It was an ambitious design. A one tube transmitter doing 50-70 kW TPO. Unfortunately, it seems the 817A was not destined to be a mass-production model.
Allen Sherrill, Chief Engineer at Curtis Media in Raleigh, NC, was the Chief Engineer of KQKQ, Omaha, when the unit arrived. His experience was "interesting."
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In May 1988, KQKQ-FM in Omaha installed a Continental 817A single-tube 60-kilowatt transmitter. This unit was intended for stations requiring a high transmitter power output without the disadvantages of combined transmitters. It replaced a combined McMartin transmitter which remained as an aux, at another location. According to Continental, there were 13 of these transmitters in use around the country in the late 80's-early 90's. All were recalled and dismantled.
|It was a big honkin' transmitter. If I
remember right, about the same size as a Harris DX50, perhaps longer.
Four "bays" as they say at Continental, as opposed to the
816-series which is three cabinets.
In case you're interested, the first cabinet from the left had the control circuits, "plasma display", exciter and driver sections. Second cab held the half-wave cavity, grid power supply, filament transformer. Third cab contained filter caps and inductors, resistor dividers for metering, and a "grid swamping" dummy load which always leaked oil. Fourth cab held the screen and plate supplies and associated rectifiers, and circuit breakers.
|The control circuits on the thing were a
piece of work. There were status tallys for EVERYTHING.
There were arc detector tallys in three places in the cavity and harmonic filter. The cavity, control cabinet and power supply cabinet had airflow tallys. Every shorting stick had a microswitch hooked to a tally. There were even tallys hooked to each circuit breaker, including the master CB (which I later realized was a waste of time, since if the master breaker pops, none of the tallys will work anyway!)
An awful lot of work went into the design of that rig, too bad it didn't work out as intended.
We installed it at KQKQ-FM in Omaha ("Sweet 98" on 98.5) in May of 1988. Removed in December of 1994, replaced by the dual 816R-6C (?) 30-kW transmitters which are surely still in use.
The transmitter room was tiny. Another FM had beaten us to the prime building space in the TV station's garage and had placed their transmitter right in the middle of the room. We had to remodel the space to remove the garage door and build a room around their equipment, which they were unwilling to relocate. So we wound up with an oddly shaped space with just enough room for the 817A, an audio rack, and a shelf for parts.
We ran into a number of problems during installation. The microprocessor control system burned up and had to be replaced. The driver section of the transmitter, which was rather complex, used solid-state amplifiers which would fail (to be fair, the modules used in these amps caused problems for other transmitter manufacturers at the time). Eventually a field service crew had to come up from Dallas and work on various problems with the transmitter, which was finally placed into fulltime service in August 1988.
We soon found that the control circuits in the transmitter were susceptible to RF and would cause unpredictable operation. Sometimes the plate voltage would shut off for no apparent reason. The manual power controller would activate itself and run the power-adjust pot to "all the way on" or "all the way off". Usually we used the automatic power control feature, but the transmitter would sometimes switch to manual on its own, with the result being either no RF output or somewhere around 120%.
One day I was working at the transmitter site and pressed the button to reset the fault tallys. This action turned off the filaments! The 817A had a two-and-a-half minute filament warmup, so we were off the air for a bit. Continental came out with an RFI filter retrofit for the controller that took care of most of the controller problems.
The filter capacitors in the power supply were a continual problem. The big plate filter cap failed frequently. There were smaller caps in the screen supply which would weep dielectric. One night, one or more of these caps apparently burst and spewed fluid all over the inside of the cabinet, causing arcs that damaged several other components. The "grid swamping" load, which was a 500-watt dummy load, continually leaked oil and burned up. We discovered that it was actually trying to absorb somewhere around 600 watts at full TPO.
The main problem with the 817A was poor tube life. The transmitter used a 4CX40,000G tetrode (about $10K new) which in our experience, just did not last very long. We probably went through about a dozen of these tubes in the six years we used the transmitter. Other users reported similar problems. The longest-lasting new tube we installed ran for about eighteen months before going soft. I did have some success having Econco rebuild a couple of these tubes, one of which ran for a couple of years or so. The tube itself weighed about 75 pounds, so changing it required some muscle.
I must commend the Continental field service and management folks who stood by this transmitter despite all its faults. As is the case with every other Continental product, they provided excellent customer service, even when it came time to put the 817A out of its misery. A complicated deal was arranged to replace the transmitter with brand-new combined 30-kilowatt 816-series transmitters (D816R-6C). We finally "retired" the 817A in December 1994, according to Continental the last of the users to do so.
When we removed the 817A, the room wasn't long enough to install the dual 30s and the control/audio rack end-to-end, as is customary. So we placed the transmitters facing each other across the room, and placed the reject load and metering panel off to the side, so it could be seen while tuning either transmitter. Worked out really well.
Again, I must give a salute to the folks at Continental who were of tremendous help during the struggles with the 817A. I got to know the field service folks pretty well during the frequent phone calls, and I learned a lot about transmitters.