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Old 4th Jun 2019, 9:08 am   #15
jjl
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Join Date: Jan 2003
Location: Ware, Herts. UK.
Posts: 841
Default Re: Marconi 2024 not powering up

Well done on repairing your PSU Chris. This a very common fault with switch mode supplies of a certain age.

I think that it's most likely that these resistors fail because they are presented with more or less the full peak mains voltage until the small capacitor downstream of them charges up, their dissipation limit may not be exceeded but they experience unusual stress for a few tens of milliseconds. Better quality PSUs seem to use special high voltage rated resistors for this function, but they still seem to fail.

I replace failed resistors in this position with 2W rated metal film types.

John
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