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Keneg

Internal power supply troubleshooting

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I am working on a very corroded beige unit.  I soaked the parts in citric acid solution to remove the rust am am now testing each part, starting with the power supply.  The 12 and minus 5 volt rails seem fine, but the plus 5 volt rail takes about 30 seconds to come up and then stabilizes at 5.207 volts.  This doesn’t seem correct.  Unfortunately, TI used a complicated circuit for the plus 5 volt rail instead of a 7805.  Anyone here willing to help me troubleshoot?

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Check the output of U4 pin 2 (which may be an oscillating signal - not sure). Does this signal change over the period it takes the 5V rail to come up? If yes the problem likely to be in the components around U4. If no, problem likely to be in components around U3.

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22 minutes ago, Stuart said:

Check the output of U4 pin 2 (which may be an oscillating signal - not sure). Does this signal change over the period it takes the 5V rail to come up? If yes the problem likely to be in the components around U4. If no, problem likely to be in components around U3.

Thanks, I will test and get back to you.

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That tested good.  5.214 volts as soon as powered on.  That connects to pin 5 of U3, which also tests good.  Any idea which component I should check first?  Resister, capacitor, power transistor, or inductor?  I’m thinking resister or power transistor.  Not sure how to test the transistors.

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For the transistors, I'd just get replacements and see if it fixes the problem.

 

Personally (but I'm no expert), I'd try replacing in order:

-- Q1 and Q2

-- C15

-- D9 and D10

-- U3

-- C11 and C13

 

I would think the resistors are OK unless they are visibly damaged.

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1 hour ago, Stuart said:

For the transistors, I'd just get replacements and see if it fixes the problem.

 

Personally (but I'm no expert), I'd try replacing in order:

-- Q1 and Q2

-- C15

-- D9 and D10

-- U3

-- C11 and C13

 

I would think the resistors are OK unless they are visibly damaged.

Thanks, I was thinking the transistors first.  Nice to have confirmation.

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I did some more testing.  Both R9 and R10 showed high resistance in circuit, M ohm in one direction and k ohm in the other.  They are supposed to be 0.56 ohm resistors.  They don’t have color markings, just 0.56 stamped on them.  Does anyone know what wattage and tolerance they are?  

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There's no sign of them having burnt? Good catch!

 

The +5V rail provides about 1A, so R9 and R10 between them need to dissipate around 5W.  3W 0.56 Ohm resistors are readily available - do a bit of Googling and see what will physically fit. Looking at the photo of the 99/4 PSU on the Mainbyte site, that has those resistors with colour codes and they are 5%.

 

If you measure the voltage at U3 pin 2, does that come up to 5V immediately? The delay is just due to C15 charging through the high resistance R9 and R10?

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Yes, it comes up to +5 immediately.  I started with R9 and R10 because they had a lot of rust on them before I cleaned the board.  Thanks for the answer, I did order some 3 watt, 1% resisters yesterday.  I figured better too large than too small.  If they don’t fit the holes, I can use jumper wires to test.

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I got the 0.56 ohm resistors in today.  Now I get 5.2 volts immediately.  Tested resistances on motherboard.  They were all in the k ohms range.  Hooked everything up, no picture.  Tried removing the VDC, but a pin broke off.  Ordered one.  I figure to test it in one of my working machines before trying it in this one.  I will need to De-oxit  the Socket and Check continuity.  I may have to replace the socket.  Pins on one side are shiny, but some of them on the other side don’t look good.  But, hey, progress.

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