On Sun, 10 Jun 2012 12:32:00 -0700 (PDT), Peter Easthope
wrote:
On Friday, June 8, 2012 8:25:14 PM UTC-7, Jeff Liebermann wrote:
You might also check the remaining capacitors with an ESR tester.
Just because the look good, doesn't mean they are good.
Do you always disconnect the cap for measuring ESR? I've seen
a mention of checking the ESR of an installed capacitor.
No. I measure them in the circuit. This works because the ESR of the
caps, even when there are a string in parallel, is much less than the
conduction resistance of most active devices that are likely to be in
parallel with the caps.
I have this ESR tester:
http://www.flippers.com/esrkthnt.html
http://members.ozemail.com.au/~bobpar/esrmeter.htm
in addition to several home made contraptions. Since I don't need to
check batteries with the tester, I have a pair of back to back 1N4002
diodes across the test leads to deal with charged cazapitors.
What's critical is knowing what the value should be. I have a chart
that helps (borrowed from Bob Parker's web site):
http://802.11junk.com/jeffl/crud/ESR.txt
Since I have a fairly good collection of replacement caps, I also like
to compare values with a brand new cap.
Since the major effort involved in recapping is often extracting the
board from its overly protective enclosure, I tend to replace first,
and test later. If one cap, of a specific type and value is obviously
bad, it's highly likely that all the caps of that type and value are
also bad. So, I replace all the likely culprits, and not worry about
testing.
Please note that it's not just the big caps, which tend to experience
high ripple current heating, that die. The smaller electrolytic have
their share of problems, especially in switching power supplies. These
are never visibly bulging and must be tested with an ESR tester.
There's also running assumption that the bulging capacitors are
somehow defective. That was true when various manufactories were
shipping electrolytics with bogus electrolyte about 14 years ago.
That's no longer the case and those caps are (hopefully) long gone.
What replaces them is marginal design and planned self destruction. In
order to save a few pennies, many manufacturers reduce capacitor
values and voltages to their absolute minimum. If there are three
caps in a row, and it will work with two caps, it's shipped with two.
The result is that the theoretical capacitor life, which is a function
of temperature and ripple current (causing internal heating), moves
fairly close to the desired lifetime. If the factory offers a 3 year
warranty, designing for a 6 year life is fairly common. The actual
failures will occur along a bell shaped curve, but the average will be
about 6 years. This is why I like to replace capacitors with the next
higher voltage rating. The ESR is actually higher, but the lifetime
is much longer.
http://www.illinoiscapacitor.com/tech-center/life-calculators.aspx
http://en.wikipedia.org/wiki/Electrolytic_capacitor#Reliability_and_length_of_l ife
When I want the board to work forever (such as for my own equipment) I
try to use tantalum or polymer caps in place of electrolytics. They're
not suitable for every application and not available in every
capacitance and voltage rating, so be sure to think about what's
important in the circuit.
http://www.capacitorlab.com/capacitor-types-polymer/
Then, there's idiot error:
http://802.11junk.com/jeffl/crud/GX520-bad-caps.jpg
I replaced all the electrolytics in the picture, and couldn't
understand why they would blow up after about an hour of operation.
When I posted the question to this newsgroup, it was pointed out that
they were all installed backwards. (Thanks Phil A.)
Now, I have a question. Is this unspecified model Deskjet worth
repairing? Even when working perfectly, the print speed, reliability,
quality, and cost per page are all problematic when compared to a
laser printer. The only thing good about an inkjet printer is the
initial cost, which is less than a laser and the power consumption.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558