HMV Model 328A

Getting down to business

I must confess I felt momentary pangs of guilt as I ruthlessly stripped the guts out of the thing!  I kept all mechanical parts like screws, brackets, plates and so on.  Whilst disassembling the unit, I was impressed with how well it was made.

The cabinet was made of veneered ply and there were many intricate cutouts to accommodate the main chassis.  Most of the panels were held together by small wooden blocks glued to them.  Amazingly, most of these were still solid, despite the age.  There's no way our modern PVA glue would last that long!

Looking at the inside of the cabinet, I began to realise that there was surprisingly little space in there.  The plan was to build most of the electronics on the original chassis, but there was barely enough room to fit a computer motherboard on top of the chassis, leaving only underneath the chassis for the remainder of the electronics.  Some of that space would be taken up by the optical drive, which could only fit in the chassis area.  The power supplies would have to be mounted on the cabinet in one of the spaces to the side of the chassis.  That left the other side for the 300W amplifier, which would have to be custom made to fit.  It became obvious that things were going to be very tight! The Chassis The metalwork was passivated steel, mostly in good condition and only parts of the top of the chassis rusted, probably due to capacitor leakage.  Fortunately, most of the ugly areas would be hidden by the motherboard. I wanted to keep the chassis as original looking as possible, with all the labeling preserved.  I cleaned the metalwork with Nifti to get rid of any dirt.  I then drenched all the metal parts in WD40 and rubbed them down to remove any grease contamination and to lightly preserve the metal.  It ended up looking OK. Mounting the Optical Drive I cut a hole in the front of the cabinet for the optical drive.  The drive was to go in place of two of the original controls, leaving me with room for two knobs.  There is almost no difference between the size of the body of the drive and the front bezel.  I had to cut the hole by hand with high accuracy.  After marking out the hole, I cut through the veneer with a sharp knife to prevent any chipping of the surrounding veneer.  Fortunately, the result was excellent, the drive fitting into the hole like a glove with no ugly exposed cut marks. I cut a bracket from an old PC chassis to secure the drive.  The elongated mounting holes easily accommodated the thickness of the wood.

Cabinet mounting system to allow chassis removal   Cabinet with chassis in place   Optical drive mounting bracket

Mounting the Hard Drive

The vibrator unit was built on a sub-chassis which was mounted with neoprene bushes to reduce vibration to the main chassis.  Amazingly, these bushes still seem to be in relatively good condition.  This assembly seemed to be the perfect for a hard drive to reduce vibration from the drive to the chassis.  Furthermore, vibrations from the subwoofer would be reduced to the drive.  And to clinch the deal, a 3.5" drive fitted perfectly on top!  The only catch was that the holes were very close to the edge - I had to carve slots to make room for the screw heads.

I am pleased to report that this system works well - you can feel vibration in the sub-chassis when the drive is running, but none in the main chassis.

I am much more concerned about the subwoofer interfering with the optical drive.  I know from working on such drives myself that they are inherently susceptible to vibration.  You can easily excite the laser assembly (which is mounted on a suspension system) to vibrate at some resonant low frequency.  I am resigned to not being able to use the optical drive when listening at high volume levels.  We'll see what happens...

Power Switch There was the remains of an old switch on the side of the cabinet which was originally used to switch on the dial lamps.  Since this was a battery operated radio, this conserved power.  (I briefly thought about adding another on the other side to act as flipper buttons for a pinball game!) The guts of the switch were long gone, but the button and bezel were still intact.  I made up a bracket to install a modern momentary action switch behind the button. Reset Switch and PC Speaker I mounted both these items in the rear of the chassis, with cable retaining clips to support the wiring down the side of the chassis.

Standby, Power, Message and HDD LEDs

Mounting brackets for
Power, Message and HDD LEDs

 

Power, Message and HDD LEDs wired

Standby LED

 

Four high efficiency 5mm LEDs will be used to illuminate engraved symbols in the cabinet speaker opening.  These are spaced 50mm apart.  The first LED is mounted on the cabinet, whilst the other three are mounted on brackets attached to the chassis.  These LEDs put out much more light than typical standard computer box LEDs.

The wiring for the Message LED and HDD LED is secured to the chassis with cable retaining clips, before going to the motherboard via standard connectors.

The Power and Message LED's outputs are the same on the GA-EP31-DS3L.  Therefore, the Power LED is controlled by the Computer Power Sense Board, described later.  The Power LED behaves like a standard power indicator - simply on when the computer is running.  The Message LED flashes slowly when the system is in the S1 sleep state and when the Dynamic Energy Saver is enabled.  The Standby LED is also controlled by the Computer Power Sense Board.  The Standby LED comes on when the system is shut down.

