[BC] Te st your wits on this

[David Senft]

Saturday night, Feb. 18th, a BE FM 20T Tx burned up the inner filament
contact ring. When the ring was replaced, the transmitter ran perfectly into
the dummy load but would not fire into a Dielectric star-point combiner. The
station combines with one other. Analysis indicated that a significant spur
would occur when operating into the combiner input. Reducing Tx power to the
lowest stable point doesn't work. All lines, elbows, patches, etc. were
inspected and swept. The combiner module was dismantled, inspected,
reassembled and re-tuned. Even with the output of the module patched into
the dummy load, the Tx will not work. A test with a 300 Watt Bext provided
an almost perfect match. As our experience with some BE Tx's with similar
combiners has shown similar symptoms, an adjustment of line-length between
the Tx and combiner will cure it. As this installation has operated twelve
years without incident, we don't suspect this cause but we've tried such
adjustments to no avail. We've inspected the site for missing ground straps,
etc. We've tried two other spare tubes with the same results. We have nine
transmitters of this model in our company. There are four such units on this
particular site, two of ours and two owned by another broadcaster. Shutting
down the other stations doesn't affect the problem. The sum total of years
of technical experience, both in-house and from one of the largest
contractors in Canada is more than 100 years. BE tech support hasn't offered
any suggestions that we haven't already tried.  
D.