Hello again my friend.
You have two possibilities:
1. The LM340 regulator is faulty. Replace it. This, however, is the less likely of the two possibilities.
2. The 5V rail being fed from this regulator has a short circuit on it and the regulator has shutdown to protect itself (and the PET). The most likely fault is a capacitor has gone short circuit (probably an electrolytic or tantalum depending upon what is fitted). Follow the +5V rail from the LM340, disconnect one lead of the capacitors you come across and check for a capacitor short circuit with your multimeter set to measure resistance. If that capacitor is Ok, solder it back in and move on. It is unlikely to be a ceramic disk decoupling capacitor. It could also be a faulty IC.
Sorry, this is a nasty fault to locate - you just have to be patient and methodical.
Dave