As stated in another blog, I have a few 7800 sent to me for repairs and eventual upgrades. This 7800 had an interesting problem where as soon as you plugged in the DC power to the console, it would immediately power on and would NOT turn off. While there are a few components in the mix that can cause, this, I've found really only two components well technically 3 that should be looked into as possibilities:
The first component and the one most people already know about, is the power switch itself. But in this particular case, it wasn't the switch as the console would still power on even with the switch removed.
The next component is the 4013 IC flip-flop. This chip controls the actual on or off signals sent through the console when you press the power button. However this had already been replaced in the past as on this console it was mounted in a socket. Removing the 4013 did NOT change the condition on this console and it would still power on immediately. This pretty much only leaves one more component...
There is a large transistor mounted in spot Q10 and labeled as an MJE210. These were used on both the 7800 and the 5200 that I know of. This transistor receives a signal from the 4013 telling it to actually switch the unregulated power over to the 7805 voltage regulator. I quickly found out this was the issue when I removed the 210 and the issue went away. This was replaced out using another one from a donor 5200 parts board as the 5200 actually has a pair of these bad boys on them. Here is what the MJE210 looks like. In this picture, the failed one is the green one on the mat and replacement from a 5200 has been installed into place. These only work installed in one direction and that is usually with the MJE labeling facing towards the back of the 7800.
So yeah, if you have power on/off issues those are the three main components to check for first as the other components are mostly passives and not as likely to go bad.