Re: Off Topic request for help with VCR supply

Denton, Adam (Exchange)

--- In, "David Wise" <david_wise@p...>
If you want to learn the theory, pick up almost any general
EE textbook at your local thrift store.
Oh it's easy. Derive the LaPlace transform of the transfer
function. Make some Bode plots and then compare all gains
with those derived from root-locus plots. Should take
an hour or two ;-)

Seriously, troubleshooting switchers is an alternative to
lifetime imprisonment for repeat offenders. It is basically
impossible unless you have a complete schematic with accurate
voltages. Since the feedback loops try to "compensate" for
anything awry (assuming the defect is not in the loop itself)
the entire circuit appears hopelessly wrong.

Generally, if you choose to bite the bullet, the approach is
to use "bias supplies" to fixate various points within the
internal loops at nominal voltages. Then hunt around for why
the loop is being "told" to deviate from it. This is NOT easy.
If you fix the loop at the wrong voltage you will fry the
VCR and/or the supply, so you need to isolate the supply and
use dummy loads on all supplies. This alone is tedious.
The currents in each load have to be correct!

The question is always "would it be faster to actually attempt
to diagnose the bad component, or to simply replace EVERY component
(other than the transformer, on the chance it's ok) 1 by 1
until it works?" Believe me, in hindsight, the latter is
often the approach to use, depending on your time and
component budgets.


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