HMV Model 328A

The Dog Box - Mains Wiring and ±15V Power Supply

Beware of the 240V!

Space inside this cabinet is a real problem and my choice of box sizes to house the Power Relay, mains distribution and the ±15V Audio Power Supply was severely limited.  Ideally, you would have the power supply in a separate enclosure.  I had to go to rediculous lengths to make things fit into one box - right down to cutting a hunk out of a fin on the heatsink to make room for the head of the transformer bolt!  With exposed mains wiring inside and things crammed around the torroidal transformer, I figure it deserves the name "The Dog Box".


Mains Distribution

The incoming mains connects to the system via an IEC connector with a built in mains filter.  A metal oxide varistor (MOV) provides additional protection against mains-born spikes.

From here, mains power is applied directly to the Computer Power Supply.  The Computer Power Sense Board, described later, detects when the computer is switched on and, after a short delay, operates a relay which applies power to the remainder of the system.

In this way, the computer controls power to the entire system.  If the power switch on the Computer Power Supply is turned off, this effectively isolates power from the entire system.

A second MOV protects the switched outputs, particularly against the spike generated by the power transformers when the relay opens.

The Power Distribution Board allows peripherals such as the external monitor to be connected to and controlled by the system.

±15 Volt Audio Power Supply

This provides power for the audio electronics.  It is a conventional analogue design with standard IC regulators.

The TO-220 7815 and 7915 IC regulators have current limiting, as well as thermal overload protection.

Note the protection diodes across the regulators, which prevent them from being reverse biased.  This is often overlooked by designers and is particularly important on supplies with connectors on their outputs.

The ±15 Volt Audio Power output connects to the Main Chassis via a 4-pin Acme "microphone" connector (J2).

The Dog Box Rear Panel


The Dog Box Rear Panel

The two green LEDs monitor the ±15 Volt outputs.  The Test Switch bypasses the power relay and allows the ±15 Volt Audio Power Supply to be tested without the computer running.  The fuse is for the ±15 Volt Power Supply only.

Pre-assembled box, before wiring
Pre-assembled box, before wiring

Regulator assembly
Regulator assembly

Box after wiring
Box after wiring

Completed box
Completed box installed in cabinet below the Computer Power Supply


The Dog Box and its associated wiring harness can be removed from the cabinet as a single entity.

The ±15V Audio Power Supply can be worked on with The Dog Box removed by applying mains power and turning the Test Switch ON.