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 50 KW three tower AM directional antenna One day, for no good reason, the DA (Directional Antenna) which had previously been rock stable goes haywire. WHAT THE! The phase is way out, or the current ratio is jumping around, nothing makes sense! Time to start cranking on those phasor handles! NOT! AM DA’s are pretty easy to work on, once you get past all the smoke and mirrors hokus pokus stuff.
The first rule of trouble shooting is cause and effect. Every DA has a common point, which is where the input impedance and transmitter power is measured. The common point is the sum of all the tower impedances as seen through the power divider. Any significant changes to the DA will cause the common point current to shift, either up or down.
What does the transmitter say? Most solid state transmitters do not like to run into loads that are significantly departed from 50Ω j0. If the common point has changed, so has the impedance and the transmitter is likely folding back to protect it’s MOSFETS. Tube transmitters are more tolerant, but other signs may exist like increased or decreased modulation levels.
What do the tower base currents say? An AM DA licenses will specify base current ratios as related to the reference tower. That is to say, take the base current of a tower and divide it by the reference tower base current. Those should be within 2% of the licensed value
What do the monitor points say? Every AM DA will have monitor points for each pattern. The maximum values will be specified on the license. In addition to that, a good station practice is to do monitor point readings several times a year and keep a log. Anything out of the ordinary indicates a problem with the DA.
1. Problem: Phase and or current ratios are out on one or more towers. All else is normal. In this case, normal operation of a DA can be confirmed by base current ratios and monitor points. Both those values should be on the license. If base currents and monitor points are normal, then this is most certainly a sample system problem. Most sample systems have either a sample loop on the tower or a toroid current transformer at the base. Working from the tower back to the antenna monitor, use a Simpson 260 VOM to check for continuity. Use a working tower get baseline information, e.g. the sample loop is a DC short, the toroid has 4 ohms DC resistance, etc. Be careful of working in hot ATU’s.
2. Problem: All current/phase rations are out of tolerance and the reflected power on the transmitter has gone up. This would indicate that a change has occurred with one of the tower impedances. Look for changes at the base tower, excessive vegetation, flooding, burned, shorted or open components can cause this. Be sure to check things like sample system and tower light isolation coils.
3. Problem: Common point current has changed one tower current/phase ration is out of tolerance. Many AM arrays have a tower that actually present a negative impedance to the phasor. In this case, the power is “absorbed” from the tower and fed back to the phasor. Check the tower base current of the out of tolerance tower. Often times, something has occurred to take that tower out of the circuit.
4. Problem: All or some of the readings are slightly out of tolerance. Not enough to indicate a real issue, more like a slow drifting over time. This can be caused by seasonal variations. Depending on the wavelength of the tower, tower bases can be very sensitive to small changes in ground conductivity, just as increased or decreased soil moisture.
5. Problem: 2 pattern station, on pattern works, the other does not. This is a phasor control issue. Most DA2 stations rely on contact closures at the tower base to ensure that the pattern switching contactors have moved into the correct position. Often times, a contactor will be hung up or a micro switch will be bad. Another possibility is a hung up contactor in the phasor, or a control circuit problem.
Those are general starting points. Often times, evaluating all of the information on hand will shed some light on the issue. AM DA’s will have the following operating indicating systems:
- Transmitter directional coupler and overload circuits
- Directional Antenna system monitoring system
- Tower base current meters
- Monitoring points
- Phasor indicator lights
Building all of those indicators into one picture will generally point to the problem. When trouble shooting begin at one end of the circuit and logically work your way to the other.
Sometimes some seasonal effect will cause the readings to be off. Before any phasor handle is turned, make a note of all the readings on the dial cranks. This way, if things get out of whack there is a restore point. If there is a seasonal variation with the antenna array, it should be well documented by previous seasons pattern changes. If not, it could be a slow deterioration of the ground system or ATU components.
As broadcasters, we don’t really hear that much about ceramic power vacuum tubes these days, as more and more broadcast transmitters migrate to solid state devices. Once upon a time, however, power tubes where the engine that drove the entire operation. Tubes had to be budgeted for, stocked, rotated and replaced on a regular schedule. Some of those dern things were expensive too.
