A few years ago In our kitchen we had this pretty good Sangean WR-5 clock radio with iPod dock. It broke but I recently fixed it and found an upgrade for it to make it actually usable again.
The little radio was great. It charged our phones, had good sound, and just did its job. Then it stopped charging our phones. And then the nail in the coffin for it…
With the upgrade to new iPhones with Lightning ports and the radio no longer charging the old devices, I put it away, to be gifted to Goodwill or just e-waste.
A few weeks ago, lost on Amazon, I discovered this great little Aurelteck 30-pin dock Bluetooth audio receiver. Just plug it into an iPod dock and turn it into a Bluetooth device. Great! Except my radio’s dock no longer charged, no longer supplying 5V to the dock connector.
Time to take the radio apart and probe around, I found there was essentially a 7805 +5V regulator feeding the dock connector and it was being fed 9V through two 1N4004 diodes. Sort of like:
The diodes were there to presumably soak up some of the voltage drop from +9V so the vreg didn’t have to handle it. The voltage regulator had kind of an oddball pinout. Thankfully the problem was that D2 (in the diagram above) was fried. So I just replaced it and the radio was powering the dock again, charging an old phone!
I soon discovered (after burning my finger) that the diode was getting very hot, like >160 degrees.
Clearly they underspec’d the diodes for the current that the phones can draw. So even though I didn’t need to fix this (the little Bluetooth adapter draws much less power), I decided to double up on both diodes to up the current carrying capacity. Tthis is normally bad practice because of potential diode thermal runaway, but I tried to make sure the parallel diodes were thermally in contact with each other and also I don’t care that much. The result was now the temp was down to a warm but not burny 99 degrees!
Good enough. Then it was just the tedious process of putting it all back together, plugging in the Bluetooth adapter, and listening to some Tycho. Now we’ve got a sweet little Bluetooth Boombox in the lab. Right next to the tiny grinder. :-)