Atari 7800 won’t turn off

[shear_xear]

Hi everyone.  I have an Atari 7800 that I picked up awhile ago that wouldn’t turn on.  I replaced the power switch and once plugged in it turns on and won’t turn off.  It is also showing that it’s taking in 12.5v or so when normally I see right around 9v.   I was wondering what could be causing this.  Anyone ever see this before?  Thanks!

[+DrVenkman]

There’s a 4013 chip in the power circuit that is essentially a flip-flop. Pressing the physical switch trips the 4013 to turn the system on and off. That’s a good place to start - if the 4013 is bad, it could give symptoms just like you describe.

 

However, I’m more concerned right now with your power supply. Did you measure the power supply under load (with the system plugged in and turned on)? If you measure a vintage transformer power supply without a load, it’s likely to give you a higher voltage.  If the system works besides not turning off, open it up, put in a game and turn it on. Measure the input leg of the 7805 voltage regulator (screwed into the heat sink) and the output leg. If you get more than about 9.15 volts or so input and anything other than about 4.95 - 5.1 volts on the output leg, your PSU could have issues and need to be replaced.

[shear_xear]

2 hours ago, DrVenkman said:

There’s a 4013 chip in the power circuit that is essentially a flip-flop. Pressing the physical switch trips the 4013 to turn the system on and off. That’s a good place to start - if the 4013 is bad, it could give symptoms just like you describe.

 

However, I’m more concerned right now with your power supply. Did you measure the power supply under load (with the system plugged in and turned on)? If you measure a vintage transformer power supply without a load, it’s likely to give you a higher voltage.  If the system works besides not turning off, open it up, put in a game and turn it on. Measure the input leg of the 7805 voltage regulator (screwed into the heat sink) and the output leg. If you get more than about 9.15 volts or so input and anything other than about 4.95 - 5.1 volts on the output leg, your PSU could have issues and need to be replaced.

I will look at the 4013.  The power supply gives me right around 9v on every other board I tested it on last night, which was about six as I’m going through all my “bad” boards with a UAV to see if I can get them going again.  Thanks!!

[shear_xear]

Oh, and I got right at 9v in and 5v out at the regulator on the other boards...

[shear_xear]

6 hours ago, DrVenkman said:

Measure the input leg of the 7805 voltage regulator (screwed into the heat sink) and the output leg. If you get more than about 9.15 volts or so input and anything other than about 4.95 - 5.1 volts on the output leg, your PSU could have issues and need to be replaced.

So I replaced the 4013 and that did nothing to remedy the problem, but then I swapped out the transformer next to the power jack (with the circular opening) and the system now powers on and off correctly.  It is still getting 12v when powered on at the jack and the 2200uf cap and power regulator, but the regulator drops it down to 5v, and all of the ICs showed 5v as well.  Will this damage the console or wear the PR out, or can the board handle this?  Thanks!

[+DrVenkman]

20 minutes ago, shear_xear said:

but then I swapped out the transformer next to the power jack (with the circular opening)

You mean the transistor? Yes, that can fail as well, though I've never seen it on one of my own systems. 

 

I'm a bit concerned that you're seeing 12V at the input to the voltage regulator. What kind of power supply are you using? 

[shear_xear]

28 minutes ago, DrVenkman said:

You mean the transistor? Yes, that can fail as well, though I've never seen it on one of my own systems. 

 

I'm a bit concerned that you're seeing 12V at the input to the voltage regulator. What kind of power supply are you using? 

Yes, sorry, the transistor.  Actually I rechecked and now it is 12v powered off and 10v powered on.  The power supply is a C0244471 with the silver label on it.  Like I said before though the same psu is giving me right around 9v on every other 7800 board.  

[+wongojack]

8 hours ago, DrVenkman said:

There’s a 4013 chip in the power circuit that is essentially a flip-flop. Pressing the physical switch trips the 4013 to turn the system on and off. That’s a good place to start - if the 4013 is bad, it could give symptoms just like you describe.

 

However, I’m more concerned right now with your power supply. Did you measure the power supply under load (with the system plugged in and turned on)? If you measure a vintage transformer power supply without a load, it’s likely to give you a higher voltage.  If the system works besides not turning off, open it up, put in a game and turn it on. Measure the input leg of the 7805 voltage regulator (screwed into the heat sink) and the output leg. If you get more than about 9.15 volts or so input and anything other than about 4.95 - 5.1 volts on the output leg, your PSU could have issues and need to be replaced.

 

I actually have a 7800 where the 210 Transistor is bad, but I haven't replaced it yet.  Unfortunately there is a lot more going wrong on that board.

 

BUT

 

Sort of related question for this thread - when testing for input voltage on a Voltage Regulator like this, do you measure the "Voltage In" and "Voltage Out" pins (Pin 1 and Pin 3) OR do you measure the Voltage In and Ground pins (Pin 1 and Pin 2).  I was recently probing around and every time I measured voltages on Pin 1 and Pin 2, I got a short and the 7800 essentially restarted.  By contrast, I can test voltage output easily by reading Ground and Voltage Out (Pin 2 and Pin 3).  I never even tried measuring P1 and P3. 

 

 

 

 

[+DrVenkman]

9 minutes ago, wongojack said:

Sort of related question for this thread - when testing for input voltage on a Voltage Regulator like this, do you measure the "Voltage In" and "Voltage Out" pins (Pin 1 and Pin 3) OR do you measure the Voltage In and Ground pins (Pin 1 and Pin 2).  I was recently probing around and every time I measured voltages on Pin 1 and Pin 2, I got a short and the 7800 essentially restarted.  By contrast, I can test voltage output easily by reading Ground and Voltage Out (Pin 2 and Pin 3). 

Not all meters are created equally, and so you never should measure using pin 2 (ground) directly. Pick another spot, well away from the VR but also on the ground plane like the RF modulator case if you haven't removed it, or the wide metal RF ground plane around the edge of the board. Use that as your ground reference and measure the input (pin 1) and output (pin 3). 

 

Reason being, some meters have so little internal resistance and minimal ESD protection so by measuring pin 1 and pin 2 directly, you are quite literally shorting the input leg of the VR. Definitely not good for the PSU or for the chips inside the machine.  

[+Stephen Moss]

13 hours ago, shear_xear said:

I swapped out the transformer next to the power jack (with the circular opening)

12 hours ago, DrVenkman said:

You mean the transistor? Yes, that can fail as well, though I've never seen it on one of my own systems. 

As the component in question is described as having a circular opening in unless a great big hole was blown in the transistor it I think they may have been referring to the inductor/ferrite.

But you are correct in that a Collector-Emitter short on either Q9 or Q10 will result in the 7800 immediately powering on and not turning off until the power unit is switched off/disconnected.  

[+DrVenkman]

1 hour ago, Stephen Moss said:

As the component in question is described as having a circular opening in unless a great big hole was blown in the transistor it I think they may have been referring to the inductor/ferrite.

It’s a transistor, an MJE210 bipolar PNP. And yes, it is has a hole in it. It is a power component and is designed to allow mounting to a heat sink, though it isn’t mounted to one on a 7800.

 

 

[+-^CrossBow^-]

7 hours ago, DrVenkman said:

It’s a transistor, an MJE210 bipolar PNP. And yes, it is has a hole in it. It is a power component and is designed to allow mounting to a heat sink, though it isn’t mounted to one on a 7800.

 

 

There are two of them present side by side on the 5200 as well just south of the RF modulator box in the lower left of it. They too aren't mounted but do receive in the brunt of the unregulated voltage from the PSU before passing it onto other components and eventually, the 7805s.