D246A - what kind of diode is this?
It is a normal diode, used for rectification of AC voltage to DC voltage. The D245A is listed here as 300 volt, 10A so the D246A is likely very similar.

To test, put meter on &quot;Diode Check.&quot; Place positive (red) on a wire, negative (black) on the metal stud.  Nothing but the meter lead must touch the metal stud (if anything else is connected to it, disconnect to test.)  It should read as a typical diode, about 0.7v.  It's okay if it reads 0.75, 0.65, even 0.6v.  It could possibly read 0.4v, but that would be exceedingly rare for the estimated age of the unit.
Now reverse the meter leads.  It should read nothing.  If it does, the diode is bad and needs to be replaced.
All of the diodes should measure the same, or very close to it. If any one measures differently than the others (say three are 0.7v but one is 0.4v) then replace them all.

It is a rather unusual part number, hard-to-find and very rare.  Something like a Vishay VS-16F40 or 1N3892 would probably be a good replacement, but it's impossible to say without examination by a professional repair tech.