No, you can't replace the 2550.
It needs to have firmware installed on it before it can update the firmware.

If you were able to connect to the programmer and read the VPP voltage, then the chip is still good.

I have tried the 40 pin adapter with a another chip and the results are the same. Voltages are 5V (good).
The VPP voltage is normally higher, but it depends on the chip you are programming. Which chip are you using in the adapter.

Is it the 840Z adapter?
Is it a 28 or 40-pin chip?
If so, verify that the ribbon cable is connected to the 10-pin header on the opposite side from the ZIF handle, and that pin 1 is next to the ZIF handle.