[Collins] 312B-4 wattmeter coupler

[Glen Zook]

I thought that I had seen "everything"!  A 312B-4 came
in for service a while back with the wattmeter being
inoperative.  Of course I "thought" the 1N82A diodes
are blown.  Well, they were.  Also, the straps that go
from the pickup link to the glass piston capacitors
was VAPORIZED as was the portion of the variable
capacitors that is on the glass.  In fact the glass
was also melted (both capacitors) and the threaded rod
that works the piston was welded in place.  Both 10
ohm 1% resistors that go from the 1N82A diodes to
ground were badly burned.

Fortunately, the other resistors and feed-through
capacitors in the circuit appear to be OK as is the
meter movement and switch.

The inside of the assembly was completely saturated
with white colored powder (what was left when the
straps and capacitor parts were vaporized).

The only way that this much damage could have been
done is to either put well over 10 KW (probably a lot
more) into a load or, more likely, running "key down"
with at least full legal power on CW (again probably
more) with no load at all on the coupler (either a bad
antenna switch or no antenna at all) for a loooooong
period of time.

Anyway, no way to treat the wattmeter coupler in a 312B-4.

Glen, K9STH

Website:  http://k9sth.com


       
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