Take the 4CX35,000A which was used in the Harris MW50 transmitters. The transmitter used two of these tubes, one in the RF section and one it the modulator. As I recall, new tubes cost somewhere north of $8,000.00 each from EIMAC. Plus, in the A models there were two 4CX1500A driver tubes. All of which could add up to an expensive maintenance cost every two years or so.
The next best option was to buy rebuilt tubes. Rebuilt tubes were about half the cost of brand new ones. Some people complain that rebuilds don’t last as long, or only last half as long as the new tubes. I never found that to be the case. I often found other factors effected tube life far greater, such as filament voltage management, cooling and by extension, cleanliness.
I can say I never had a warranty issue with ECONCO tubes. I cannot say that about EIMAC, as during the late 90′s and early 00′s (or whatever you call that decade) I had several brand new 4CX3500 tubes that were bad right out of the box. These days, ECONCO and EIMAC are both owned by CPI.
I spoke with John Canevari from ECONCO who had a lot of information. For example, as the tube ages, the filament gets more flexible, not less. Most ceramic power tubes use a carbonized tungsten filament containing some small amount of thorium. As the tube ages, the filament can no longer boil off enough electrons and the emission begins to drop off. That is the normal end of life for a power tube. Occasionally, some catastrophic failure will occur.
There are many steps in the rebuilding process:
- Dud is received from the field, the serial number is recorded and the tube is tested in.
- The tube is prepped by sand blasting the sealing rings
- It is opened
- Filament is replaced. In 60-70% of the cases, the grid is replaced. In those tubes that have a screen assembly, 20-60% of those will be replaced.
- Interior of the tube is cleaned
- Tube is resealed and tested for leaks with a gas spectrometer
- Tube is placed on the vacuum machine. Tubes are evacuated hot, smaller tubes take 12 to 24 hours, very large tubes can take up to one week.
- The tube is nipped off of the vacuum while still hot. When the tube is fully cooled the vacuum scale is normally around 10-12
- Exterior of tube is cleaned and replated. Silver for tubes that are socketed and Nickel for tubes that have leads.
- Tube is retested to manufacture’s original specification or greater.
After that, the tube is sent back to its owner or returned to stock. John mentioned that they are very proud of there vacuum tube processing machines, so I asked if he could send along a picture. They certainly look impressive to me, too:
 Vacuum tube processing machine, photo courtesy of ECONCO Not exactly sure which tube type these are, but they sure to look like 4CX15,000:
 Vacuum pump working on rebuilt ceramic power tubes, photo courtesy ECONCO Econco has been in business since 1968 and rebuilds about 600-1,000 tubes per month. In the past, broadcasters used most of the larger tube types. However, with the majority of broadcast transmitters shifting to solid state, other markets have opened up such as industrial heating, military, research and medical equipment.
One of the unfortunate signs of the times is increased theft of valuable materials. Copper, while not as expensive as it once was, still fetches a fair amount at the scrap dealer. One local telephone company has been having a difficult time keeping their aerial cables intact in certain areas. For radio stations, the situation is compounded by remote transmitter sites with lots of copper transmission lines and buried ground radials around AM towers. Reduced staffing levels also means that the weekly trip to the transmitter site is now every two weeks or perhaps once a month or even less.
Site that are not visited or monitored very often are prime targets for copper theft. Forget asking the local constabulary to patrol more often, the few times I tried that I was met with a blank stare.
A few common sense type things that I have learned over the years may keep your site intact:
- Keep up appearances. A neglected transmitter site is more likely to attract the wrong type of attention from the wrong type of people. Clean up any rubbish, dead equipment, keep the weeds and trees cut down, etc. If a site looks well tended and often visited, a thief may think twice about lifting valuable metals.
- Along with #1, keep things buttoned up. Secure all transmission lines to ice bridges, remove any dead lines, etc. If there are ground radials poking out bury them, same with ground screens, copper strap, etc. Out of sight, out of mind, leaving this stuff exposed is asking for somebody to come along and give a tug.
- Fences and locks. Towers are required to be fenced and locked to prevent electric shock hazard. It is also a good idea to fence the building, generator and fuel tank if possible.
- Post all sorts of warning signs, RF warning, high voltage, no trespassing, under video surveillance, pretty much anything to deter trespassing and vandalism.
