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shear_xear

Atari 7800 only 2.3v off power regulator

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Hi everyone.  I have an Atari 7800 that is giving me a dim LED and only 2.3ish v out of the power regulator (with close to 10 going in).  Anyone have any ideas about where the problem might be?  Thanks!

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Also, the power regulator and three caps have been replaced (along with the power button switch.  

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2.3V off?  Sale!

On a serious note, is the regulator heating up?  Because with 10V in, it pretty much has to be a short somewhere.  Feel around for other hot components too.  Just try not to burn your fingers...

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Actually both power supplies I have on three 7800s all show 10v in.  The working two have 5v out.  

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Try replacing the MJE210 transistor located at Q10 as that is the next major component upline of the 7805. Also, 10v in might actually be a tad low. Most of my 7800 PSU put out about 14v into the system. Can you measure the input voltage from FB2 in the upper right area just below the large filter cap? That is the actual input voltage from your PSU before the 210 transistor and from there the 7805 regulator.

 

Also do you still have the RF shielding on? Be curious if anything changes with it removed as that would indicate some stray lead on the bottom of the PCB could be shorting against the RF shield.

 

 

 

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So FB2 is right around 10v and the transistor I swapped from another board.  Both times (before and after the swap) it was about 10.4v in and 9v out.  

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So still the same results then as before? Still only getting 2.3v from the output side of the regulator?

 

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And all of your 7800 PSUs only provide about 10v on the input? Again, that seems low to me. I've had a few 7800s that wouldn't seem to kick in until the voltage was over 12v on the input. No idea why but didn't think much of it at the time since the original PSUs put out at least that and more in most cases.

 

Have you checked the 4013 to make sure it is switching fully on to the system? Still seems damn odd that you only get 2.3v out on the regulator though. But again, I've only seen that when I didn't have enough input voltage going into the system to begin with.

 

What voltage do you get on the main +5 rail along the middle of the console? Assuming this is NTSC we are talking about right? It is the left thick trace down the middle of the mainboard.

 

 

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I’m getting the same 2v on the middle rail.  How to I verify the 4013 is working correctly?

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Posted (edited)

I just installed a different 7805 and now everything is working perfectly.  I’m not exactly sure why the other new pr was giving me problems, but it’s all looking good now.  Thanks!!

Edited by shear_xear

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8 hours ago, shear_xear said:

I just installed a different 7805 and now everything is working perfectly.  I’m not exactly sure why the other new pr was giving me problems, but it’s all looking good now.  Thanks!!

It’s not entirely unheard of for new voltage regulators to be dead on arrival, especially in this era of super cheap/made by the billions mass productions. 7805’s are crazy cheap so quality control isn’t always the best, unfortunately.

 

Anyway, glad you got it figured out!

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