12 April, 2018

Power regulator works as polarity protection

Step-down converter based on LM2596. Note the damaged chip
Ok, now I've done the test. My QRPLabs U3S runs off a 12 Volt power supply. There are two step-down converters, one for 5 Volts for the processor and another adjustable one for the power amplifier, if one can call 0.2-0.5 Watts a power amplifier. See picture of these voltage converters in this post.

I happened to make a new cable for 12 Volts which had the polarities inverted - and puff - there was a noise and absolutely no response from the U3S. I feared that I had blown the entire circuit. As my power amplifier was turned off, only the 5 Volts supply was affected and upon inspection I found that the voltage converter had a destroyed chip.

Since since these step-down converter modules are so cheap, I had a spare. Luckily for me, the U3S worked as it should after the replacement. So the LM2596 can take a reversed polarity and sacrifices itself in order to protect the rest of the electronics. Nice!

This post first appeared on the LA3ZA Radio & Electronics blog.

04 March, 2018

Deteriorating ceramic filters due to DC

Tandberg Huldra 10
Tasos, SV8YM, has written about "The Mysterious Case of the Withering Filters". This seems to affect not only ham radio transceivers, but FM stereo receivers as well.

Tandberg from the 70's are collectors items and since I actually worked one summer at Tandberg in the early 70's they bring back good memories for me. The latest generation of receivers (2nd version of Huldra 10, Huldra 11, and Huldra 12) had ceramic filters for the 10.7 MHz intermediate frequency for FM. It is also known that these filters deteriorate leading to reduced sensitivity over time.

SV8YM has pointed out that ceramic filters deteriorate due to DC on the terminals, especially the output terminal and that this leads to electromigration. In the Huldra 10, both filters have 7.1 V DC on the input. Filter F1 has 0 V DC on the output, while F2 has 2.1 V on the output.