Debugging gets quite hectic without a correctly pitched probe header, rendering two hands and ten fingers inadequate, but the new probes I ordered several days ago has arrived this morning — what better day to make the final push?

Three more assemblies. This has become my first PCB prototype batch that all got assembled.

Instead of the original intended speakers, I went for ones of a larger type, also rated 8Ω 1W, but free from distortions at the desired amplitude. The sounds came out quite clear.

Spread some cleaning agent on the boards, and wash it away with water. Everything was good, except that the speakers all got stuck onto the board. It would not move a bit, failing all attempts with tweezers, jumper wire headers, and safety pins. I don’t know the exact reason, but I suppose it might be the organic compounds in the solution melting the plastic…? Maybe I can try re-applying the cleaning agent to see whether it can be removed this way. There is a Chinese idiom that goes, “to remove the bell from a tiger’s neck, it calls for the very person who tied it there” (解铃还须系铃人). Well, it may as well apply here.

That aside, now that everything seems ready…

One last remaining issue (for this revision) is that the device makes a continuous buzz if plugged into certain types of power sources (e.g., USB receptacles on power strips). A laptop’s USB outlet presented no problem. I can’t find my portable power bank, so that remains unknown, but I speculate that it should work fine and the noise comes from mains hum. But that is left to be checked against later!