Tuesday, July 27, 2010

Discombobulator Block 5: Tethys

Presented with appropriate mood lighting: the long-awaited Block 5 of the Discombobulator, Tethys!

The currently installed complement consists of:
Here's the usual sequence of build photos. I finally used up all the scrap 3/8" plywood (left over from a years-ago repair job at our previous house) that I've been using to build the bases of these things. I decided I wanted something a bit sturdier anyway, so I got some 5/8" plywood. Here, the rail pieces have been cut and are laying on the base, and the front rail has been glued in position and is clamped on while the glue sets.

All of the rail pieces are in place except for the top rail. The two rear posts (to hold up the top) were cut from a piece of dowel that I had laying around from some long-ago job.

After building the last block (Iapetus), I swore that I'd use its power supply configuration from now on. So of course I changed it again. However, the reason I did so was because I acquired a used Power One triple-voltage supply from a regular poster at Muff's for a very reasonable price. The Power One supplies are functionally and physically interchangeable with the Condor supplies that I usually use. Both are considered good brands; I usually use Condor because that's the brand that Mouser carries.

Here's the supply and the MOTM-990 power distribution board, in place but not yet mounted on the base. It was a little tight, and I wanted to get all the wiring connected before I screwed them down. The supply came with leads already soldered on the +/-15V side, and a power cord installed on the transformer. Unfortunately, someone had used a shielded cable for the power cord. Never ever use shielded cable for power! I had to remove that and make a new one out of zip cord.

Here is the completed power supply installation. The +/-15V comes from the part of the supply to the right of the transformer; the last owner had configured it for 12V, and I had to track down and remove two soldered jumpers to convert it to 15V. Fortunately, the instructions for how to do so are printed on the back of the chassis. The +5V comes from the smaller board to the left of the transformer. Everything is wired to the MOTM-995 distribution board, which has connectors for both MOTM 4-pin and 6-pin standards. The 6-pin connectors have the +5V and can be used to power a Dotcom or similar module, by constructing an adaptor cable. The line cord comes in at the bottom left of the picture and has an in-line AGC standard fuse holder on the hot side. I fuse these at 1A.

Some close-ups of the installed modules. First, the bank of MOTM-310 VCOs. I bought these as a package; I intend to use them for FM experimenting.

The Mankato Filter. This is an unusual filter, designed originally by Thomas Henry, that produces different response characteristics depending on which of the output jacks in the big circle you plug into. When the resonance is turned up to self-oscillation, it becomes an 8-phase sine wave VCO; each output jack is 45 degrees advanced from the previous one.

The Encore Frequency Shifter. Unlike a pitch shifter, a frequency shifter shifts each partial in the input signal by the same amount, in terms of Hz. This means that, unlike the pitch shifter, it does not maintain the harmonic relationships that are present in the input signal. Frequency shifters are complex circuits and are usually very expensive; Encore figured out how to build a less expensive one by using a microprocessor to generate internal control signals. It is capable of doing quite brutal things to a signal! The MOTM-formatted version was out of production for a long time, but when Encore announced they were doing another run of them early this year, I jumped on it.

I'll have reviews and sound samples of these coming up over the next two weeks.

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