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Mtlatc
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Plug in my board to tested wall plug. On 5V pin I’m getting about 0.9v on 12v pin I’m getting about 26v and on -5v pin I’m getting 0. What would be your troubleshooting start point?  FWIW all caps look good. Thanks. 

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Well ...There are at least two different types of voltage regulator boards for the 4A. We are talking about the TI-99/4A?

 

It appears that the switching supply uses a full-wave rectifier. While the linear supply does not.

So after checking the transformer, the next stop is the rectification section.

 

         Linear:

image.jpeg.815cc5d3097d01408115ae03aec0b0fe.jpeg

       Switching:

image.jpeg.881a67778c02423a685ca6f9f9270995.jpeg

 

No unusually bad smells?

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18 minutes ago, acadiel said:

Yep, verify that the switch is working... and then possibly check other voltage points.  Check out the Sam's Technical manual here:  

 

Can't thank you enough.  I have been searching all over for this type on information on the power board!!!!

 

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3 hours ago, HOME AUTOMATION said:

image.thumb.jpeg.f80ea3bf9d5ef76e6823465089b58294.jpeg

image.jpeg.47cc98e4cd2f751e008efaeca496a766.jpeg

My first thought would be TIP31(TO220), and the passive components in series between it and the +5V output pin.(L4,R9,R10)

 

...perhaps an underside shot of the board, would reveal something.:ponder:

 

C0B0B58B-FF78-487D-90BD-784D4404DDF0.jpeg

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On 1/4/2023 at 4:04 PM, HOME AUTOMATION said:

image.thumb.jpeg.f80ea3bf9d5ef76e6823465089b58294.jpeg

image.jpeg.47cc98e4cd2f751e008efaeca496a766.jpeg

My first thought would be TIP31(TO220), and the passive components in series between it and the +5V output pin.(L4,R9,R10)

 

...perhaps an underside shot of the board, would reveal something.:ponder:

Going to give you the big assist on this.  Long story short I replaced the TIP31 and the UA723CN (which I managed to smoke during troubleshooting) and it only cost me about $1.80 (USD). The board is now functioning as designed and at the proper voltages.  Now just need to plug it in to something and see where I am.  Thanks!

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