We have this guy that works for us who is atypical. We call him Pete because that is his name. The other day, he was slacking off on the job again, this time figuring out how to take a nap in a transmitter:
Pete working on a Harris FM25K
What are we going to do with him?
Actually, he is rebuilding the grid tuning section (AKA input tuning section), which is no small matter. Soon, we will have this 26 year old transmitter running good as new, or better than new. It already sounds much better on the air than it did before, the input tuning is broader and there is much less AM noise.
Currently, it is running about 70% power while we wait for a replacement amplifier from Silicon Valley Power Amps.
They say the first thirty years are the hardest, perhaps it is true. This Harris MW1A transmitter turns 31 this year:
Harris MW1A AM transmitter, WINE, Brookfield, CT
Truth be told, these are not bad units. They have some quirks, however, the overall circuitry is simple, the design is simple enough and parts for repair are readily available. They require regular infusions of RF transistors, but those are easy to change and are inexpensive to buy off the shelf from places like Mouser or Allied.
It is on the air as the main transmitter for WINE-AM in Brookfield, CT. This is Harris’s first solid-state AM transmitter design, based on the work of Himmler Swanson. This is not a PDM transmitter, rather, each module has RF transistors and audio transistors. The output of all twelve modules is combined for a carrier output of 1,000 watts with +125% modulation. Harris calls this PSM (Progressive Series Modulation), which is sort of high-level modulation.
This is also the only transmitter that I know of where blown fuses can cause damage to the RF devices.
The RF output transistors and audio transistors are still available from Harris. Non-OEM parts for this include the 2N5038G for the RF transistors and the MJ15011 for the audio transistor. Inside the front of the transmitter is a row of incandescent light bulbs that glow increasingly as the various transistors go bad. At 1,000 watts carrier power, the ratio of PA volts to PA amps is 52.5/22.5 respectively. If that ratio is off by any measure, there is a problem.
The other thing with this transmitter is it is very sensitive to any kind of VSWR. Any change in the output impedance will quickly make itself apparent. My Harrisburg MW1A had two ATU settings, one for winter and one for summer. It was a slightly tall tower on 1230 KHz, thus any change in the ground system (e.g. snow cover) would upset the tower base impedance.
The other thing that goes bad is the large Rotron fan at the bottom of the cabinet. They go bad about every 10-15 years or so.
The owner has spent some money on this particular unit, rebuilding and replacing several modules with new transistors, etc. Will it last another thirty years? Depends on if the RF and audio devices remain in production.
Radio World, bless them, has yet another article about the public’s lack of awareness regarding HD Radio®. Calling it a “lack of awareness,” is overly kind and I think they are missing the point. It would be better phrased as “apathy” or “indifference.”
There is a general misconception in the world that one either loves or hates something. That is not true, the opposite of love is indifference, not hate. The public has voted, with their wallets, for things like 3 and 4G wireless devices, satellite radio, iPods, and other entertainment venues. Why? Because HD Radio® is not an advance, it is a repackaging of old ideas with slick marketing. The general public has viewed the great digital radio conversion with a jaundiced eye, opting to sit on the fence and wait for something better. What has iBiquity given them?
The technology itself is a step backward with many band-aids needed to affect the same coverage area as analog FM. A technology that has poorer building penetration, less coverage area, and mobile reception issues with no appreciable difference in sound quality or program material offerings. A power increase from 1% to 10% analog carrier power (20dBc to -10dBc) hasn’t really made a difference. Now, studies are underway looking at asymmetrical sidebands and same-frequency repeater networks for FM IBOC. All of these things, are not to improve radio reception, but rather to achieve the same coverage as analog FM.
The AM HD Radio® has even greater issues.
There is nothing at all surprising about the public indifference toward HD Radio®.
Man, this is taking longer than I thought it would. We moved the Harris FM25K last week, all went well. The only hangup, as you can see, is the harmonic filter and the height of the racks next to the transmitter. The transmitter had to go on a 4×4 to get the filter up over the racks. The output from the transmitter to the harmonic filter cannot be changed in any way, shape, or form (e.g. adding a little bit of line section to the top of the transmitter), or else the transmitter will not run. So, up on 4×4’s it is.
WRKI WINE transmitter room
There we were, all ready to turn the transmitter on. Press the high voltage on button, lots of volts but no current and no power output. Seems something is wrong with the outboard IPA driver (over in the bottom of the rack, that thing pulled out with the manual on it).
The IPA is a Silicon Valley Power Amplifier 500-watt unit, which replaced the internal IPA driver about ten years ago. The tube in the Harris FM25K needs at least 390 watts to drive the transmitter to full power. Unfortunately, this particular amplifier was not in the best environment prior to the recent move. It was sitting in an unconditioned building on top of the backup transmitter in high heat and humidity. According to the manufacturer, such abuse is bound to take its toll sooner or later. The latter being, of course, the night we want to turn the thing back on and go home.
Time to drop back and punt. I found an old RVR 250-watt amp at a sister station nearby, which was also in pretty bad shape but repairable. That unit was pressed into service temporarily and with 200 watts drive, the old 25K put out about 11 KW. We need to affect permanent repairs to the RVR power amp before we place it into temporary service. I don’t want any 2 am phone calls. The Silicon Valley Power Amp needs to have the amplifier module sent back to the manufacturer and rebuilt. They will refurbish the entire thing for something like $900.00 plus shipping. Considering what it does, that is worth it.
This is a little short cellphone video of the turn-on at half power. This is a very loud transmitter, as such, I think the audio is a little distorted.
When this beast gets up to full power, I will update this, again.