- Add video cameras with a video recording device since most theft occurs during non-working hours. Last year, the company I used to work for traded a video surveillance system for the studio location.
- Compensate a neighbor to keep an eye on the place and call you if they see any suspicious activity. It doesn’t even have to be money, I once worked out a deal with a neighbor for some T-shirts and CD’s. That was the best alarm system we ever had.
In the long run, keeping all the copper parts where they belong is a great way to avoid those annoying “the station is off the air” phone calls not to mention the expense of replacing damaged transmission and ground systems.
My former employer thinks he knows better than anyone what to do in every given situation. ”Mister,” as he is “affectionately” known, has a legendary cheap streak. When I worked for the company, every year there would be a debate on whether we should plow and maintain the road to a certain transmitter site. Mind you, this is not just any transmitter site, but the transmitter site of the number one billing station of the entire group of 35 stations. It is located in the wealthy suburban setting of market Number One and bills more than most of their other markets combined.
Naturally, when I was there, I put up a stiff fight to make sure the road got patched and plowed. Oh they would scream and nash their teeth about how unfair it was, and can’t we do this or do that, etc. This went on every year for the entire ten years I worked for the company. For my part, I just ignored it. Back in October of last year, when I was first starting to see the handwriting on the wall, there was this clandestine meeting with the other residents on the road which I was not invited to. You see, the lower half of the road has houses on it. Mister thought that the residents of the road should chip in for the road plowing. When they refused (because they were already plowing the lower part of the road themselves) he said we would absolutely, positively ,100% not be plowing the road this winter.
I departed the company in January. Since then, the upper part of the road was not touched. Then came last week’s blizzard. Prior to the blizzard, the generator fuel tank was 9/10 full. The power went out on Wednesday during the first storm. I called the General Manager for the radio station on Friday and told him that the generator would need fuel soon and asked if the road had been plowed. He said they were working on it and it should be done on Saturday. I told him that we needed a fuel truck up there ASAP otherwise they would be going off the air. He said he was on it.
Sunday morning at 6:30 am, the generator ran out of fuel. Naturally, my phone rings. I begin calling around all the fuel oil companies in the area to see if I can get a Sunday delivery. I finally arrange something and we also get a 4WD pickup with a 100 gallon day tank to meet us there. When I arrive at the site at 9:30 am, a backhoe was just starting to clear the upper part of the road. The snow is knee deep and there is a layer of ice under it. It took until about 1:30pm to get the road cleared enough to get the 4WD pickup, with tire chains near the generator to transfer fuel. Then, because the fuel pump sucked air, we had to bleed the injectors, reset the faults, etc. We finally got the generator started around 2:00pm.
So, let us compare costs:
Plowing the road cost about $800.00-$900.00 per storm. This year, there were five to six storms where the road needed to be plowed. Total $4,500.00
Last Sunday, the station was down for about 7 hours. I’d say that station likely bills $150.00 per unit on a Sunday morning, 10 units an hour so they lost $10,500.00 by being off the air. Then there is the backhoe needed to clear the road. A backhoe was needed because there was so much snow on the road that a regular snow plow could not move it, especially plowing up hill. That cost $1,500.00. Then there is my overtime and the guy with the 4WD pickup, another $1,440.00. Total cost to plow the road and get the station back on the air, somewhere in the neighborhood of $13,440.00.
So, yeah, Mister is really saving money. How’s that working out for you, a$$hat?
As promised, here is the AM transmitter site maintenance check list. This is for a generic directional AM station with a backup transmitter, generator and an RF STL.
 Broadcast Electronics AM6A transmitter Usual disclaimers apply.
AM site Maintenance checklist
Weekly Maintenance:
A. Visit site, Check following:
- Check critical transmitter values against last logged value
- Check forward/reflected power on main transmitter
- Check and reset any overloads
- Check signal strength on STL against last logged value
- Check generator fuel level
- General check of building, look in all rooms, inspect for damage from vandalism, Leaking roofs, obvious signs of trouble, take steps to correct.
Monthly Maintenance:
B. Visit site, Check following:
- Do a full multi-meter log, (includes tower phase angles, loop currents), run backup transmitter into dummy load.
