"The Real Andy" .pearson@wayit_dot_com_dot_au_remove_the_obvious_ to_reply
wrote in message ...
On Thu, 10 Jun 2004 19:35:06 +1000, "Peter"
wrote:


"The Real Andy"
.pearson@wayit_dot_com_dot_au_remove_the_obvious_ to_reply
wrote in message ...
On Thu, 10 Jun 2004 16:21:46 +1000, "Peter"
wrote:

Sorry, I meant a capacitance meter of course.

The result for all of them were very close to the specifications,
except
the
one I replaced. That one was showing higher readings after I held it
my
hand for a while. But I guess that is not much an indication.
However
replacing it did change the symptoms slightly but did not fix the
problem.

Peter

snip

Sound like a cap that is in the power supply. See if you can locate a
rectifier, or perhaps the four diodes that make a rectifier and
replace. A faulty rectifier can often stress a cap to the point of
failure.

Thank you, that sounds like an idea worth trying.

I can locate the rectifier no problem (a square bit with four legs I
think),
but I have a question since as you can probably tell I am still learning
as
I go along.
If the rectifier was for the power supply and it was faulty, wouldn't
that
stop everything from working?

If one diode in the rectifier goes open circuit, it can create enough
ripple to damage a cap over time.

The cap will try and smooth the ripple. If you draw to much power from
the cap, it will discharge and will take time to charge up again, the
more ripple, the longer the charge time. Likewise, the more power you
draw from the cap the longer it will take to charge. If you have an
open circuit doide in the rectifier, it may only be charging it half
the time.

In AU (i think you are there) the mains is 50hz. When you rectify a
50hz sinewave you invert the negative part of the sinewave into a
positive, effectively giving you a 100hz voltage, like here
http://hyperphysics.phy-astr.gsu.edu...ic/rectbr.html
If one diode is open, you only get half of that voltage, just like the
second diagram on this page
http://hyperphysics.phy-astr.gsu.edu...ectifiers.html




Or is it that since the rectifier is faulty, it immediately damaged (or
stressed above the limits) the new capacitor I put in?

The new cap will be fine.

Secondly, is there a way to test a rectifier to see if it faulty?

You can test using the diode range on the multimeter. If you don't
have that you can test it using the ohms range. Each diode in the
rectifier should only conduct one way. If it doesn't conduct either
way it has an open circuit diode.

The bridge rectifier is drawn here
http://hyperphysics.phy-astr.gsu.edu...rectbr.html#c2


Thanks again for your help. I think there may be a light at the end of
the
tunnel yet. :-)



Thank you for the info and the links.
I had a look at them and here is what I have found out.

When I looked at the circuit again I got very excited because I noticed that
the rectifier is actually linked to the cap I just replaced.
So I knew I was on the right track.

But when I removed it from the circuit and measured it, the rectifier
checked out OK.
I measured about 0.48V in one direction and 0V in reverse between each two
pins.
However as someone else in this thread has suggested, it may not show as
faulty unless it is under load.

So I guess the best thing to do is to replace it and see if that helps?

Thank you again, really appreciate it.