[Collins] 312B-4 wattmeter coupler

[Dr. Gerald N. Johnson]

On Thu, 2007-08-16 at 11:54 -0700, Glen Zook wrote:
> 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
> 
> 
Lots of continued RF or how about lightning? It would tend to leave a
characteristic set of points as it melted the surface while not melting
the center and the intense magnetic fields pull up tits of the melted
metal.

An RF or AC would not make the tits because the magnetic fields would be
much smaller and the melted metal would go deeper or all the way
through. So the melted stubs would be rounded.

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer