.. Is there a soild state replacement for the rectifier tube 5U4 in a
HQ-145xc?

Seems I remember such a tube being available for around $20 from an
internet source.

73

John

The tube replacement always seemed like a good idea to me, so
back in the good old days, I did just exactly what many responders
here talk about.
I replaced the 5Y3 (I think) in my RME 4350 with a resistor and
a couple of nondescript silicon diodes (before the advent of the
extremely 1N4007), and after fidgeting with the voltage output
by adjusting the series resistor, found a new hash on the received
signal.
It's been a while, now, more than 40 years, but if I remember
correctly, I tried RF chokes, additional filter capacity, smaller
ceramic disk capacitors (condenser, actually, but what the heck)
and I finally went back to the original 5Y3 tube.
As I remember (and probably have the scars to prove it) the
voltage dropping resistor got pretty hot.... had to use a
wirewound resistor to handle the heat.

Old Chief Lynn, W7LTQ