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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 5th Oct 2015, 11:38 am   #1
wireless_john
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Default Apple Macintosh 512k

Apple Macintosh 512K. Mine has just died! Completely dead.

Does anyone have a circuit diagram for the international version (240V) of the power supply board. I have the US version but the component numbering is slightly different and the more I go in to it the more confusing it gets.

I have mains going in OK. The fuse is OK and I'm getting 333VDC after the first bridge rectifier. I've looked at all the obvious things that happen with these, poor connector joints etc but no joy so far.

Thanks

John
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Old 5th Oct 2015, 4:40 pm   #2
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Default Re: Apple Macintosh 512k

I've battled with the similar board in a UK Mac+, but I have not unpacked my notes, etc after a house move (almost a year ago). So this will have to be from memory...

Look for a couple of resistors in series (I think it's not just one resistor) from the +ve end of the mains smoothing capacitors (+ve side of the bridge rectifier you mention) going into the chopper circuit. About 100k each. 9 times out of 10 you will fine one is open-circuit. Just replace it, there is no other fault that caused it to go.
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Old 5th Oct 2015, 5:53 pm   #3
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Default Re: Apple Macintosh 512k

Hi Tony,

Thanks for that. I've been poking about with it again this afternoon and went to look at the +5V, +12V and -12V on the connector that goes to the logic board and found to my surprise that they were all there! Hadn't expected that. I was hoping that maybe just the 5V would be missing or something like that.

So I went to the logic board, resoldered all the connections and reseated the connector. I'd already done that on the analog board. Ta Da! It sprang in to life! Great I thought. Went to put it back in the case, one more check before I put the screws in - nothing.

Out of case and back on the bench - now all the DC voltages have disappeared on the connector - so frustrating!

I'll look at those resistors though - thanks.

John
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Old 5th Oct 2015, 6:00 pm   #4
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Default Re: Apple Macintosh 512k

If the supply is starting up at all, it's not those resistors. Those are to provide enough power for the chopper circuit to start before you get outputs from the chopper transformer.

This sounds a lot like dry joints somewhere. Maybe just resolder the lot, its not a complicated machine...
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Old 5th Oct 2015, 6:49 pm   #5
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Default Re: Apple Macintosh 512k

Out soldering iron again tomorrow ...
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Old 5th Oct 2015, 8:06 pm   #6
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Default Re: Apple Macintosh 512k

<sigh> Just went out to lock the garage up and noticed that the Mac had suddenly come on. I'd left it plugged in and no low voltage DC anywhere </sigh>

Start again tomorrow - it's making me cross now!
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Old 6th Oct 2015, 8:35 am   #7
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Default Re: Apple Macintosh 512k

Dredging the depths of my work with Macintosh over 20 years ago, I'm 100% certain that you have dry joint issues. We had them on the early ones even when they were nearly new, mostly due to heat build up at the top of the vertical monitor board becausde there's no effective heat control.

Take the board out (mind you discharge the EHT) and examine the solder joints with a jewellers loupe - you will find some cracked joints in there.

Also clean the connector to the deflection coils, that gets hot and dirty which makes for a high-resistance connection.
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Last edited by mole42uk; 6th Oct 2015 at 8:36 am. Reason: Extra information I just remembered.
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Old 6th Oct 2015, 8:58 am   #8
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Default Re: Apple Macintosh 512k

Thank you for that Richard. I can't think it's a component failure issue now. Soldering iron and magnifying glass today!!
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Old 10th Oct 2015, 6:05 pm   #9
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Default Re: Apple Macintosh 512k

Fixed! Hairline cracks in the soldering around the switching transistor!

Thanks all for the advice (and encouragement!)

John
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Old 10th Oct 2015, 6:06 pm   #10
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Default Re: Apple Macintosh 512k

Great news! Well done, those cracked solder joints are difficult to see.....
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Old 10th Oct 2015, 7:38 pm   #11
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Default Re: Apple Macintosh 512k

They were very difficult. I had looked once as well. Eventually I went to measure the emitter voltage and it sprang in to life just as I touched it. Another close look and hairline crack!

I could have just resoldered the whole lot I suppose as was suggested earlier and that would have fixed it and that's probably what I would have done in the end. But it is nice to know what and where the problem lay isn't it.
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