In the late 1980s the television station I worked at was still using an early 1970s transmitter, an RCA TT-50FH (50 kW, Series F, High-band VHF).
The transmitter was made with three cabinets: two 25 kW amplifiers, A and B, on the left and right, and a control cabinet with aural and visual exciters and intermediate power amplifiers (IPAs) in the center. The amplifier outputs were combined externally to produce the full 50 kW (Figure 1).
Figure 1 The TV transmitter was made with three cabinets: two 25 kW amplifiers, A and B, on the left and right, and a control cabinet with aural and visual exciters and intermediate power amplifiers (IPAs) in the center.
Every four or five months we’d notice intermittent black lines running through the video. Apparently, this had been an ongoing problem for several years, with the problem originating in the A amplifier. The transmitter supervisor brought me out to the transmitter site, and we’d use his standard procedure, as follows:
- Split the transmitter, so amplifier B fed the antenna and amplifier A fed the dummy load.
- Slide the IPA chassis out from the center cabinet and remove its top.
- Turn all of the adjustments on the IPA to a minimum.
- Follow the IPA procedure in the maintenance manual to set up the IPA for proper operation.
- Close up the IPA, slide the chassis back in place, and recombine the transmitter amplifiers.
This worked every time, eliminating the black lines for another few months.
After I saw this happen two or three times, I got a little suspicious, especially since the IPA adjustments always ended up exactly where they had started. I asked the transmitter supervisor how he came up with the fix. He learned it from his predecessor, who probably learned it from his predecessor.
This fix didn’t seem right. It had more of a feel of a bad connection than an electronic component failure.
I took a look in the back of the transmitter, at the IPA’s connections. The IPA used a loop-through input, which allows a one signal to feed multiple devices. If that’s not necessary, the output is terminated with a 75-ohm resistor matching the characteristic impedance of the coax cable.
In more modern equipment, if you consider the 1980s modern, the loop-through is made with a pair of BNC connectors on a circuit board. In this transmitter, RCA built the device with N-connectors on a bracket. See Figure 2.
Figure 2 The IPA used a loop-through input, which allows a one signal to feed multiple devices. This was built with N-connectors on a bracket where the output is terminated with a 75-ohm resistor
When I checked the connections, I found the cables were tight on the chassis-mounted jacks, but the jacks themselves were not. The hex nuts on the rear of the bracket had loosened up over the years, so the ground connection, which depended on the metal bracket, was poor. We tightened the nuts, and the transmitter behaved itself for the rest of its life, well into the 1990s.
So why did the standard procedure fix the problem for a while each time? It didn’t, of course. It was the sliding back and forth of the chassis that was shaking up the cables and connectors and restoring a good ground connection, even if only slightly and only for a while.
Those were the good old days. With analog TV you could see the problem in the video or hear it in the audio. HDTV transmitters, on the other hand, just go dark and silent when there’s a problem. But those are stories for another day.
Robert Yankowitz retired as Chief Engineer at a television station in Boston, Massachusetts, where he had worked for 23 years. Prior to that, he worked for 15 years at a station in Providence, Rhode Island.
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