R136 is "drive level". Over-driving can produce distortion at the driver stage (cause of "barely intelligible audio", likely mistaken as FMing?), and if it continues can burn out the driver's emitter resistor. For the BITX40 this is R141, 10 ohms. It seems the original surface-mount resistor's power rating is too close to the maximum normal drive level, so some users are experiencing this burn-out. There's not that much actual material to it, so it won't give off a significant smoke or smell as it fails, therefore there's not likely to be much warning ... The BITX's Final is Class A, hence the hefty quiescent current. The Hendrix may be Class B or an AB, so likely to be more efficient that way. Summary: don't push it - these wee devices ain't 4CX250Bs! ? ;)
