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By Paul Thurst, on February 1st, 2012
This transmitter is in service at WSBS, Great Barrington, MA as a standby. It was new in January of 1975.
 Gates BC250GY transmitter, WSBS Great Barrington, MA This was running into the dummy load for testing, which we try to do periodically.
 Gates BC250GY AM transmitter audio section The audio section is a pair of 8008′s 810′s running in parallel. This goes through a modulation transformer to the RF section.
 Gates BC250GY RF section The RF section consists of another pair of 8008′s 810′s running a parallel. The plate voltage for these tubes is 1,250 VDC which is fairly tame, all things considered. The transmitter is dirt simple 250 watt carrier power, 125% positive peak capable. It is not the most efficient unit under the sun but it can still be repaired with off the self parts.
 Gates BC250GY Schematic This is a somewhat faded schematic. The schematic shows a single 833A as the final, however, this particular transmitter has a pair of 810′s for the final, as shown in the above picture. Ham radio operators love these things as they are easy to convert to 160 or 80 meters for AM phone use. The bigger brother to this unit is the Gates BC1G, which is also pretty simple unit using 833A tubes in parallel with 3,500 VDC plate voltage.
By Paul Thurst, on January 25th, 2012
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 It is on the air as the main transmitter for WINE-AM in Brookfield, CT. These are not necessarily bad transmitters, although they do seem to require regular infusions of MOSFETS to stay at full power. 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 MOSFETS and audio transistors. The output of all twelve modules are combined for a carrier output of 1,ooo watts with +125% modulation. Thus, I would call it low level AM 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 2N5038 for the RF MOSFETS and the MJ15011 for the audio transistor. Inside the front of the transmitter is a row if incandescent light bulbs that glow increasingly as the various MOSFETS 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 in 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 MOSFETS etc. Will it last another thirty years? Depends on if the RF and audio devices remain in production.
By Paul Thurst, on January 6th, 2012
I have been spending my days in Brookfield, Connecticut, dragging transmitters around and reconnecting them in various ways. The WRKI-FM WINE-AM transmitter site is finally moving into the “new” transmitter building at the base of the tower. Today, we moved WINE.
WINE was first signed on in 1963 on 940 KHz from a 170 degree non-directional tower on top of a pretty high hill. That same tower serves as the antenna support for WRKI, which signed on in 1957. The station runs 680 watts daytime, however since it is non-directional, it has some pretty serious power reductions at night. The post sun set power drops in two steps, 450 watts for the first hour, then 189 watts for the second hour, followed by 4 watts night time.
The 4 watt night time signal goes about 2-4 miles before it becomes unlistenable. The Post Sun Set Authority (PSSA) allows the station to stay on the air with at least some coverage up to about 6:46 pm in the winter time and 10 pm in the summer, which is better than nothing.
The problem is, the Harris MW-1A transmitter goes down to 250 watts and no lower. In order to make the night time power, the station switches to a dissipation network to burn off 246 watts of RF, at 50% percent AC-RF efficiency, which just ends up being a waste of power. Further, the station engineers have been ignoring the PSSA because there are too many steps and it was easier to just switch to night power at sunset.
What we decided to do instead, was install a small low power night time transmitter, a Radio Systems TR-6000. The MW1A can then be set to use the low power level for the first step of the PSSA, then switch the dissipation network in for the second step of the PSSA, finally switching in the night transmitter at the proper time.
 Harris MW1A AM transmitter, WINE 940 KHz, Brookfield, Ct This is the Harris transmitter, new Circa 1981, was cleaned up and moved into the new transmitter building.
 WINE Parallel dissipation network and dummy load The dissipation network. This will have to be reconfigured for the proper power levels, once the night transmitter is installed. The dissipation network is on the right, a dummy load is on the left. The two large RF contactors switch the dissipation network in and out, or select which transmitter is feeding the antenna/dummy load. This is the really, really old school way of doing it. Most transmitters manufactured after 1990 or so can run at any power level, making a dissipation network unnecessary.
Before re-installing the dissipation network/dummy load, we lined the enclosure with copper mesh. I don’t want that thing interfering with any of the other equipment nearby, which would be the STL receivers, satellite receivers or Town of Brookfield police dispatch radios.
