit will just beep continuously with no ram connected.

I/R: Desolder one or both leads of the I/R diode. If the transistor is shorted, current will go to zero as it has to go through the diode.