Tag Archives: FN376

HP 85685A: another mains filter failure – Schaffner FN 376

With the 85685A repair complete, the instrument was subject the an extensive test, to make sure all is in good order and working stable. Well, it did work well for a while, then – PUFF! The mains filter blew, one of the infamous Schaffner filters that is designed to blow up after about 20 years of service. Schaffner is one of the only companies I stay away from for any design – it is a Swiss enterprise, but too many of their devices failed in my hands – their filters are often the first parts to fail, in high grade test equipment.

The 85685A, like most other HP gear, has the mains filter combined with a voltage selector switch.

Cutting it open, you can see the Wima MP3 cap, 47 nF, 250 VAC. The MP3 are metallized paper capacitors, rated for X2 (mains) service. All embedded in some black resin.

Copper wires of the choke showing trough.

New filters that match the FN 376 are hard to find, and new-old-stock, well, these filters might fail again. So I decided to go for a new filter, a Schaffner-free solution.

This will be the new filter – a ID-10AC-S, available for little money, and seem to be pretty good for their current rating.

The internals… the filter elements are nicely encapsuled in a two-shell plastic case. No potting compound!

The X2 capacitor, and the choke…

Transplanted to the Schaffner filter body… and wrapped with Cu tape, soldered closed, for EMI shielding. All well grounded!

From the datasheets, you these filter should have 20-30 dB loss at 1 MHz. Let’s put it to a test!

For the new assembly, tested with a 3585A, about 12 dB loss at 100 kHz, 30 dB loss at 1 MHz, should be good, and no modification necessary to the 85685A.