I recently got access to an old linear 22 volt 4 amperes current limited power supply from an old telephone station. Due to it’s relative simplicity, I decided to reverse engineer it and try and make it a variable lab bench power supply. I attach the original schematic and the modification I tried to this post. However, I have two problems: first of all, for some reason I can not lower the voltage below 7,8 V. The second is that the voltage changes rapidly if I rotate the initial path of the pot (from 8V to 23V aprox.) and then spents about 3/4 of the entire turn from 23V to 30V. I could really use some help, since I have tried changing the feedback resistor and potentiometer value and I always get the 7,8 V threshold.
PS: the circuit with the 7805 is the old one, the one with the 317 the modified one.