[Hallicrafters] SX-42 transformer problem - help!

[Julian Bunn]

A strange problem has developed in my SX-42, which was working fine last 
night.

When I turn the set on, I can hear bad fizzing/gurgling noises from the 
mains transformer, so I turn it off
quickly! There is no smoke, just this fizzing noise.

I've removed the 5U4G tube and the octal shorting plug to narrow down 
what's going on, but I'm
not getting very far.

When the set is on, I measure the voltages to be 420 VAC between 
transformer pins 2 and 6,
6.2 VAC between pins 7 and 9, and 4.9V between pins 8 and 10. This is 
with no 5U4G present,
and no power to the other tube heaters and dial lamps.

I'd be most grateful for suggestions of what to look at next ... I 
desperately don't want to fry
the mains transformer! Why might it have developed a problem overnight?!

Thanks,
Julian