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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

Scoping a SMPS primary side...

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tekman

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Scoping a SMPS primary side...

wrote:
Since the equipment under test is being powered via an isolation
transformer you should ground the scope probe ground to the negative
side in the hot chassis in order to obtain proper waveform
measurements.... no problem, no sparks, not an issue.
Obviously voltage and current are still present and standard personal
safety proceedures are still required, but because the scope is safeyy
grounded there are not hot chassis issues since they are floating
thanks to the isolation transformer.
electricitym

All correct. An isolation transformer is always a good part of your
personal safety.

The UC384x type supplies I hadto repair have had these usual sympthoms:

1. Dead UC 3842. Change.
2. Freid resistor at chip output (in the connection to the gate of the
FET). Replace.
3. Dead FET. Replace.
4. Capacitor about Ucc of UC384x: Leaky or lost capacity. This is often
the "twit..twit...twit" sound of the startup.

google for the article of Keith Lofstrom (or similar , may be my memory
is not correctly remebering): "fixin scitching poer supplies".

hth,
Andreas

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