7800 wont work, help?

[ZippyRedPlumber]

So I felt like playing my 7800 yesterday, hooked it all up. It would not turn on, let alone show the led light. After I unplugged it, I discovered that both AC adapter and System itself heated up. Is there something I'm doing wrong? Should I plug it to a wall outlet instead of a power strip?

[+-^CrossBow^-]

Well I would start with testing the voltage output coming from the AC adapter as while it is normal for them to get warm when in use and plugged in, they shouldn't get overly warm or hot to the touch either. The 7800 only gets warm once it is powered on and the voltage regulator inside it starts to do its job providing steady +5 volts to the system. But if there is a problem with the VR (And it does happen on the 7800), then that could be cause for the issue.

 

The process I try and follow is called ASAP. Meaning...

 

Always

Start

At

Power

 

And then you follow it from there. If the power adapter is putting out about good voltage and even voltages as high as +14 from the power supply isn't unusual, then you have to make sure the power switch isn't fault. (Another common issue on the 7800s). There is other stuff in that mix as well, but again start with making sure the power supply is good, then the voltage regulator, and then the power switch.

 

 

[+DrVenkman]

@-^CrossBow^- (did you change your name and remove the section mark? It’s a lot easier to type now, lol!) gives excellent advice. Of course, you must have a volt meter. In this day and age, 30+ years after these consoles were built, it should be mandatory for people to have at least a basic digital multi-meter to check the basics when things inevitably fail. Fortunately most fixes are cheap and easy, and you can figure out a lot with just a $15 - $30 meter.

 

So if you have a meter and the power supply seems okay per -^CrossBow^-‘s suggestion above, when you open your console, check the 7805 voltage regulator. The pins on the 7805, looking from the front of the console toward the back where the regulator is screwed into the heat sink, are Input, Ground and Output. With the system plugged in and turned on - the power LED should light -place one test lead of your meter on the ground plane of the board (the large metal border that runs around the board where the RF shield usually rests) and one on the output pin of the regulator, the pin on the right looking toward the back of the board. You should get something very close to 5V (usually 4.95 - 5.05 or so is fine; a little bit higher (under 5.1V) won’t damage the components, but lower than 4.95 usually indicates there’s a short somewhere - possibly a damaged chip - pulling the voltage low. 

 

If the power supply is okay and you don’t get power to the board at all - the LED doesn’t light when you press the power switch - you might very well have a bad switch. I’ve never had to replace a switch on my 7800’s so I’ll leave suggested replacements for others. I do know that similar very small switches are still used today for all kinds of things, though I don’t know about the exact size used on the 7800.  

[+-^CrossBow^-]

On 10/5/2019 at 10:26 AM, DrVenkman said:

@-^CrossBow^- (did you change your name and remove the section mark? 

Yes after having the § symbol in my username for nearly 20 years, I decided (Now that I'm a sub), to change it to something that is easier to type. Although the new forum software having the ability to easily mention or hashtag..etc with the @ symbol and then the name makes this an easy process as well.

 

@ZippyRedPlumber, have you had any luck or checked on this further? 

[ZippyRedPlumber]

@-^CrossBow^-

 

Not yet, have to use the voltage output test first