Re: Apple II+ power supply woes

[adrianw@guinevere.st.nepean.uws.edu.au (Adrian Whichello)]

adrianw@kay.st.nepean.uws.edu.au (Adrian Whichello) writes:


>Hi Netland

>I have an old 2+ which runs the model railway, among other things.
>Last week it's power supply gave up. I wasn't there at the time,
>I've found that leaving the computer permanently switched on is
>good insurance - the power supplies tend to die between cycles if
>eft off for long periods. Anyway, I opened it up and found that
>an electrolytic had swelled up so much that it had torn it's connections
>out of the circuit board. I replaced it, but all the power supply
>wants to do now is go click click click ie it won't oscillate
>properly. It does this if plugged into the motherboard or not.
>Any suggestions about what else could be wrong. I'm not wild
>about diving into a circuit that is connected directly to the
>house supply without some clues.

>Adrian.

>ps surely this must be a FAQ by now - I have several old supplies
>that are dead for one reason or another.


Hi Netland

     had a go at the power supply over the weekend.  I was successful, because
I typed this on the Apple.  There was nothing wrong with the bridge rectifier
and input side - there was over 300V on the collector of the big transistor.
None of the inductors had burnt out and all the diodes and transistors checked
out OK with a simple multimeter check.  In fact nothing looked wrong at all,
physically.  There was just hardly any (~0.5VDC) voltage on the output side.
I was getting desperate!  I decided to check out the voltage reference section
- I faked 5VDC (from the bench power supply) on the anode of the TL-431C and
up came the power!  So then the penny dropped, the capacitor the temporarily
shorts out the 4.7K 1% resistor at power on wasn't doing it's job.  I replaced
it and away we went, all systems go.  Anyone else found this?

Adrian.