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Atari 7800 Powering On / Off issues...



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 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.

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I never really understood why GCC designed the 7800 with this type of power-up system. Having the console always powered and using the flip-flop to just start sending power to the rest of the system. Maybe there is something I'm missing, but when I designed my portable 7800, I changed the circuit to just have a standard power switch which turned the system on/off, without this transistor or flip-flop. Don't want to always be discharging the lipo battery. :)

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Likely just a hold over from the 5200 since it works on the exact same setup? Only in the case of the 5200, you have 2x as much to deal with since it has twin VRs switched on by a pair of those 210s. So you can get a 5200 that looks to power on but still dead because only one of the two VRs comes up due to a faulty 210 that should be driving the power over to it.


That said, I've only had one 5200 where I had to replace that 210 while I've done this like 3 times over the years in various 7800s. Then again, I've serviced a LOT more 78s than 52s so I'm sure the stats are the same between them.


You know in thinking about this, I'm sure they did this because of how they handle the reset to the system? They actually reset the 5200 and 7800 using the reset pin from the 4013 IC to quickly cycle the power off/on this way.


I only really learned about this after getting my BackBit Pro cartridge because for full use of the cartridge, you have to attach a reset wire off the 4013 ICs back to the BBpro cartridge so the button on the cartridge can control the soft reset function and work as intended.


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I had to create a different reset circuit for the portable, which allowed me to not implement any of the original 7800 power-up/reset design.

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