Chassis Power connectors

Power Connectors

I cut two brackets from aluminium angle to mount two Acme "microphone" connectors.  These two connectors will provide power for the chassis electronics.

The 5-pin connector is for the Computer Power Supply, whilst the 4-pin connector is for the ±15V power supply.

The computer motherboard and other computer peripherals will connect directly to the Computer Power Supply, using the standard connectors.

End view of chassis showing the DB25 connector and Chassis Ground Terminal.  The brackets below are for mounting the Interface Electronics, described later.

Cabinet Connectors

The DB25 connector will be used to connect to the Cabinet Wiring Harness, which includes things like the Power Switch and the cabinet LED lighting.

The Chassis Ground Terminal will be used to connect to things like the Computer Power Supply case and optical drive case.

Work light LED

Work Lights

Three 30mA 20,000mCd White LEDS are mounted on brackets on the rear of the chassis.  They are wired in series and fed from Ch1 of the 10 Channel LED Driver Board, described later.

These brackets, in conjunction with the retaining bar for the computer expansion boards, provide a stable base to place the entire chassis on its back to work on, whilst protecting the rear connectors and switches.

 

Chassis Illumination LEDs

Chassis Illumination

Three coloured LEDS are mounted on the chassis bars, wired in series and fed from Ch2 of the 10 Channel LED Driver Board.  These run at 30mA.  The aqua LED is relatively inefficient.  The yellow and orange LEDs are 70mA 35,000mCd types, running well below their optimum level.  There is a slight colour shift due to this, making them a more golden shade.

 

Dial Lamp Switch

Dial Lamp Switch

Just below the Reset Switch is an interesting hole in the chassis marked "Dial Lamp Switch", originally a connector to the cabinet-mounted switch.  Its labeling proved irresistible to me - I made up a bracket, mounted on spacers, to mount a switch which overrides the LED Controller Board and turns on both the work lights and chassis illumination.  It could come in handy one day when groping around the back of this thing, trying to plug something in.

Mounting the Motherboard

 

 

The original chassis had two bars which were used to support the dial assembly.  I wanted to retain these bars to support the display and assist in chassis removal.

The motherboard was to mount on top of the chassis, adjacent to the vibrator unit.  There was just enough room to fit a standard ATX 305mm (12") width motherboard.  The first problem was one of the chassis bars was in the way.  I moved the bar closer to the edge of the chassis.  This also meant enlarging a cutout in the cabinet to allow clearance for the bar.

The next problem was the motherboard depth.  Standard ATX form factor motherboards were too big.  I needed to find something with only 210mm depth.  Even then, it would still overlap the chassis at the front, but at least there was room inside the cabinet.

I decided to make a clear acrylic panel for the rear of the chassis, with all the necessary cutouts for the cards and connectors.  I created a CAD drawing to be used by a laser cutter to make this panel.

The exact placement of the motherboard was critical.  The rear of the motherboard needed to be 5mm from the rear of the chassis, to allow for the thickness of the acrylic panel (2mm), the thickness of the internal re-enforcing bracket for the chassis bars (1mm) and clearance for the bottom of the expansion card brackets (2mm).  I worked out that the bottom of the motherboard needed to clear the top of the chassis by 12mm, to allow room for the bottom of the expansion card brackets.  This avoided having to cut rectangular slots into the chassis.  I used threaded metal M3 x 10mm metal spacers to mount the motherboard, with additional washers to get the 12mm clearance.

Securing the Expansion Cards

To secure the bottom of the expansion card brackets, a 5mm thick aluminium bar, cut to shape, was mounted on top of the chassis, under the motherboard.

To secure the top of the expansion card brackets, I made a bar from aluminium angle, bolted to the chassis bars.  This is different to a standard setup, the screws bolting from underneath and using the outer cutout of the expansion card brackets.

The disadvantage of this setup is that you have to loosen all 7 bracket screws and remove this bar in order to remove a card.  To make things easier, the top bar is attached to the chassis bars using M3 screws into tapped holes.

Finally, a 20mm wide trim bar was added to cover the bottom anchor bar and the join between the rear acrylic panel and the top of the chassis.

The result was that the expansion cards were secured more accurately and tightly than in a standard computer box.

Laser-cut clear acrylic rear panel

Rear view with the motherboard and clear acrylic panel installed