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

[aurelio]

(04-24-2020 02:58 PM)Paul Berger (Canada) Wrote:  Since you have one of the version that is switchable and I am guessing it is a later version that does not have screws that fasten the circuit board onto the bottom, however I believe that the holes in the board and the bottom of the shell are still there, even if the screws where not present. having the board fastened to the bottom makes it a lot easier to measure things inside the charger. I would wonder what the voltage is across the capacitor, also note these transformers are not capable of providing a lot of current so if the capacitor fails short it will drag the whole thing down.

Paul.

My version is the one with the screws, I'll post a pair of pics, I've already replaced the cap without success

Edit: PICS in the second with a new cap            

[aurelio]

(04-24-2020 02:58 PM)Paul Berger (Canada) Wrote:  Since you have one of the version that is switchable and I am guessing it is a later version that does not have screws that fasten the circuit board onto the bottom, however I believe that the holes in the board and the bottom of the shell are still there, even if the screws where not present. having the board fastened to the bottom makes it a lot easier to measure things inside the charger. I would wonder what the voltage is across the capacitor, also note these transformers are not capable of providing a lot of current so if the capacitor fails short it will drag the whole thing down.

Paul.

Hi Paul, hi all, the version of this charger is the one wich has screws that fasten the circuit board onto the bottom.
At the end the guilty was the Q3 transistor faulty, I replaced it with a NTE 184 (MJE800 not available), now the voltage is 5,2 V(unloaded) and the value is correct when compared with another charger. I connected the wall adapter to a calculator and now it works fine.
I have expecially to thank my friend Ignazio who provided me with an original backup transformer because the one installed was also faulty, this time I'm the "guilty" ... this would explain why, after the first attempts, the adapter was not able to provide any output voltage.

It is not easy to find for sale a transformer which fits perfectly to the board, this made my job easyer, 1000 times.
Summarizing I've replaced in this journey a transformer a capacitor (I swapped the original 400uF with a modern 470uF) and the Q3 power transistor

Thanks to all again
Aurelio