Monday, March 20, 2023

Lessons from the Electronics Bench: Trust Nothing.

I've finished building the first stage of my Elenco AM/FM radio kit. One of the key features of the Eleneco kit is that it comes with instructions for testing each module that you've built. This helps ensure you don't get too far along in the build process before discovering issues. More importantly, it helps ensure that you understand what you're actually building, which for me is what this exercise is all about.

One of the first tests I was supposed to run was the 'Output Bias Test.' Basically, you pop in the battery and turn the radio on. You then check the voltage coming out of the audio amplifier I just built.

Even as a electronics novice I could appreciate that this test was confirming that at least power was flowing through the radio.

I grabbed a random 9 volt battery from our battery pile. I plugged it in. I turned on the radio and used my volt meter to test the output. Nothing.

I didn't panic.

I looked over the circuit and realized I'd never installed the IC amplifier into the IC socket. I did this and repeated the test. Still nothing.

I looked over the circuit, all seemed fine. Then it hit me: I bet this random battery is dead. I poked around and found a sealed package of Duracell, 9 volt batteries. Now we were talking. I popped one of those into the radio.

Nothing. The volt meter still read 0.


If this were a software problem I'd debug it by picking something that I knew was true and confirming that it was so. I knew very little about the circuit I'd just soldered together, but I did know that a 9 volt battery should be spewing out 9 volts. So I tested the batteries.

The first, sketchy battery, was indeed dead. It read 1 volt when I measured it with my volt meter. To my surprise, the second battery wasn't much better: it was brand new, but read only 4 volts.

I grabbed another from the package and tested it: 8.8 volts. That seemed better. I put that battery in the radio and turned it on. A got a hum from the speaker! And check it out, here's my volt reading from the audio amplifier's output:

The manual says that my meter should read between 3 and 6 volts. My meter read 4.34. Success!

Lesson learned: Trust Nothing.

Before you boycott Duracell, take a closer look at the above pic. The expiration date of my new-in-box battery was 2020. Yikes, those are ancient. No wonder the first was a dud. Onward!

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