Failed H7864 PSU

Jarratt RMA robert.jarratt at
Wed Nov 5 00:48:30 CST 2014

A little while ago I posted about a H7864 PSU (from an rtVAX 1000)  that
had failed on me with a loud pop and no apparent physical damage. I
discovered which component has failed and it is a large transistor, Q301,
but I have yet to find out why it failed.

To help, I have been working out the schematic for the area of the PSU
that failed, which is labelled Primary Control Module, and situated after
the rectifier and before a large transformer that appears to isolate the
following stages of the PSU. The aim is of course to use the schematic to
attempt to work out the cause of the failure and repair that too before it
blows up the replacement transistor or damages some other part(s).

The schematic is here: (png) or here: (Eagle schematic). I may not have drawn it
logically, and may have some bits wrong, as it is quite difficult to
derive the schematic and it is easy to make mistakes. I don’t know how to
identify zener diodes, so all diodes are drawn as ordinary ones. I was also
unable to draw the transformers correctly as I don't know their spec and

I have found some of the capacitors in the circuit to have a high-ish ESR.
C407 in particular has a high ESR, C404 has a high-ish ESR, and C303 needs
re-forming but I don’t yet have equipment that can produce a high enough DC
voltage (should be able to do this in a few days time). My plan is to
replace all three of these. Whether these would cause Q301 to pop I don’t
know, what do people think?

There are also some heat marks around D408 and possibly R415 (both seem to
test OK).

Are there any other bits of the circuit that I should check that might
cause Q301 to pop?

Any advice really appreciated!



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