(click thumbnail)Fig. 1: The defective cap in this IF can is seen in the foreground.
(click thumbnail)Fig. 2: After desoldering, there wasn't much left except the capacitor leads.
(click thumbnail)Fig. 3: A little car wax applied to satellite dishes will make even wet snow easy to remove.
Laverne Siemens is the director of engineering for Golden West Media in Canada. A client station of his in Saskatoon, Saskatchewan reported that one of his Marti STL-15C receivers at a repeater site would squelch off in windy conditions. The other receiver at that site — which gets its feed from the same antenna through a splitter — was rock-solid regardless of the weather conditions.
An initial test of the receiver on the bench proved nothing. It squelched off only when the signal got down to under 4 uV. It was returned to the field under the assumption that the problem must be with the splitter system. The receiver failed again after just a few days.
At this point the customer tested the splitter with a spectrum analyzer and confirmed that it was working fine. There was plenty of received signal, so the receiver got shipped back to Laverne's shop again.
This time Laverne promised the client that the receiver would not be returned until they found something definitive. The first test was what Laverne called the “air force” treatment: plenty of chassis flexing and bumping followed by cold spray and the hot air of a hair dryer.
This test got the signal to drop, and the problem was finally narrowed down to the LC filter can on the IF filter. If enough cold spray was applied right on the can, the signal would drop to the point of squelching off. This was a drop of about 30 dB.
Laverne and his staff confirmed it was the IF filter by monitoring both the input and output of the filter with a spectrum analyzer tuned to 10.7 MHz. After it was in the failed condition it took only a slight tap on the side of the can — or a few seconds of heat from the hair dryer — to get it working again.
In 27 years of servicing Marti gear, Laverne never had a reason to open one of these LC filters. However, by this point they had spent enough time on the unit that Laverne just wanted to get it fixed and back in service.
They replaced the entire filter card. Now there was nothing to lose by further damaging the old card, so he cut it open. Inside, and seen in the foreground of Fig. 1, is a cracked 2.7 pfd capacitor. It fell apart as soon as it was desoldered, shown in Fig. 2. Based on the appearance of the capacitor it had likely been that way since day one, or somehow cracked after it was soldered in place.
Laverne's hunch is that under windy conditions there were minor vibrations of just the right type that got transferred down the coax to shake the capacitor and cause it to go intermittent, which in turn caused the drop in signal level. The next step is to replace the defective cap and tackle soldering the can back together so he has a spare on the shelf.
Thanks, Laverne, for an interesting repair lesson.
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Roberta Barmore is a senior RF tech and chief operator for WTHR(TV/DT) and WALV(LP) in Indianapolis. Referring to an archived Workbench column at radioworld.com, Roberta raises a red flag about where the garden-variety RTV sealant (common at hardware stores) is used around copper wire and steel chassis.
She writes that most common “hardware store” RTV outgases acetic acid as it cures, which can cause corrosion and eventually lead to big problems. Some versions of RTV don't do this, for instance automotive types labeled “oxygen sensor safe” and Dow Corning's 748 non-corrosive sealant. At WTHR, the staff has standardized on the latter with good results.
Roberta says she was lured to TV 19 years ago, having started her broadcasting career in radio. Most days she still misses radio; and although the TV budgets are (usually) larger, the headaches are larger still.
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R.C. Woolfenden is operations director at WFAX(AM), one of the first Christian/religious block-formatted stations in the country, located in suburban Washington, D.C.
R.C. just read the Workbench submission on APC UPS sensitivity with respect to generators. The station has two APC units, purchased this year, that have this problem. R.C. looked for a sensitivity switch, or adjustment, on the two offending units but could find none. He e-mailed APC and found that not all uninterruptible power supplies have this feature; and given the widespread use of generators at both studio and transmitter sites, it's a feature you may want to consider including in your list of specifications when purchasing a UPS.
Without the ability to adjust the UPS, adding the isolator-power conditioner is the next step to keep the UPS happy with generator power.
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A reminder that a coating of car wax will make removal of even wet snow a snap.
If non-engineers are tasked with satellite dish snow removal, take a few minutes to explain that heavy-handedness can ruin the dish, costing the station hundreds if not thousands of dollars.
Make sure the snow removal staff is armed only with a broom and they understand that banging on the dish with the broomstick will affect dish performance by knocking the parabola out-of-round. The few minutes of training will pay back tenfold in keeping satellite formats on the air, and you at home during snowstorms.