- Start and run generator for 5 minutes, check block heater, hoses, belts, oil and antifreeze levels
- Calibrate remote control meters with transmitter meters, log it*
- Check all tower fences for integrity and locked gates*
- Complete Items 3, 4 and 5 under weekly maintenance.
Quarterly Maintenance:
C. Visit site, Check following:
- Complete 1 through 5 under monthly maintenance.
- Check all air filters, clean or replace as needed.
- Check frequencies of all transmitters, STL receiver, and log.
- Complete quarterly tower lighting and painting inspection*
Bi-yearly Maintenance:
D. Visit site, Check Following:
- Complete 1 through 5 under quarterly maintenance.
- Conduct monitor point readings for all directional antenna patterns*
- Check base current readings for day/night towers. Ratio.*
- Clean backup transmitter
- Place backup transmitter on air and clean main transmitter.
Yearly Maintenance:
E. Check all licenses and authorizations for accuracy. Make sure that all renewal cards etc are in public file and are posted at control point.*
F. Visit site, Check following
- Complete 1 through 5 under Bi-yearly maintenance
- Equipment performance measurements (NRSC, Harmonics, frequency)*
- Complete service of generator
- Complete Inspection of towers, check for vertical and plumb, check guy wire tensions, retension as needed.
- Check property for anything out of the ordinary
- Repair driveway as needed
General maintenance that is completed on an as needed basis
- Re-fill fuel generator fuel tank when drops below 50 percent
- Empty trash, sweep floors, dust.
- Cut/remove vegetation inside tower fences, spray herbicide as needed
- Water proof tower fences every 2 years
- Paint exterior of building
- Replace tower lights*
- Paint towers*
*These are FCC inspection items, pay close attention if you do not want a fine.
That is it, a .pdf version of this file can be downloaded here.
Almost all radio stations use a tower of some sort to support their transmitting antennas. These towers need maintenance from time to time and only qualified people should perform maintenance on towers. Hence, the tower company is formed.
 405 foot guyed tower with ERI FM antennas Over my years of experience, I have dealt with many different tower companies, from one man operations to big corporations that have multiple crews out in the field on any given day. I have discovered that not all tower companies are created equal. Not only do tower climbers need to be in good physical shape and be trained correctly in all tower climbing safety procedures, they also need to be good mechanics so they can actually repair things on the tower. Climbing a 470 foot tower to repair a strobe light is all well and good. Once the climber gets to the strobe light, he needs to be able to disassemble it without dropping parts or breaking things, trouble shoot if needed, install new parts and re-assemble the unit, again without dropping or breaking anything.
Applying a RF connectors, installing a FM antenna or STL antenna, repairing light fixtures or conduit all require some amount of manual dexterity and concentration. Assembling high powered antenna requires close attention to detail. Any pinched O rings, cross threaded bolts, bent bullets and the antenna will have problems, likely at the worst possible time.
The sign of a bad tower company is if it’s climbers cannot carry out those tasks with one or at most two climbs. I have a situation on a tower where our FM station is a tenant. The tower has a strobe light failure near the top of the tower where our FM antenna is located. They have climbed the tower no less than four times to repair this, and it is still not fixed yet. Each time they climb, the station has to reduce power to protect the tower climbers from excessive RF exposure. Each climb it takes them several hours longer than anticipated to finish their work.
A good rule of thumb, If the defective part cannot fixed in the first two climbs, then the entire strobe unit should be replaced on the third climb. Even though the strobe units are expensive, by the time they get done paying for all this tower work, they could have bought two new strobes. Today will be the fifth climb and there is no guarantee that it will be fixed.
I advised the tower owner that they should be looking around for another tower company because these guys aren’t exactly setting the world on fire.
 Nautel V-10 Power Supply Gone is the day when the radio station engineer had to trouble shoot down to the component level, often crawling in and out of transmitters to get at the suspected part. I for one, spent many a long night at a transmitter site chasing some weird combination of symptoms down to the $0.34 1N914 diode in the directional coupler (see previous post about the MW-50).
It is a skill set now mostly confined to manufactures’ repair departments, for which they charge a pretty penny. Nowadays, the technician simply slides out one module or circuit card and slides in another. If that doesn’t fix it, panic ensues. I know of several class C FM radio stations that are now relying on the computer guy to fix transmitters, because, you know, it’s cheaper.
