I turned on my FT90R the other day and the fuse blew next to the radio.
The FT90 has, like many Yaesu radios, a diode protecting against reverse polarisation at the power input. If the radio is reversed, the diode will short out. This is a catastrophic failure: the diode burns, but the failure mode is a permanent short.
What not to do:
- * Remove the diode
- * Remove the fuse, short it, etc
A reversed radio where the diode has failed can be fixed for less than a dollar, the damage done without diode would render the radio useless even for parts. Believe me, I've seen it multiple times.
I think (but cannot verify right now) that the diode also protects against overvoltages.
What to do? First, make damn sure polarity is OK, then test if the diode has failed (this is a lot easier if you temp remove the PCB - not difficult). If it's the diode, find out what happened.
If not, you'll have to find where the short is. But the protection diode is a good first guess with more than 75% chance.
Whatever you do, only tinker if you have a power supply with current limiter that can be set *very* low so that a short, etc doesn't cause burns.