Schematically, it looks like this:
 WINE 940 KHz Brookfield, CT night time dissipation network This is the picture behind the transmitters, shows the coaxial cable feed through ports and the dissipation network on the wall.
 WINE WRKI transmitter room, behind the transmitters It is a work in progress, so forgive the mess.
By Paul Thurst, on November 15th, 2011
Still in use as the main transmitter after 42 years at WCKL 560 KHz, Catskill, NY.
 CCA AM1000D transmitter, WCKL Catskill, NY The last seven years or so, it has not had much use, the station being caught in some strange LMA with Clear Channel, then sold to the Black United Fund of NY something or another. They basically had it dark, turning it on for a few days each year to as not to loose their license. Finally, they LMA’d it to Family Broadcasting (not to be confused with Family Radio). There are rumors of a sale, but it remains to be seen.
They have been broadcasting an eclectic, free form programming style which appears to be the work of mostly volunteers.
The station was first licensed in 1970, thus this is the original transmitter:
 CCA AM 1000D name plate, WCKL Catskill, NY Towers are 446 feet tall, which works out to 90 degrees at 560 KHz.
 WCKL 560 KHz antenna array The station is licensed to Catskill, but the transmitter site is located in Hudson, across the river. With the current ownership situation in flux, I would characterize the operation as “tenuous.”
The transmitter itself is a pretty simple high level modulation tube type transmitter. It uses 4-400 tubes, like the RCA-BT1AR transmitters and is build around a similar design, which makes sense as they were designed and built by former RCA engineers. One of the CCA principles, Bernie Wise, still makes Energy Onix transmitters about 10 miles away in Valatie, NY.
Parts are fairly generic and still available. Things like the modulation transformer may be harder to come by, however, Goodrich Electronics, Harbach Electronics, Energy Onix and others will be able to steer one in the right direction. I’d put up a schematic if I could find one.
I find these older tube type transmitters often sing with modulation, especially the higher frequencies. That sound and the soft sound of the blower moving air is the sound of radio, at least to me.
By Paul Thurst, on October 19th, 2011
Since the FCC waved some of its rules regarding carrier power and carrier shift on the AM broadcast band, AM stations are now able to implement MCDL or DCC (Dynamic Carrier Control) technology to save money on their electric bills. This technology has the potential to save tens of thousands of dollars for higher powered AM stations (high power=greater than 10 KW carrier level).
On a standard AM broadcasting station, the carrier represents two thirds of the energy being transmitted, with the modulation index containing the other one third. The carrier contains no information; it is simply there on the center frequency at the power level authorized by the station’s license. Thus, if the carrier can be reduced without effecting the quality of the broadcast reception, it will reduce to overall power consumption of the transmitter. In areas where electric costs are high, the savings can be substantial.
There are various ways to accomplish this. The first is called Dynamic Carrier Control (DCC), where the carrier voltage is reduced during moderate modulation levels (between 20-50%) and restored during peaks. This reduces the output power during average modulation, restoring most of it during quite periods and peaks. The next is Dynamic Amplitude Modulation (DAM), which is similar to DCC. The most savings will noted with less heavily processed programming such as talk radio because the higher the average modulation density is, the less the MDCL circuit reduces the carrier voltage level. The little graph in the diagram shows the reduction in the carrier voltage vs. modulation levels.
 Nautel DAM block diagram, courtesy of Nautel, Ltd. Finally, Amplitude Modulation Companding (AMC) reduces the voltage in both the carrier and modulation product during peaks. This results in better savings for higher density modulation indexes. It is also the most transparent of the three schemes, as the carrier is restored to full power during periods of low or no modulation levels. During peak modulation, the reduction does not drop the power level below the un-modulated carrier level. The little graph in the diagram shows the reduction in the carrier voltage vs. modulation levels.
 Nautel AMC block diagram, courtesy of Nautel, Ltd Nautel has done extensive work on MDCL and includes several algorithms in their NX series transmitters. For older Nautel transmitter models such as ND, XL, XR and the J-1000, there is an outboard exciter, which is in a one rack unit chassis. Older transmitters may need a simple field modification to create a DC coupled audio input. The cost for the upgrade is approximately $5,000 USD, however check with the regional Nautel sales rep.
Once the system has been installed, there are several things to be aware of:
- Modulation monitors may not work properly, especially older units, which will show significant carrier shift and have carrier alarms. Belar AMMA-2 modulation monitor is specifically built to work with MDCL transmitters.
