Sometimes there is just no way around it, especially with some modern equipment:
Hard restart, Nautel VS2500 transmitter
This Nautel VS2500 transmitter got all cranky after lightning struck the tower (or nearby) on Friday night. Thunderstorms in February are not unheard of, but they are unusual, at least in the Northeastern United States.
Nautel VS2500 FM transmitter, WBEC-FM, Pittsfield, MA
Anyway, the transmitter would not reset or restart via remote control, therefore, we had to ride the chair lift to the top of the hill and pull the plug to reset its logic and start over again.
Bousquet Ski Area Chair lift
At least the trip up to the transmitter site was scenic. We had to wait a day for the winds to calm down, but all in all, not a terrible day. Did I mention the scenery?
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.
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.
Every good transmitter, tube transmitters, in particular, require harmonic filtering. The last thing any good engineer or broadcaster wants is to cause interference, especially out-of-band interference to public safety or aviation frequencies. All modern transmitters are required to have spurious emissions attenuated by 80 dB or greater >75 kHz from the carrier frequency. In reality, 80 dB is still quite high these days, especially in the VHF/UHF band, where receivers are much more sensitive than they used to be. A good receiver noise floor can be -110 dB depending on local conditions.
The principle behind a low pass filter is pretty easy to understand. The desired frequency is passed to the antenna, while anything above the cut-off frequency is restricted and shunted to ground via a capacitor.
Low pass RC filter
In this case, the resistor is actually an inductor with high reactance above the cut-off frequency. Often, these filters are lumped together to give better performance. This is a picture of an RVR three-stage low pass filter:
RVR three-stage low pass filter
RVR is an Italian transmitter maker that sells many transmitters and exciters in this country under names like Bext, Armstrong, etc. The inductors are obvious, the capacitors consist of a copper strip sandwiched between teflon insulators held down by the dividers in between the inductors.
Schematically, it looks like this:
Low pass filter schematic diagram
For the FM broadcast band, a good design cutoff frequency would be about 160 MHz. This will give the filter a steep skirt at the first possible harmonic frequency of 176 MHz (88.1 x 2 = 176.2).
Values for components:
Capacitors
Value
Inductors
Value
C1
20 pf
L1
74.7 nf
C2
54 pf
L2
75.1 nf
C3
54 pf
L3
73.9 nf
C4
20 pf
The inductors are wire, or in this case copper strap, with an air core. It is important to keep the transmitter power output in mind when designing and building these things. Higher carrier powers require greater spacing between coil windings and larger coil diameters. This particular filter is rated for 1 KW at 100 MHz.
