The H7874 is the power supply  used in the BA4xx and R400x enclosures, such as several machines in the VAX 4000 series. I have a VAX 4000-500 in a BA440. I had some problems with this power supply that seemed to get fixed after replacing some leaked electrolytic capacitors. I still have intermittent problems with the power supply shutting down occasionally, particularly if the machine has not been powered on for a while.

It is an astonishingly complex power supply and very hard to disassemble too. A post on the classiccmp mailing list requested information about this power supply. I have partially reverse engineered the schematic of the 12V output board so I am posting it here. It is unlikely to be correct either.

H7874 Power Supply – 12V Output Board