Hunting season is upon us. It’s a good time to monitor your line pressure frequently. The dry air or nitrogen that pressurizes most FM and some high-power AM lines keeps moisture from forming and causing a breakdown or flashover of the inner conductor to the grounded outer conductor.
(click thumbnail)Fig. 1: Burned and clean lines.
But a single bullet from a gun, aimed from outside your transmitter site fence, can puncture the line, or worse. Fig. 1 shows a cross section of line that was hit, then burned. Contrast the discolored spacer and dark interior on the left with the bright copper on the right.
If your dehydrator runs continuously, or you suddenly lose a tank of compressed nitrogen, suspect that a line has been hit. Sometimes, if there is slack in the line as it comes off the tower, you can hear the slug rattle around as you gently jiggle the line. This is a repair that can’t wait until spring. If the line hasn’t flashed over already, it’s an accident waiting to happen. And you know it will happen on Christmas Eve, or during morning drive the first day of the next book!
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Jack Quinn, an independent engineer out of Santa Rosa, Calif., offers historical perspective and opinions on why all new or rebuilt vacuum tubes do not always work in some FM transmitters, as first discussed in our Oct. 6 Workbench.
There is a natural tendency on the part of engineers constantly to improve their product. Couple that with MBAs constantly looking for cost savings in manufacturing, and you have a formula for problems.
All of the 4CX5000A and the 3CX5000A7 families originally were designed by EIMAC. The internal elements were supported on metal cones, and the grids and filaments were spot-welded to them. The anodes used two matching metal flanges, one brazed to the copper anode, the other brazed to the metalized ceramic envelope end. The two were then heli-arced together to form the output vacuum seal.
Over time, EIMAC, its imitators and, to some extent, rebuilders have sought ways to reduce costs. In some cases, engineers made changes to the internal structure. Small changes can affect the input or output circuit of the original transmitter design. The original design engineer built his transmitter around the parameters of the tube he had. The tube became, therefore, an integral part of the RF circuit design. This included the internal cone’s support inductance and capacitance, same for the filament support, and the final anode seal configuration. Messing with any of these parameters at FM or TV frequencies is a guarantee for trouble.
Because tube engineers were not transmitter engineers, and visa versa, EIMAC had a separate highly expert Marketing Application Engineering Group, which would go out and help the transmitter manufacturer in marrying the two. In addition they kept their own tube production engineers from making any changes that were not comparable with the original design. This group also included a Specifications Manager who had the authority to stop production or shipping, if any of the tube parameters got out of limits. Those tubes were destroyed and production started only when QC got the variance under control. All of this and much more, thanks to the founders Bill Eitel and Jack McCullough (hence the name EIMAC).
Those well-intended changes do not generally effect AM or HF (shortwave) transmitters. But if you get a structure that is not the same as that for which the FM transmitter originally was designed, in many cases your only out is to select tubes, or even sometimes to modify the input/output circuit.
Markets change, companies change, time marches on. Fortunately, as George Badger has stated, it does work if you specify the transmitter make and model. Keep that in mind the next time you specify a replacement tube.
Reach Jack Quinn at firstname.lastname@example.org.
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I worked with Jim and Larry Schropp on several transmitter projects. The father-son team comprising Schropp Electronics Services is a full-service broadcast engineering firm serving Charlotte, N.C., and the surrounding areas since 1982. Jim is following in his father’s footsteps in providing broadcast engineering service to stations in this region.
(click thumbnail)Fig. 2: The Insulation Displacement XLR doesn’t need a soldering iron.
Jim writes that he enjoyed the article about XLR connectors, and ways to make life easier while attempting to solder them. He also offers an alternative view: Why solder them at all?
Jim has been using a new type of XLR by Neutrik. It is an IDC, or insulation displacement connector, and Jim has found it to be of excellent quality. The connector is available in male and female varieties.
Schropp Electronics Services built a multi-million dollar studio complex using only these XLR connectors and has yet to experience a failure. Fig. 3 shows the red wire laying in the cradle of the insulation displacement jaws. When the connector is screwed together, the wire is forced between the jaws for a firm connection.
In addition to a reliable connection, Jim writes that once you have assembled a few of these connectors, you generally can complete an XLR in 35 to 45 seconds. Conversion to these connectors has saved him an incredible amount of time that would have been spent soldering.
He recommends that you read Neutrik’s instructions for the XL’s, as they contain important tips for assembly. See www.neutrik.com/images/ock/downloads/Media_996806503.pdf
(click thumbnail)Fig. 3: Wires are ready for final assembly.
There are two part numbers for these connectors:
Neutrik IDC Male XLR NC3MEZY-NI
Neutrik IDC Female XLR NC3FEZY-NI
If you’re looking for a good source, Jim suggests Mike Phelps at SCMS; visit www.scmsinc.com
Jim Schropp can be reached at email@example.com.
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