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Ultra-LD Mk.3 135W/Channel Stereo Amplifier, Pt.2

Completing the wiring & making adjustments

By Greg Swain

WE LEFT OFF last month with all the modules mounted in position, except for the two Ultra-LD Mk.3 power amplifier modules. We also described the transformer mounting.

Once the transformer is in place, remove the adjacent side panel of the case by undoing the five self-tapping screws. Be careful during this procedure – the screws will initially be tight and one slip with the screwdriver is all it takes the leave an unsightly mark on the panel. Placing several layers of masking tape around the screw heads while you undo them is a worthwhile insurance policy here.

The next step is to run the 230VAC wiring. This wiring must be installed in a professional manner with all connections securely crimped and insulated.

Removing the side panel gives you good access to the chassis to install the mains wiring. This involves crimping a number of fully-insulated spade connectors onto various leads and it’s important to use a ratchet-driven crimping tool for this job (see panel last month). After crimping, you must also check that each connection is secure and that all wire strands are inside the connector – something that’s particularly important with mains wiring.

Begin by routing the blue and brown primary leads around the rear of the transformer and securing them with several cable ties (see photos & Fig.1 in Pt.1). The blue primary lead can then be trimmed to length (about 225mm), stripped of 5mm of insulation and terminated in a fully-insulated spade connector.

If you want to be doubly-fussy here, it’s a good idea to cut a 17mm length of 5mm-diameter heatshrink and slip it over the blue lead before crimping on the connector. The heatshrink can then be slid over the rear section of the connector and shrunk down using a hot-air gun (but take it easy with the heat; gently does it).

This connector can now by plugged into the Neutral terminal on the back of the IEC connector. That done, fit another cable tie to secure both primary leads close to the end of the heatshrink.

Next, cut a 450mm length of brown 32/0.20mm mains-rated cable and terminate one end in another fully-insulated spade connector (and again add the heatshrink). Plug this into the Active terminal on the IEC connector, then feed both this lead and the brown transformer primary lead down a 360mm-length of 5mm-diameter heatshrink.

That done, adjust the heatshrink so that it is about 20mm from the end of the fully-insulated spade connector and apply some gentle heat from a hot-air gun to shrink it down. Push the leads and the heatshrink down flat against the chassis as you do this, so that the leads sit side by side.

This not only keeps the leads together but also double-insulates them where they run around the mains transformer and under the righthand power amplifier module, to keep everything nice and safe.

The switch end of this cable should now be pushed through a rubber insulation boot (eg, Altronics H1472), after which the individual leads can be fitted with fully-insulated spade connectors. Once again, slip a 17mm length of 5mm heatshrink over each lead before crimping on the connector, then push the heatshrink into position over the end of the connector and shrink it down.

Click for larger image
Fig.6: the Earth lead from the IEC connector is secured to the case via an insulated crimp eyelet as shown here. The top nut serves as a lock-nut, so that the assembly cannot come loose. Make sure that the crimp eyelet makes good electrical contact with the case.

That done, secure the mains wiring to the floor of the chassis using four 5mm P-clamps but leave the P-clamp nearest to the power switch out for the time being. Orientate the P-clamps that are fitted at this stage as shown in Fig.1 (and the photo on page 63) and secure them using M4 x 10mm machine screws, flat washers and nuts.

Important: as with the audio input cable, you must also install two M4 flat washers under the head of the screw that secures the P-clamp under the power amplifier module. This avoids the possibility of shorting the end of this screw to the underside of the amplifier.

Finally, you can complete the mains wiring by installing the main green/yellow Earth lead prepared earlier. This plugs into the Earth terminal of the IEC connector, while the other (eyelet) end is bolted to the main chassis earth point using an M4 x 10mm machine screw, two star washers and two nuts – see Fig.6 above.

The second nut on top locks the assembly into position and ensures that the assembly cannot possibly come loose.

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