What software are you using to program/erase them?. If you've got them set to use the internal oscillator you need to switch them to programming mode very quickly - before the oscillator can start. My software WinPicProg incorporates a Vpp delay, and if it's set too high the oscillator can start and this prevents programming the chip.
I bought the P16PRO40 with a 74L506 pre-assembled, but don't know
how to set up the hardware so I inverted the last three (attachment)
The program LED doesn't lite when trying to program a PIC so have been using IC Prog. (It inverts output, clock and input)
I also installed the Port95nt driver.
I use Win XP.
Would like to use your program if I could determine how to setup the
hardware.
I do use the internal osc & the protected mode is off.
WinPicProg has fixed settings for both versions of the P16PRO40, simply select the one you have from the pulldown menu - you don't need to use the custom settings.
This one PIC still won't give up it's code nor allow new code to be programmed. I am putting it away and moving on to purchase a new one.