Re: Apple ][ original power supply restoration (Pt 2)...

["Luke Forat" <iceman@nospamdigidude.homeip.net>]

    Nama,

    switching PSUs (especially older ones) usually need a load at least on
the 5V reference line - did you put a load on the 5V when turning it on? You
can stick a 10 Watt 10-5Ohm resistor on the 5V line instead of hooking your
Apple to it to avoid unpleasant surprises. Then turn the PSU on and see if
stops "squeaking". If there is no load the feedback circuit will
continuously cut off and on the output in a neverending loop: in the case of
the Apple PSU if I recall correctly it will squeak as it is frequency
controlled (not real PWM with variable duty cycle): the load determines the
frequency through a pseudo VCO which receives the feedback from the 5V
output so the higher the load/current, the lower the switching frequency.

   Please try to put a load on the +5V and see if it works by measuring the
output voltage(s). Remember, the reference voltage is the +5V line, always
check this voltage first. It must be within 5V +/- 5% for the logics to be
happy.

   Luke

-- 
"Nama" <lord_philip@mac.com> wrote in message
1143630692.248836.53080@i40g2000cwc.googlegroups.com">news:1143630692.248836.53080@i40g2000cwc.googlegroups.com...
> Hi again.
> Here is one for you elecrical geniuses on the list.
>
> I have just replaced all the bigger caps in my power supply.
> C6, C7, C10 thu C14 and C17 thu C20, and C22.
>
> (side note: it's amazing how much smaller capacitors are now. They are
> almost 1/3rd the size they were in 1977).
>
> When turning the unit on it chirps! Maybe at a rate of 4 chirps a
> second.
>
> I read somewhere that this could be a sign of a short, or signify
> another problem. I have looked for shorts and can not see anything
> obvious. and all resistors look solid and not burnt in any way.
>
> Taking voltage reading I get the following:
> +5V line = 1.6V
> -5V line = -0.8V
> +12V line = 1.8V
> -12V line = -0.8V
>
> ...these voltages also pulse a little in time with the chirps.
>
> I was wondering if anybody would be able to suggest where I should look
> next? Rectifier maybe?
> Unfortunately I don't really have any serious testing tools beyond my
> trusty multimeter.
>
> Any help will be (as always) greatly appreciated.
>
> Cheers
> Phil
>