HMV Model 328A

Computer Power Supply

Tapping into the Computer Power Supply

  • The +12V rail is used to power the LED lighting and audio muting relays.  The maximum current drawn from this rail is less than 1A.
  • The +5V rail is used to power the LED Controller Board and the System Control Board, both described later.  The total current draw is negligible.
  • The +5VSB rail is used by the Computer Power Sense Board to power the Standby LED.  The current draw is around 1.5mA.
  • The Power Good signal is generated by the Computer Power Supply and is used by the Computer motherboard reset circuitry.  Here, it is also used by the Computer Power Sense Board to detect mains failure.

The Power Good signal (grey wire) appears on pin 8 of the ATX power connector.  The +5VSB rail (purple wire) appears on pin 9 of the ATX power connector.  The only way of gaining access to the +5VSB rail and the Power Good signal is either on the back of the motherboard or by cutting into the Computer Power Supply wiring harness.

I decided to leave the motherboard untouched and instead modify the Computer Power Supply, which needed other modifications anyway.  The original unit had plenty of spare peripheral power looms.  I had to extend one of these slightly to reach the Blu-ray/DVD drive by cutting the required extra length off a spare loom.  I used the remains of the cut off loom as the basis for the Computer Power Connector loom which goes to the Main Chassis.  I wired this to a 5-pin Acme female line "microphone" connector (J1), which connects to P1 on the chassis.  The spare loom also had an orange wire for the 3.3V rail which this project does not use, so I cut this off completely.  (I hope this wasn't a mistake!)  I also added two more wires to tap into the Power Good signal (grey wire) and +5VSB rail (purple wire) of the main motherboard power loom.  I took no chances here, cutting the original wires completely to add heatshrink to cover and protect the soldered joints.




Other Computer Power Supply Modifications

Since this system is in a wooden cabinet, I added a green/yellow ground wire, attached to the grounding bolt in the Computer Power Supply.  This attaches to the Chassis Ground Terminal.

To mount the Computer Power Supply in the wooden cabinet, there was enough space to drill a hole in the bottom rear of the unit.  Access to this screw is through the fan.  A magnetised screwdriver is mandatory!  It was necessary to bolt a small bracket to the rear of the unit for the second screw.  Lack of space and access to this area, coupled with the delicate operation of screwing into the limited thickness wooden side of the cabinet made this job extra tricky.

The down side of these Computer Power Supply modifications is that when (not if) the unit finally fails, the new one must also be modified in the same way.

Modified Computer Power Supply harness
Tapping into the +5V Standy and Power Good wires
 
Modified Computer Power Supply harness
Modified Computer Power Supply harness with the additional Mains Ground wire and the Computer Power Connector (J1)
 
Front mounting screw
Computer Power Supply front mounting screw
 
Rear mounting bracket
Computer Power Supply rear mounting bracket and screw