Step 2: Reviving the +5V PSU

Parent Category: Stories Category: VAX 11/750: The Resurrection of a Comet Written by UlliH


How one can deal with old power supplies

There are two ways to deal with old power supplies in general:

  • Either do a go/no go smoke test (connect it to mains, switch it on and see what happens)
  • or do careful systematic tests to assure functionality of separate subassemblies one by one.

I did it the "quick and dirty" way with my VAX-11/725 (I was lucky - the only fault was a short in the line-in filter), but I didn't want to tempt fate again this time with the VAX-11/750.

So I studied the 7104's schematics and "Power System Technical Description" to learn about the way it works.

I used the following print sets and manuals:

  • EK-PS750-TD-002 VAX-11/750 H7104 Power System Technical Description
  • MP-01022 875 Controller Field Maintenance Print Set
  • MP-01021 H7104-D 5 V Power Supply Assembly Field Maintenance Print Set
  • MP-01020 H7104-C 2.5 V Power Supply Assembly Field Maintenance Print Set


Both PSUs (+2.5V left, +5V right) still in place and uncleaned.


This is the +5V PSU (DEC # H7104-D) opened. On the left two "hand grenades" (about the size and about the impact when exploding).


Both boards (5V Bias/Control and +/-15V Regulator) removed; left side: Output Assy - down right : Bottom Panel Assy

The H7104 PSUs are of a modular design. This is very advantageous because you can test different assemblies separately and one after another.