PS

Mounting the 317 regulator and TIP32 transistor to the heatsink.

Construction

For most electronic enthusiasts the mechanical work involved in a project is usually the tedious bit. We would expect most constructors to purchase a pre-punched and drilled chassis, but if you want to do it yourself or plan to use a difterent chassis, then start by carefully laying out and marking up the metal-work. Component placement is not critical, but we would suggest you keep a strict division between the mains com-ponents and wiring and the rest of the circuitry and components. If you’re using the same chassis, or something similar, then our Scotchcal front panel artwork can be used as a template for the front panel. General placement of components can be determined from the photographs. The 184 x 70 x 160 mm chassis we used could have done with some bracing of the front and rear panels. Some smali brackets could be madę up from scrap aluminium pieces to do this job, if you wish. Ifyou do this, tackle it first, but make surę the brackets won’t foul any components attached to the panels.

If you’re using the same chassis we used and all holes are prepared, First thing to do is apply the Scotchcal front panel. We madę up a metal Scotchcal, rather than plastic, as it’s morę durable. Carefully lift the backing from one edge and align the edge on the chassis panel. Peel off the backing and carefully smooth the Scotchcal into place across the panel making surę it’s correctly aligned as you go. When it’s in place, smooth out any bubbles by carefully rubbing them toward an edge. You can cut out the holes with a sharp penknife or modelling scalpel.

We also madę up a metal Scotchcal label for the meter scalę. Disassemble the meter and carefully apply the Scotchcal to the original scalę, trim the edges if necessary and then re-assemble the meter.

Mount the meter to the front panel first, otherwise you will have great difficulty reaching the nuts that secure it as they will be obscured by other components. Then mount the LEDs, switches, pots (and their knobs) and out-put terminals. On the rear panel, mount the mains fuse and install the mains cable with its clamp grommet, leaving enough of the cable protruding inside the case so that it reaches the terminal błock. Mount the terminal błock on the chassis bottom next, then terminate the mains cable. Make surę the green and yellow earth lead is the longest so that it’s the last to break in the event of a catastrophic accident.

Insert the four pc board mounting bolts next. We used 13 mm long 6 BA bolts. Put two nuts on them to space the pc board up from the chassis. Cut a shield’ for the mains wiring from a 70 x 70 mm piece of heavy cardboard, bend up one edge about 8 mm in and secure it

under the rear mounting nut nearest the mains terminal błock by punching a hole in the appropriate place in the bent-up piece — see the internal photograph.

Assemble the 317 regulator and TIP32 transistor to the heatsink next. Insulate each with a mica or plastic thermal washer and bolt insulators. Using a multimeter, check that the metal tag of each device is not shorted to the heatsink. Bend the leads of each device up from the heatsink.

TOP VIEW    WASHERS

Internal view of the power supply showing mounting and terminating details for the regulator and transistor. These are assembled to the heatsink as shown above and the heatsink mounted to the rear panel using bolts above and below the cutout visible here.

Now you can attach flying leads to all the components mounted on the front panel and between the mains terminal błock and fuse to the mains switch. Put heatshrink tubing over the mains fuse and mains switch, ensuring they are well covered. Make surę all leads are long enough to reach their destination. Solder C7 to the terminals of RV1 and D9 to the output terminals, as shown in the overlay and wiring diagram.

Mount the power transformer next, oriented such that its 240 Vac input terminals are away from the pc board. Terminate the leads from the mains switch to the transformer primary con-nections, using heatshrink tubing again to shroud the terminals.    ►

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