The big blue looking thing is a large capacitor. The batteries on the CPU board are the only ones. I know you checked the batteries, but the easiest thing to try is replace all 3 with new batteries just to be absolutely sure. When you pull them out, inspect the battery holder for signs of current or old battery acid leakage, as that can cause problems and damage traces. If it looks fine and with brand new batteries you still have the same problem, get a multimeter and test for 4.5 volts DC at the blocking diode just to the right of the batter holder. It's D25. FIRST TURN OFF THE MACHINE. To test for 4.5v at D25, put your meter to DC volts, put the black lead on the ground test point, which is on the top right of the batter holder closest to the holder (the test points are little metal loops on the board). Put the red lead on the non banded side of the diode D25 (you'll notice one side of the diode has a silver band on one end. That's the banded side). You should see 4.5 volts. Then move the red lead to the banded side of the diode and you should see 4.2 volts. If you get no reading on the banded side, the diode has failed. You can replace it with a 1N4004. If all voltage readings are looking good, move your red lead to the B+ test point, which is just to the right of D25 and below the black capacitor. You should see 4.2 volts. If voltage is present, test to see if the voltage is on the ram chip. Just below the B+ test point you'll see 3 chip sockets. The third one down is the ram chip, which is chip 5D. Put your red lead on the last leg on the bottom right of the chip. You should see 4.2 volts. If you have that voltage present, but still the same open door message, the ram chip itself might be bad. You might also check all the solder pads on that chip socket.
If at any point in the chain you aren't getting the voltage, you need to look at the traces leading to that part or the part being tested. If no voltage is on the non banded side of D25, make sure you are getting continuity to it from the battery holder. The battery holder itself could be the issue. I've had battery holders test good and look great, but replacing still fixed the issue. If no voltage on the banded side of D25, the D25 diode is bad. If you get no voltage at test point B+, but it's present on the banded side of D25, you need to test for continuity from the banded side of D25 to test point B+. Look at the traces for damage. If no voltage at lower right leg of D5 ram chip but present on test point B+, you'll need to test for continuity from test point B+ to lower right leg of D25. Look for a damaged trace.