Within the late Eighties the tv station I labored at was nonetheless utilizing an early Seventies transmitter, an RCA TT-50FH (50 kW, Sequence F, Excessive-band VHF).
The transmitter was made with three cupboards: two 25 kW amplifiers, A and B, on the left and proper, and a management cupboard with aural and visible exciters and intermediate energy amplifiers (IPAs) within the heart. The amplifier outputs have been mixed externally to provide the complete 50 kW (Determine 1).
Determine 1 The TV transmitter was made with three cupboards: two 25 kW amplifiers, A and B, on the left and proper, and a management cupboard with aural and visible exciters and intermediate energy amplifiers (IPAs) within the heart.
Each 4 or 5 months we’d discover intermittent black strains operating via the video. Apparently, this had been an ongoing downside for a number of years, with the issue originating within the A amplifier. The transmitter supervisor introduced me out to the transmitter website, and we’d use his commonplace process, as follows:
- Break up the transmitter, so amplifier B fed the antenna and amplifier A fed the dummy load.
- Slide the IPA chassis out from the middle cupboard and take away its prime.
- Flip all the changes on the IPA to a minimal.
- Observe the IPA process within the upkeep guide to arrange the IPA for correct operation.
- Shut up the IPA, slide the chassis again in place, and recombine the transmitter amplifiers.
This labored each time, eliminating the black strains for one more few months.
After I noticed this occur two or 3 times, I acquired somewhat suspicious, particularly for the reason that IPA changes all the time ended up precisely the place that they had began. I requested the transmitter supervisor how he got here up with the repair. He discovered it from his predecessor, who most likely discovered it from his predecessor.
This repair didn’t appear proper. It had extra of a really feel of a nasty connection than an digital part failure.
I took a glance behind the transmitter, on the IPA’s connections. The IPA used a loop-through enter, which permits a one sign to feed a number of gadgets. If that’s not obligatory, the output is terminated with a 75-ohm resistor matching the attribute impedance of the coax cable.
In additional trendy tools, should you contemplate the Eighties trendy, the loop-through is made with a pair of BNC connectors on a circuit board. On this transmitter, RCA constructed the system with N-connectors on a bracket. See Determine 2.
Determine 2 The IPA used a loop-through enter, which permits a one sign to feed a number of gadgets. This was constructed with N-connectors on a bracket the place the output is terminated with a 75-ohm resistor
After I checked the connections, I discovered the cables have been tight on the chassis-mounted jacks, however the jacks themselves weren’t. The hex nuts on the rear of the bracket had loosened up through the years, so the bottom connection, which relied on the metallic bracket, was poor. We tightened the nuts, and the transmitter behaved itself for the remainder of its life, properly into the Nineteen Nineties.
So why did the usual process repair the issue for some time every time? It didn’t, after all. It was the sliding forwards and backwards of the chassis that was shaking up the cables and connectors and restoring a great floor connection, even when solely barely and just for some time.
These have been the great outdated days. With analog TV you possibly can see the issue within the video or hear it within the audio. HDTV transmitters, however, simply go darkish and silent when there’s an issue. However these are tales for one more day.
Robert Yankowitz retired as Chief Engineer at a tv station in Boston, Massachusetts, the place he had labored for 23 years. Previous to that, he labored for 15 years at a station in Windfall, Rhode Island.
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