Wednesday, November 26, 2008

MOTM-820 Assembly

I'm resuming a project that I started last spring, and then put in the shelf due to house-related projects.  This is a circuit board for a Synthesis Technology MOTM-820 voltage controlled lag processor:

At this stage, I have just installed the white power connector at the lower right, and the ferrite beads immediately above it.  All resistors and caps are installed, but none of the semiconductor devices are yet.

In this photo, the ICs are installed and I'm installing transistors.  There are two stuffed here, one in the center of the board, and one at right center; they are not soldered yet.  When I solder discrete devices, I generally like to do at least two at a time, and preferably 3-5 at a time, so that I can alternate between devices, doing a pin on the first one, a pin on the second one, etc.  That reduces the heat buildup in the devices.  

Here is a closeup of the solder side of the board.  The bending of the leads is called "cinching" the leads; it keeps the device in place while it is being soldered.  I bend them in different directions to help prevent shorts.  After it is soldered, the excess lead lengths are cut off.

With all of the transistors in, this phase of assembly is complete and the board is ready to wash.  In its kits, Synth Tech divides board stuffing and assembly into two phases: all of the parts that can tolerate water immersion are installed first, using an organic acid-based solder.  This solder does a good job of removing oxidation from pins and board traces, but it must be washed after soldering to remove the acid flux.  After the final wash, the remaining components are installed using a "no-clean" solder that doesn't have to be washed, but requires more care in soldering.  The board, ready for wash:

This weekend, I'll do more work on it and post more photos.  Once the potentiometers are installed on the board (they go along the top edge in the photo above), it'll start to look more like the final product.  

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