- When making field strength readings, the MDCL circuitry must be turned off to get accurate readings.
- For stations running IBOC, the amount of carrier power reduction may need to be experimented with, as the effect of the carrier reduction may cause the transmitter to exceed the NRSC mask.
Currently, only Nautel and Harris are selling MDCL transmitters. I spent several minutes poking around the Harris website and looking through their product brochures for the DX series transmitters and no mention of DCC o MDCL was found. I’d be happy to include any information from Harris if it were made available.
By Paul Thurst, on September 29th, 2011
The Nautel AMPFET series transmitters date from the early 80′s through early 90′s. They were Nautel’s first attempt at MF Broadcast transmitters and were quite successful. This particular transmitter was installed in early 1990 at WBEC in Pittsfield, MA:
 Nautel AMPFET 1 AM broadcast transmitter I believe Nautel got started making MW transmitters for Marine Radio stations, Aeronautical and Marine radio beacons, and similar equipment. Their early equipment is very rugged and designed for rough/continuous service. The early solid state broadcast transmitters like the AMPFET were not hot plugable but who cares, they almost never break. The design is simple, efficient and it sounds good on the air.
Early transmitters were housed in racks that were much shorter. In later versions, the racks became larger to standardize the transmitter size with comparable units of the day. Inside this cabinet, there is a lot of empty space.
The design is modular, RF modules and power supplies can be removed from the transmitter for repair, unlike the Harris AM transmitter products of the same or later periods.
There later AM transmitter versions built on the AMPFET experience.
By Paul Thurst, on September 13th, 2011
Coming to an AM transmitter near you. The FCC announced that starting immediately, stations can employ Modulation Dependent Carrier Level or MDCL technology on AM transmitters. According to Public Notice DA 11-1535 (.pdf):
Use of MDCL technologies requires a waiver of Section 73.1560(a) of the Commission’s Rules, which sets upper and lower limits for an AM station’s operating power. We hereby establish procedures for AM broadcasters to seek a rule waiver in order to use energy-saving MDCL technologies.
Several transmitter manufactures offer some some version of MDCL in their newer models with the ability to update some older models. Harris Corporation offers something called “Amplitude Modulation Companding” (AMC) and “Adaptive Carrier Control” (ACC). While Nautel includes an option called “Dynamic Carrier Control” (DCC) on all NX series transmitters with the ability to upgrade some older transmitters. Continental offers Controlled Carrier level Modulation (CCM) on later model shortwave transmitters by installing SSM modulator, which can be retrofitted.
Nautel’s Dynamic Carrier Control (.ppt) (.pdf available here) reduces the carrier level during moderate modulation periods. The effect of this is to increase the perceived loudness at the receiver. During higher modulation periods, the carrier is increased to prevent distortion. The net effect is between 3 – 6 dB carrier reduction. During quite periods, the carrier is returned to full power to reduce noise.
 Nautel Presentation on Dynamic AM carrier control The potential savings is from 20-40%, which for a 10 or 50 KW station, would represent a significant reduction in the power bill. For a 50 KW station running an older transmitter, the savings would fall in the $37,000 to $56,000 per year range. In some cases, smaller stations may be able to get rid of a demand meter, which would also represent a significant savings. The threshold for demand meters around here is 5,000 KWh per month.
The FCC further notes that:
The reduction in AM signal power at certain modulation levels inevitably exacts some penalty upon audio quality. Depending on the content of the audio program, MDCL algorithms may introduce some audio distortion or may decrease the signal-to-noise ratio in the receiver. In addition, MDCL algorithms may erode coverage slightly at the fringes of the AM station’s protected service area. Both the long experience of transmitter manufacturers and broadcasters abroad, and the initial reports from experimental operations in Alaska however, indicate that such adverse effects are generally imperceptible.
Would be especially true for higher powered stations that stand to save the most money.
By Paul Thurst, on September 10th, 2011
This transmitter is found at WBEC in Pittsfield, MA. It is still in operation as a standby transmitter for that station and has a manufacture’s date of 1955. The Broadcasting Yearbook places WBEC first on air in March of 1947. This would be the second transmitter the station installed.