To be fair, most engineers are contractors and many of those simply do not have the time to trouble shoot to the component level. So, they ship everything back to the factory then pass the cost on to their client.
Then of course, most circuit boards these days are surface mount systems, which are hard to work on if you don’t have the right tools. Normally an expensive temperature controlled soldering station is required, as well as a magnifying glass.
All of these things combine make circuit board work something to be outsourced. Unfortunately, a night spent trouble shooting was often a great learning experience. I have done some of my best work when my back was up against a wall and I was out of options.
I make the attempt to fix things locally, unless the transmitter or other item is under warranty or not having a spare/attempting to trouble shoot will take the station off the air. I think it is important to keep abreast of technology and keep my trouble shooting skills up to par. Besides, I find it gratifying that at least I can still fix things.
In the never ending saga of things I have not yet seen, we had a 350 kcml 3 phase underground feeder burn out yesterday at our AM transmitter site.
Actually, it happened the day before, around noon. I received a call from the remote control that we were on the backup generator. Upon arriving at the site, I found several trees down on the three phase primary down the street. I figured that was the cause. After checking the generator fuel supply, oil pressure, temperature, phase volts/amps, I decided that everything was okay and the power company would be along shortly to restore power. I then continued up the road to our FM site to do weekly maintenance.
Upon returning to the office several hours later, I looked at the utility company website. They have a pretty cool interactive map application that shows all outages and give restoration times. The area around our transmitter site showed no outages, therefore I figured it had been cleared.
I committed two errors here:
- Not calling the utility company myself to ensure that the outage was reported. I assumed that the tree across the three phase primary was the cause, it was not.
- Not calling the transmitter site remote control to check the generator status after I checked the website.
To be honest, I don’t know if I am coming or going these days. With seven radio stations, each with it’s own transmitter site, three of them Directional AM stations, and three studio locations spread out over a 75 mile stretch, it is difficult to keep up with the small details. Did I mention that I am solo, the engineering assistant position was cut two years ago. But, I am not here to make excuses…
The net result is the generator ran all night long. The next day, when I checked the transmitter in the morning, I was surprised to find the generator still running. Unfortunately, I had an FM station on low power (see post below) that needed to be taken care of first. When I finished replacing the RF module in the FM transmitter, I made my way to the AM site.
I called the power company then checked the generator fuel, the propane tank was down to 10%. Yikes, better get this taken care of fast. I will say the power company showed up pretty quickly. After some measurements with a hand held meter, it was determined that the under ground feeder was open between the pole and the transmitter building.
The line man was not at all surprized, in fact, he called it before he even went up in the bucket truck. After some back and forth with his supervisor, who came out in a pick up truck, it was decided that they would run a temporary overhead feed to the meter can.
 temporary overhead electrical feeder They also did some research in there records and discovered that they (the utility company) own the underground cable and therefore they would dig it up and fix it. That’s nice because otherwise it seems like it would be an expensive repair job. On a station that makes not a lot of money.
We have a Harris Z5-CD transmitter for one of our FM stations. Brand H is not my preferred make, however, it was already installed when we bought the station, so I have to live with it.
This particular site gets hit by lightning strikes often. Normally, it does not affect anything until the transmitter gets turned off for maintenance. Then, almost invariably, when turning the transmitter back on one of the modules will fail. Most often this is manifest when one of the two power supplies shut down causing the transmitter to run no more than 20% power.
The way this is trouble shot is to slide each module out and turn the transmitter back on. When the power supply stays on, the bad module has been located. A confirmation test is to check the MOSFET for a short circuit between Drain and Source. This short circuit condition puts a direct short on the power supply causing it to crow bar and turn off.
So, once the bad module has been located, and the spare module is installed in the transmitter, then what? Most engineers call Harris and ship the module back for repair. Most engineers don’t want to mess with unsoldering a surface mount MOSFET and soldering a new one in. I find it moderately entertaining to fix things myself, so I do not do what most engineers do.
 NXP BLF177 MOSFETS The MOSFET in this particular module is the BLF177, made by NXP. Harris will sell you one for quite a bit of money. You can also buy one from Mouser for about half the cost.
