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This is the front panel (upper) of a Harris SX-1 AM radio transmitter. The SX-1 (aka "Sucks 1") was the first generation hybrid analog-digital AM transmitter which Harris produced. Meaning that the control and drive section was all digital, but the final amplifier stage was still brute-force analog.
What you see in the photo is a "lights-out" shot of me looking through the window of the front panel with a 2001-era digital camera on long-exposure. So, yeah, potato.
What's really meaningful here is that this is a photo of 12 low-wattage incandescent light bulbs. Each one is driven by the reflected RF energy from the combiner which couples the twelve solid-state amplifier modules into the main output.
Yup. This photo captures the essence of tube-heads and DSP engineers collaborating on one of the weirdest radio transmitters in history, right around the cusp of Y2K. The analog folks decided that running the reflected power through an incandescent light bulb filament on its way to ground was a good way of measuring cabinet balance, thus totally screwing the digital guys out of any chance of capturing that signal in a meaningful way.
Reading what these filaments said, and adjusting the PA balance, was therefore the last, and without question most bizzare, analog control on any Harris transmitter ever made.
But after a while, you got really good at it. And, in retrospect, despite the fact that it was designed out of pure malice, this might have been the best PA balance control indicator I have ever used. You really developed an indescribable feel for when the modules were in perfect balance (which, in this shot, they are not.)
What I left unsaid is that those light bulbs were the only path to ground for the combiner's reject ports.
What was their impedance at 1Mhz? Who knows.
If you know what that means, then the laughter and ***-puckering will mean something to you.
If you do not, then to put it simply: a bulb being burned out or unscrewed (which, to be fair, would never happen under ordinary circumstances) would be really bad.
^ I think I see the problem. There is supposed to be more than 20psi of oil pressure when the engine is running at high RPM under load on a racecourse.
Oh, WGN... Can't even spell William Shartner's name correctly.
10-13-2021, 12:58 AM
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