[clubjoker]

- The RX+'s 3.3V linear regulator (IC3) and its metal pad (heatsink I assume?) on the PCB get very hot to the touch, too hot for me to touch for long - is that normal?

This is a bad sign. You should try to fix it as soon as possible, don't keep the unit running until this issue is fixed. Under normal conditions the 3.3V regulator runs  barely warm.
The first thing to check it power supply voltage, make sure it is no more than 9V.
Also, make sure that you always have connected the 9V external power supply before connecting the USB cable to the Meduino board. Failing to it that, will destroy your board. Please carefully read this info:

1 - http://www.meteocercal.info/forum/Thread...een-warned

2 - WeatherDuino Pro2 PLUS Receiver - Notes about Assembling and Power On

Werk, thanks very much for the advice. It's good to know that the 3.3V shouldn't be hot, even though not happy news!

Supply voltage to the RX+ is measuring 8.6V on my multimeter, and jumps around a a bit - 8.5V, 8.7V, ... I wonder if the 7809 I rigged up to provide it is oscillating and not regulating properly. Hmm.

I was careful to remove the relevant diode on the Meduino before I soldered it onto the RX+ PCB and powered it up.

I'll investigate more on the power supply side (maybe try a bigger capacitor on the output of the 7809 regulator for the RX+) and report back.