 Harris FM Z series transmitter PA module with cover removed Once the parts are obtained, the worst part of the entire job is unsoldering the old MOSFET. This takes some patience and skill. What I found works best is to melt some solder on the foil leads and get them good and hot. Since this MOSFET is already destroyed, we don’t have to worry about heat etc. The one thing you do not want to do it actually break the MOSFET open. That is because it contains beryllium oxide, a known carcinogen. Once all the solder is liquid, carefully pry the foil up with a small screw driver. There are several components that have to be moved to work on this.
 Harris Z series PA module with MOSFETS removed After the old MOSFET is removed, clean up the solder pad with a solder pump and solder wick. I like to use a little liquid flux on the solder wick, it makes things go faster.
Once all the old solder is cleaned off the solder pads, I brush a light coat of liquid flux in the pad. Again, this makes things go faster.
 Harris Z series FM transmitter module new MOSFETs waiting to be soldered The new MOSFETS are very sensitive to static discharge, so I always use a static drain wrist band when handling. I place both MOSFETs on to the circuit board. I then solder them on using as little heat as possible from the soldering iron. Again, the MOSFETs are sensitive to heat and one can easily be destroyed if it gets too hot.
 Harris Z FM series PA module repaired This is the module with the new MOSFETs soldered in. I use defluxing compound to remove all the extra flux. Once it cools off, I test the new module with a DVM:
 Harris Z series FM PA circuit board under test, resistance is 3.3 Mohm If the MOSFETS are good, they will have an internal resistance of around 3.3 MΩ. If the module is bad the MOSFETS will read only a few ohms if shorted:
 Harris Z series FM PA module under test, DVM reads 1.6 ohms That is how you do it. I think Harris charges $775.00 per module to repair. I fixed this one for $240.00, but that is not the reason I did it. I did it for the fun that was in it.
Every now and then something goes wrong. One of the nicer features of a solid state transmitter is a soft failure mode. For example, the loss of a single RF module may bring the transmitter down to 95% power vs 100% power. In a tube transmitter, the failure of the tube would mean 0% power.
This happened recently when a transmitter was turned off for tower maintenance. Upon restart, an RF module failed. Unfortunately, the spare RF module had already been used due to a lightning strike in early July. So we were down a few percent on the output meter until another RF module was ordered and installed. The station was running at 94% power according to the external watt meter. That equates to about half a dB power reduction over normal operations, which is really insignificant.
Naturally, the fact that the transmitter was at low power gave the program director another excuse to pile on. First I received this e-mail:
Getting reports out of both XXX and XXX that it’s nothing but the The (competing station) on 1xx.1 – the tropo is going pretty good right now. I’ll monitor on the ride in but needless to say we can’t fix this soon enough. We’ve been bad in XXX County for the last two weeks and I just assumed tropo and stayed quiet – could this module have been out for a while?
Where are we on a software controller so we can log in and monitor stuff like this?
To which I responded:
The module problem arose after the transmitter was turned back on, so no, this has not been a problem for the last two weeks, it has only been a problem since Sunday Morning at 11:42 am.
As I said below, the new module was ordered and as soon as it arrives, it will be put back in.
I then received this e-mail:
With all the shadowing in our contour and the short-spacing, we just can’t afford to loose 1db without tangible effects. We need every nanowatt possible everyday – especially in the summer.
So, Mr. Smarty paints there thinks that 0.00000001 watt makes a difference. The absurdity of that statement is un-measurable. Why not a femto watt or a yacto watt? Here was my politically correct response:
- I understand you want the transmitter fixed.
- I have done everything humanly possible to effect repairs including calling Harris on my day off to order parts and have them shipped.
- e-mails of this type do not make things go any faster, and are in fact, counter productive.
I hope one day, he reads this blog.
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Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
~1st amendment to the United States Constitution
Any society that would give up a little liberty to gain a little security will deserve neither and lose both.
~Benjamin Franklin
...radio was discovered, and not invented, and that these frequencies and principles were always in existence long before man was aware of them. Therefore, no one owns them. They are there as free as sunlight, which is a higher frequency form of the same energy.
~Alan Weiner
Everyone has the right to freedom of opinion and expression; this right includes the freedom to hold opinions without interference and to seek, receive and impart information and ideas through any media and regardless of frontiers
~Universal Declaration Of Human Rights, Article 19
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