December 4, 2020

Drum Machine Part IV - PCB Assembly

After a long two weeks of waiting for the Drum Machine PCB to arrive in the mail, I immediately got soldering. It took about three hours to assemble the first PCB among its many transistors, resistors, capacitors and potentiometers. Unfortunately there was an error in the input stage of the device. It appeared as thought the transistor amplifiers were being fried for an unknown reason.

I spent an enormous amount of time probing the PCB, replacing components and performing various tests. No success. In a last-ditch effort to confirm that my nearly complete 4-year engineering degree was not wasted; I rebuilt the entire circuit on a breadboard for the second time. Sure enough that worked just fine, but that didn't solve any underlying problems. I began building the Drum Machine again, this time on a fresh PCB. I worked in stages, testing successively. This fresh PCB-Drum Machine worked exactly how it should. Other than reducing the decay caps from 100 microfarads to 3.3 microfarads, the overall design remained intact.

First fully assembled PCB of the Drum Machine
First fully assembled PCB (the one that doesn't work) of the Drum Machine.

Here’s a few notes I took along the way about PCB design.

  1. Include a ground solder pad or ground header pin for easier debugging.
  2. Show component values and names on both the schematic and physical PCB to make it easier to locate parts.
  3. Label potentiometers with their function.
  4. Do a Youtube search: “How to solder SMD components.”
  5. Make sure to leave room for wires when placing solder pads near mechanical components. The LED solder pads are a bit too close to the front panel of SDHH0001 V1.
  6. Use the PCB house’s SMT assembly service! It is absolutely worth the few bucks.
  7. Use panel mount components that can be soldered directly to the PCB without wires.