On Thu, 31 Mar 2016 13:16:59 -0500, you wrote:
Thanks Jerry - I'd be buying a 7000 series time base off feabay for
that one IC if not for you! I'd be very curious as to the version
difference too.
The 7000 series timebases which use that IC are commonly available
very inexpensively. The 7B53A for instance are very common and often
included with other purchases.
I have some notes on which custom ICs were used in which products but
none for the timebase ICs.
With all the mention of 'touch as little as possible' it's changed my
thinking and I'm thankful to hear it after realizing what's involved
with just moving one board out of the way yesterday. A lot different
than the servers I manage that are meant to have parts swapped - no
soldering irons there.
That is the general rule I follow but if I do have to replace
something which requires a lot of work to access, then I am likely to
test or replace other parts which age poorly or are suspect. This is
especially the case with aluminum electrolytic capacitors which dry
out over time.
What has people's experience been with the giant power filter caps?
Do they tend to be bad after this many decades from age? Or is it
really dependent on chance and hours on the unit?
In linear regulators, the input capacitors are sized based on
capacitance to achieve a given ripple at 120 Hz which results in them
operating significantly below their rated power so they last a long
long time. Linear regulator output capacitors are also significantly
derated in this respect however they are much smaller so do wear out
sooner.
Capacitors used in switching regulators are a different story.
Can these be tested 'in-circuit'? I'm guessing that excessive ripple
would point to needing to focus on these but saw on a rehab thread it
seemed someone had swapped a few and was curious why or if it's a
matter of course.
For linear regulator input capacitors, just measuring the ripple
voltage and minimum voltage while operating is sufficient to verify
their condition.
For an in-circuit measurement, if the regulator's output current can
be known by for instance measuring the value across the already
included current limit resistor, then the value of the input
capacitance can be determined from the ripple voltage. Some
capacitance meters can make a measurement of capacitance and
dissipation or ESR in-circuit.
David Hess