A strange behaviour when my classic calculator is connected to a wall adapter

[aurelio]

The machine (a 67 with a battery pack)) does not turn on if the wall adapter is plugged in ...
if I unplug it the calculator instead turns on and works correctly, what do you think, something something wrong with the charger?
If I swap it, the calculator works normally (I mean turns on)

the same problem with another classic calculator: the same behaviour.

I tested the voltage on the terminals of both the adapters following what i read on this previous post, I mean:

for the first adapter the "outside pins" is 4,9 V (supply for the calculator's logic chips). on the other outside pin (a constant current source to charge the NiCd.) the open-circuit voltage is around 16,2V.

for the second one respectively 5,3 and 16,1V

It seems these values are both in the range, where must I check forward ?

thank-you for any advice
Aurelio