Scott Christensen is one of many engineers who wrote about enjoying John Huntley’s pictorial on installing ground rods (Workbench, Aug. 11). However, Scott adds that driving ground rods using any kind of a hammer or post driver is way too much work.
Scott’s solution: Get yourself a big drill with a chuck large enough to grab on to the dull end of the ground rod. Your local rental place should have these; once you use this method, you might just want to buy one.
Scott sets up a stepladder, climbs it and just drills that baby in. He was able to drive three 10-foot rods for his ham tower through somewhat rocky soil in a matter of minutes. No sweat, no cussing.
The best part is that when the rod hits small rocks, the “wiggle” of the point tends to get past the rock rather than forcing you to stop, pull out the rod and try driving it a few inches away. And if you do need to back the rod out, it slides out easily while turning.
Scott used a regular drill; you could also use a hammer drill. He’s betting that would work even better.
Scott Christensen is a radio communications technician for Ramsey County Consolidated Dispatch in St. Paul, Minn.
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Ted Fuller is back with a solution for owners of the Harris SX-2.5 who encounter problems with those blue capacitors.
One of Ted’s clients runs a Harris SX-2.5 full-time. On three occasions the four blue capacitors that combine to make C4 have exploded.
The first two times, spare capacitors were on site. The next time, a startled station employee, on seeing sparks and smoke coming out of the top of the transmitter, called the fire department. The firemen opened both front and back doors to the transmitter, then told the employee that the whole top section of the transmitter had burned up.
Though this was something of an exaggeration, Ted knew he needed a permanent solution to the capacitor failure problem. Fortunately, the fire department did not spray anything into the transmitter. The station operated on a backup Gates BC-1T until repairs were completed.
Ted checked the Harris SX-5 manual and saw what looked like a Type 293 mica capacitor in the C4 position. That would probably work well in the SX-2.5, but over the years Ted has had quite a bit of trouble with the use of mica capacitors in high-stress circuits. In those cases, Ted usually doubled or tripled the capacitor current rating from the original value. To do so in this case would be expensive.
Considering the high voltages and currents involved and the lack of space in the top of the transmitter, Ted decided to try to find a good used vacuum capacitor to replace C4. Once he acquired the vacuum capacitor, it took some time to decide on the location and the best way to mount it.
Fig. 1 shows the PA section before the new capacitor installation. You can see the damaged blue capacitors to the left, and the empty space in the middle where Ted mounted the vacuum capacitor.
Fig. 2 shows the replacement capacitor mounted to the chassis and connected to the circuit. Note that coil L7 had to be relocated to make room for the C4 capacitor. The combination of C4 and L7 make up the center of the RF output tee section, but also functions as a third harmonic trap.
(Left) Fig. 1: Destroyed blue capacitors required a novel solution. (Right) Fig. 2: Relocating a vacuum capacitor offers additional safety margin to prevent future failures.
Prior to placing the PA section back into the transmitter, Ted retuned L7 with a HP 3325A signal generator, a small resistor and an oscilloscope. Ted checked the tuning again, once everything was back together.
The overall job turned out to be a big task, especially since the top assembly had to be removed from the transmitter for the modification, and then reinstalled. So far Ted’s efforts have worked; there’s been no further failure from this section of the transmitter.
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Ted’s company, Fuller Electronics, services not only transmitters but all kinds of broadcast equipment including one of the first digital STL systems, the Dolby DSTL.
I mentioned the Dolby system here in July and heard back from several engineers who are still using them including Dennis Snyder, director of engineering for Hall Communications stations in Burlington, Vt. His pair, a master/slave combination, serves one of his FM stations.
Another DSTL reader wrote in saying he’d discovered noise on his receiver output and was having trouble determining its source. Ted offers a tip that may help anyone in this situation. He suggests that if you have two systems or a master/slave on the same frequency, try paralleling the receivers, as fed by one transmitter, and see if the problem continues on the outputs of each receiver.
If the noise shows up on both receiver outputs, you can pretty much assume it is transmitter-related, possibly old power supply filtering capacitors.
If the noise only appears on one receiver and the second receiver is clean, the noisy receiver is the culprit.
Ted Fuller notes that this technique is a good first step to locating whether the problem is related to the receiver or transmitter. At the least, given that the internal electronics of the DSTL are complex, this can help to narrow the problem.
Many of the Dolby ICs are no longer available, though some parts are; in many cases, the units just need an alignment, which companies like Fuller Electronics can do.
John Bisset marked his 40th year in radio in broadcasting recently. He is a past recipient of the SBE’s Educator of the Year Award. Reach him at[email protected]. He can be reached at (603) 472-5282. Faxed submissions can be sent to (603) 472-4944.
Submissions for this column are encouraged and qualify for SBE recertification credit.