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Design Fault in 6632B, 6633B, 6634B and 66332A leads to fatal error

Question asked by Dr_Frank on Oct 5, 2016

Hello, I have discovered a fatal design error in these PSUs.

After turning on, the PSU directly shows " protected mode", and errors 1, 2, 3, 4, 5, 80, 11. 

The internal analogue +/-15V supply fails, blowing F300 and F301,with shorted / burnt tantalum caps C496, C497.

Root cause are several basic design errors.

At first, tantalum capacitors should not be used at all in this place, i.e. directly at transformer power entry, due to safety aspects. There is no current limiting resistor in series with these caps.

The main error is a missing junction - dot in the schematic, so that on the PCB, both caps have NO connection to ground. (see appended picture from actual service manual.)

That may lead to a misbalance of the distribution of the overall 55V, across one of both capacitors, which are rated 35V only, and may ignite it, due to overvoltage.

This fatal error will happen sooner or later, maybe after years, or under humid conditions, both causing bigger difference in leakage currents.

 

A repair note should be published, at least to add the missing connection to GND, and better to additionally replace these capacitors by safer types, i.e. MLCCs.

As this design error strongly affects the reliability, and due to possible fire of the Tantalum caps also the safety, an application note should be published, despite the coming termination of these products.

Frank

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