01 September 2018

FM radio for the morning routine

The "old-style" replacement RX-16F
I was appointed an apparently simple task from a family member: have an FM radio in the bathroom to listen to the morning news. It is basically a single frequency FM receiver outputting audio through a loudspeaker. Possibly as small as possible and requiring as little maintenance as possible. Something: "flip a switch and listen to the news."

If we were in the 1980's I would buy an analog tuned pocket radio powered with two AA batteries. Unfortunately they don't seem to be produced anymore, replaced with energy-hungry digitally tuned radios (with a varying degree of user friendliness user interface). Used devices have all sort of problems.

4 years ago I bought a small cube powered with a rechargeable Lithium cell (Kevler KP-520PLL). It would receive FM just fine with a big problem: current consumption. Even when powered off it would drain the battery flat in a week (2mA when off, 48mA when on). What is worse is that it looses memory of the last tuned station when the battery dies and the single-button tuning system makes it really annoying to reset way too often.

Then in Marzaglia ham flea market in May 2018 I picked a 1970's analog tuned FM radio that sounded good in the field, marked RX-16F. This one, while lasting forever on a couple of modern AA batteries, suffers a long forgotten (and solved) problem: the hand proximity effect! If you hold the receiver in your hand it would behave perfectly, but looses the tuning and strength (no AGC!) when left on the table. It's fine for listening to the football match while walking downtown, but not practical for the bathroom morning routine.

I need a better solution.

Back of the FM radio.
RX-16F circuit.