In a discussion about testing Eurorack power cables, user mskala of North Coast Synthesis posted a link to his strip board power cable tester. I decided to build one like his. As luck would have it, I found a perf board in my parts bin the exact size needed! I located all the parts, which I had already on hand.

For a complete theory of operation, see North Coast Synthesis.

A few small modifications

I had a bunch of LM2940CT positive 9 volt regulators on hand. It’s pin-compatible with the LM7809, but has different filter capacitor requirements. I added a 10uf cap on the input and increased the cap on the output to 100uf. I added an orange LED as a power indicator.

I had a bag of red LEDs, so I used those instead of the green ones, with 2K current resistors instead of the 820 ohm. I did have on hand the Kingbright 57EGW red/green LEDs, so I was able to exactly replicate the circuits for testing the three power conductors. It just means that when all is good, they light green and the others are lighting red of course. Unfortunately, the red LEDs turned out to have a narrow viewing angle, so they’re brightest when looking straight on at them.

Construction

I was able to almost perfectly replicate the component layout of the North Coast board. The addition of the orange power LED threw it off a little.

I tested about two dozen power cables. All tested good.