 Gates BC1J in service as backup at WBEC, September 2011 The transmitter has been in service for 56 years, which is remarkable. This was made back when Gates was just Gates (no Harris) and AM radio was still king of the airwaves. TV was coming of age, FM radio still had a ways to go until widespread acceptance by the general public.
This transmitter doesn’t get to run very much, the third transmitter installed at WBEC is a Nautel AMPHET-1.
By Paul Thurst, on September 8th, 2011
The case of the blown fuse, or rather the blown up fuse:
 Blown 10 amp fuse on Harris SX5 PA board F32 is blown to small bits and had to be vacuumed out of the bottom of the transmitter. The reason why is the pair of MOSFETS connected to that circuit were shorted. Of course, the reason for the shorting MOSFETS needed to be investigated. What I found was on the under side of the PA board where the brass stand off attacked to the torroid combiner board, the nuts attaching the stand off to the combiner board was loose and there was a big arc mark.
I tightened everything up and replaced the MOSFETS, marking them with a pen in case they short again, in which case the drive section needs to be closely examined.
By Paul Thurst, on August 20th, 2011
I give you joy, the unmitigated joy and shear pleasure of the Harris SX 5 AM transmitter. This particular unit dates from 1984 and is installed at WUPE in Pittsfield, MA. It has a few issues of late.
 Harris SX 5 medium frequency AM transmitter The first of which is the unbalanced or out of ratio condition of the PA current and voltage. When changing power levels, the PA current and voltage are supposed to track together. When they do not, it is an almost sure sign that one or several of the MOSFETS in the PA are shorted. Shorted IRF-350 MOSFETS are indicated by blown fuses on the PA boards and should be replaced in pairs. The reason for the damaged devices also needs to be investigated. It is entirely possible that the site receives a lot of lightning, which can cause this damage. It could also be heat related, as the site is not currently air conditioned. The other possibility is under drive conditions.
The MOSFETS turn on and off at a rate of 1/(carrier frequency (hertz)) times per second. If they are under driven, they will go full on and short circuit. Minimum drive is 27.5 volts peak to peak, anything less than that is marginal and can lead to destruction of the PA devices. Under drive indicates an issue with the oscillator, which has it’s own set of peculiar failure modes.
Since this is an older unit, all of the large electrolytic capacitors are also suspect and need to be replaced. There are three power supply capacitors in the bottom of the transmitter, two 76,000 μF 40 VDC for low voltage and one 7500 μF, 350 VDC for high voltage. The modulator section also has six 5100 μF 350 VDC capacitors, collectively known as “dynamite sticks” due to there explosive potential if installed incorrectly.
 Harris SX 5 modulator section While replacing the dynamite sticks, I noticed this PDM pickup board has a whole burned through it. This is a part of the modulator section and if it burned completely open, would likely cause all sorts of problems with this transmitter, likely spurs all around the dial, distorted modulation or perhaps overload and fail altogether.
 Harris SX 5 transmitter damaged PDM pull up board I managed to fix it with a jumper between what is left of the circuit board trace and the capacitor mounting bracket. I soldered the jumper to the board face and soldered the wire lug. After scraping all the oxidized metal off of the capacitor mounting bracket, I attached with a screw. The board itself needs to be replaced, if it is still available from Harris, which it may not be as support for this transmitter was dropped in 2008.
 Harris SX5 PDM pull up board temporarily repaired with wire jumper The worst, and I mean worst possible situation with these transmitters is some type of control malfunction. The control boards and oscillator are in that large vertical pull out drawer. God protect and preserve the digital control and S and M boards, as they are a major headache to troubleshoot. They have 7300 TLL (5 volt logic) that controls all functions and only little problem will cause the entire transmitter to shut down.
Other SX series transmitter tips can be found here.
I didn’t get to replacing the blown devices because of a looming electrical storm, which precludes working inside of transmitters. I’ll get back there next week and finish the job.
Update: I finished the repair job today 8/24. There were 16 blown MOSFETS on the PA boards. I checked the drive levels on the input side of the RF torrid load resistors and it is with normal range. I also found this snake in the bottom of the transmitter across the HV shorting bar.
 Small grey rat snake, electrocuted by Harris SX 5 transmitter Could have been chasing diner. Overall, the site needs help. The air conditioner is coming next week.
 Harris SX 5 transmitter fully operational After repairs, the transmitter is back at full power and modulating +125% again.
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