Is the main rectifier diode a black ractangular box
with 4 legs, one
labled +, one labled -, and two labeled with s-shaped swigly lines?
Should I get continuity between the s-shaped swigglely lines?
That is actually a diode bridge, four diodes in a single package. Feed
AC in at the pins marked with a sine-wave (the wiggly lines) and get
full-wave rectified DC (modulo diode drops) out at the + and - pins.
"Continuity"? Maybe. If you apply a voltage there - of either
polarity - that's more than about two diode drops, you will see current
flow, through the rectifier and into the rest of the circuit (probably
charging the primary filter caps). Depending on the instrument you
use, this may look like continuity (because your measurement instrument
does not supply enough current to actually charge the caps
significantly), or it may look like a peculiar sort of resistance that
varies depending on which scale you set the meter to.
That's assuming the diode bridge is in full working order. If one of
the diodes has failed open, you'll see the above for applied voltage of
one polarity and no continuity for voltage of the other polarity. If
one of the diodes has failed shorted, you'll see the above description
for one polarity and something similar but with about half the voltage
developed across it for the other polarity. (The latter could well
explain blown fuses, as it amounts to an almost dead short across the
input for half of the AC cycle - it's just a question of whether the
fuse or the diode blows first.)
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