Sunday, July 5, 2009

Continuing on Discombobulator Block 4

The adhesive is dry and the bottom rail is secure on the base. Time to add the side rails and the rear posts. I drive in flat-head screws through the bottom of the base (after drilling pilot holes first).

A couple of brads from the nail gun keep the side rails and rear posts from wanting to rotate.

At this point, all the woodwork is complete, except that I haven't put the top rail on yet:

Now it's time to add the power components. The three items you see here are: at left rear, the tall item is a Condor dual-voltage linear power supply that will provide the ±15V. The small object in front of it is the +5V switching supply. This is a return to the power supply configuration that I used for block 1. For blocks 2 and 3; I used a Condor triple-voltage linear to supply all three voltages, but the tri-voltage supply is large, heavy, and expensive, and I haven't noticed any difference in audio quality using a linear for the +5V vs. using a switcher. (Most modules, if they use +5 at all, will use it for noisy things like logic circuits or LEDs anyway.) At right is the MOTM-990 triple-voltage distribution board:

The Condor has posts for soldering on the wires to connect it to the distribution board. Below, the soldering is done: the red wire is the +15V, the green is the -15V, and the black is the return. Note in the photo above that the distribution board has terminals at both ends. You want to make sure you put the power in at the end of the board where the filter caps are; the terminals at the other end are for expansion. The photo below shows this. The MOTM-990 board is new to me; all of the previous Discombobulator blocks used a MOTM-960 for the ±15V, and a separate terminal strip for the +5V. That's kind of a pain in the rear to connect if you need the +5. One of the modules intended to go into this block is an MOTM-650 MIDI interface, which is one of the new series of MOTM modules that needs +5, and it will come with the six-pin power cable that mates with the 990.

A closeup of the +5V switcher. I didn't know until I unwrapped it that it needs a three-position MTA-156 connector for the AC input, and a two-position MTA-156 to connect the output. I considered just soldering directly to the header pins, but thought better of that. Unfortunately, the only MTA-156 connectors I have on hand at the moment are four-position. Yes, I know I can just let them hang off the ends, but I want this to be a clean installation. So off to Mouser to order some. And until they come in next week, I'm at a stopping point on the electrical work.

Now to put on the top rail. The distance between the bottom and top rails is a critical measurement. It needs to be just barely tall enough to allow the stiffening tabs on a Dotcom-format module to slip in between. If it's more than about 1/4-inch taller than that, the mounting holes on MOTM modules won't line up with the rails. And if it's short, of course, Dotcom modules won't fit. So I test-fit before I put the top rail on:

It turns out my side rails are very slightly short, about 1/16 inch. So for spacing, I'll insert these little split lock washers in between the side rail and the top rail; the mounting screw will hold it in place.

Here's the result; a pilot hole is drilled through the top rail and then into the end of the side rail, and a flat-head screw driven. Later, a piece of Velcro will go over the screw to hold the top on, which is why we use a flat-head screw that can be turned down flush.

And the Dotcom module just fits:

Meanwhile, my supervisor DJ keeps a close eye on things. Or not.

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