Troubleshooting Apple II Plus -- System Board or Power Supply?

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Troubleshooting Apple II Plus -- System Board or Power Supply?

Hello,

 

When I turn on my Apple II Plus, the system doesn't boot and the power supply makes a rapid ticking noise.  When I disconnect the power supply, I can read the correct voltages on the Power Supply connector.  At this point, I am unsure as to whether the power supply isn't putting out the proper current or there is a problem on the system board.

 

From +5vdc to ground, on the system board, measures around 218 ohms.  I have all the cards removed, the power supply disconnected, and the keyboard disconnected.  218 ohms sounds rather low to me.  Is this expected or did something blow out on the system board?

 

Thanks!

 

Brian

 

CVT
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Your PSU cannot deliver the

Your PSU cannot deliver the required voltage under load and it most likely has a bad electrolytic capacitor on the low voltage side. Easiest thing to do is to recap the electrolytics. You can either stop when you hit the bad one or change them all. I did the latter when I had this exact problem with my Apple II+ PSU.

 

If you have a 5 or 1o watt 12V bulb, you can test the 5V rail under load. The PSU should be able to sustain the load of the light bulb.

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I'll have to see if I have

I'll have to see if I have something to load the power supply down.  I just replaced all the PSU electrolytic capacitors.  I didn't smell anything, so I sort of figured that wasn't the problem.

 

Is there a recommended resistor to use to load the power supply as opposed to a light bulb?

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reifsnyderb wrote:I'll have
reifsnyderb wrote:

I'll have to see if I have something to load the power supply down.  I just replaced all the PSU electrolytic capacitors.  I didn't smell anything, so I sort of figured that wasn't the problem.

 

Is there a recommended resistor to use to load the power supply as opposed to a light bulb?

 

 

Two comments, first I agree with what CVT said, but your motheboad would be a load so the supply and that souldn't click when  connected. Sounds like there may be a short somewhere causing the supply to reset.  What model is the supply? Many II+, except  maybe the very earliest, came with the Astec AA11040B which should be able to deliver stable output on all pins when unloaded. The same is not true of the earlier II supplies, but AA11040B can.

 

Also the idea of laoding with a lightbub is possible,  5V would be the line to load. But you're getting into black magic waters here. The supply should really have loads on all outputs bue 5V may/should be enough to stablize the control loop. This isn't really a move for the novice or at least I don't think it is.  

 

That said, CVT's comment about a dead cap could be a short and that sounds like a good place to start and that's much more relevent if you just recapped the board. Check you got the polarities correct. 

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Well, I am positive that the

Well, I am positive that the capacitor polarities are correct.  I double and triple checked them when I installed the capacitors.  The power supply is the AA11040B.  The outputs look good when unloaded.  The meter doesn't show a dead short on the board but shows 218 ohms across +5 and ground.

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Did you measure the

Did you measure the resistance of the other voltage lines?

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I did not.  I thought that

I did not.  I thought that the resistance across +5 and ground sounded awfully low and posted here asking if anyone knew.  If it is too low, then I've got to figure out what's wrong on the system board.

CVT
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It is not low at all. 200

It is not low at all. 200 ohms on 5V will only draw 25mA as a resistive load and consume 1/8 of a watt. It basically means that you don't have a short on the motherboard.

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Thank you!  It sounds like it

Thank you!  It sounds like it's my power supply then.  I've got one of those Mean Well power supplies coming, and a 7905 sitting on my table, so I'll go that route.

 

Thanks!

 

CVT writes:

It is not low at all. 200

It is not low at all. 200 ohms on 5V will only draw 25mA as a resistive load and consume 1/8 of a watt.

 

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