Engineering Radio

Old blue, I like to call them, the Harris 1980’s model transmitters with black faces, white cabinets and blue trim. I have yet to find one that I really like, the FM 25-K is, well okay. Sort of like that 200,000 mile jeep that works, most of the time, and it’s paid for.
This particular FM-25K transmitter is located at WIZN in Charlotte, VT.

This transmitter was new in 1987.  It had a bad day yesterday, deciding to throw a temper tantrum and trip the HV power supply breaker.  Fortunately, the station has a back up transmitter.  When we arrived, we found the HV power supply feed through insulator at E1 arced over and broken.  Again, fortunately this station’s management believes in stocking spare parts and a replacement was on hand.

This is part of the RF filter for the HV power supply. This happened once before, about two months ago. The replacement insulator then was used, so that might be a factor. Two months ago, both capacitors in the Pi filter and the HV power supply cable (RG-8 coax) was replaced all the way back to the rectifier stacks.

The FM25-K can produce spontaneous high frequency oscillations if not tuned properly.  We looked at transmitter output with a Rode Schwartz spectrum analyzer and found it to be clean.  Exactly why it blew out another feed through insulator is a bit of a mystery.  Since the first replacement was a used part, we surmise that it may have been cracked.  If this replacement insulator arcs, there needs to be a full investigation.

As I said in the beginning, I have found these transmitters to be okay, not the best, not the worst.  Most of the problems I have encountered with the K series FM transmitters had to do with the controller cards.  There are two, one analog and one digital.  That’s what Harris calls them anyway.  Like the SX transmitter, and the MW transmitter to a certain extent, the control circuits are way over complicated and full of +/- 5 volt CMOS logic.   Having that type of control logic connected to a radio tower (e.g. lightning rod) is asking for trouble.

I have been the road warrior lately, if you haven’t noticed a certain decline in the blog posts… One place that seems to keep pulling me back is Randolph, VT, which is about as close to the geographical center of Vermont as one can get and still be on a roadway.   There resides the silent FM station formerly known as WCVR, to be returned to the air as WXVR by Vermont Public Radio.

WCVR went on the air in 1982 and was a community oriented country station for 17 years.  Then in 1999 it was sold to Clear Channel and things began to go down hill.  Over the next decade the station transfered ownership four times.  Finally ending up with absentee landlord Vox radio.  By the time the station was sold to Vermont Public Radio last May, the years of neglect were compounded and the main transmitter was no longer running.  As is typical of any good slum lord honest radio owners, the transmitter site was raided before the transfer and things like spare parts, a backup transmitter and dummy load were removed.  As one engineer from VPR noted, the only thing of any value is the Shively antenna.

This story probably repeats itself a thousand times over throughout the country as small market, formerly community radio stations are left to die on the vine by big time corporate radio gurus in Atlanta, San Antonio, and Las Vegas.

Said station has a McMartin BF5-K transmitter that is not currently running and by the accumulations of dirt, debris and other evidence, has not run in quite some time.

The beauty of a McMartin FM transmitter is it is grounded grid.  Can’t get much simpler than that when it comes to FM transmitters.  The downside is, of course, McMartin has been out of business for almost thirty years.  Thankfully, Goodrich Enterprises is still around and still supports them.

The first order of business was cleaning out the filthy, and I mean absolutely filthy building.  Several hours with a broom, dustpan and shop vac got rid of most of the dirt and made my skin less likely to crawl.  Then came the fateful attempt to run the transmitter.  Loud arcs, power supply hum and dimming lights revealed that all was not well.  All of the fluorescent lights were out, new bulbs did not fix the problem.  So, to the Grainger to pick up new fixtures and install them.  Now, at least, we could see what were were doing.

Next, step by step trouble shooting of the High Voltage power supply.  Step one, resistance checks on the HV transformer and filter chokes to ground.  Next forward and reverse resistance checks on the rectifier stacks.  All of those looked good.  Next, we isolated the HV transformer and the rectifiers and turned the transmitter on; no problems.  Next we added the metering and filtering capacitors and turned the transmitter on; no problem.  Finally we found the problem on the HV power supply RF filter up on the side of the PA enclosure.  In a McMartin FM transmitter there is a little box mounted on the outside of the PA enclosure that holds half the parts in this circuit.  Taking that box off revealed a bad 200 pf 7.5 KV doorknob capacitor that was shorting to ground.  Lots of arc marks, soot, debris and other stuff makes me think that this problem had been going on for a long time.   I can kick myself for not taking a picture.

Hopefully this thing will run for a few month while a replacement is sought.

Compounding that issue is the leaking transmission line connector at the bottom of the antenna, which was fixed, but there appears to be another leak somewhere else as the line still does not hold pressure for very long.

VPR is going to broadcast their Classical Music format on this station, starting as soon as we can make the transmitter run.

UPDATE: Pictures:

300 foot WCVR tower, Randolph Center, V

Transmitter building:

WCVR Mc Martin BF-5K transmitter:

Arcing power supply filter section, the bad door knob capacitor has been replaced, still evident are the arc marks on the PA cavity:

Mc Martin BF-5K transmitter on the air:

We’ll see how long that lasts.

Nautel, Ltd., which always seems to be a forward thinking company, has their very own YouTube channel. Okay, that is not such a big deal, I have my own YouTube channel too.

There are lots of video on how to configure a NV transmitter, which, as I have said before, fancy GUIs are all well and good, I am more concerned with the MOSFETS generating the RF.  I would forgo the GUI in favor of more more reliability, but that’s just me.

I would put the DRM+ video up on the blog, but the embed function is disabled.  Anyway, I enjoyed watching many of these, you might too.

4CX35,000C ceramic vacuum tube

There are still many hollow state (AKA tube type) transmitters floating around out there in the broadcast world.  High power, especially high power FM transmitters are often tube types and there are many good attributes to a tube transmitter.  They are rugged, efficient and many of the well designed tube units can last 20-25 years if well maintained.

There downside of a tube transmitter is tube replacement.  Ceramic tubes, like a 4CX20,000 or 4CX35,000C cost $6-9K depending on manufacture.  A well maintained tube and last 3-4 years, I have had some lasting 8 years or more.  My personal record was for a 4CX35,000C that was a final PA tube in a Harris MW50A transmitter.  The tube was made by EEV (English Electrical Valve, now known as E2V)  and lasted approximately 84,000 hours, which is 9.58 years.  When it finally came out of service it looked like it had been through a fire, the entire metal plate body was dark blue.  I took it out because the power was beginning to drop a little and it was making me nervous.

This was not accident, I did it by maintaining the filament voltage, keeping the tube and transmitter clean.  The tube filament supplies the raw material for signal amplification.  Basically, the filament boils off electrons, which are then accelerated at various rates and intensity toward the plate by various control grids.  The plate then collects the amplified signal and couples it to the rest of the transmitter.  When a tube goes “soft,” it has used up its filament.

I had a long conversation about this one day with Fred Riley, from Continental Electronics, likely the best transmitter engineer I have ever known.  At the time, the consensus was to lower the tube filament voltage no more than 10%.  On the 4CX35,000C, the specified filament voltage is 10 volts, therefore, making it 9 volts was the standard procedure.  What Fred recommended was to find the performance “knee,” in other words, where the power began to drop off as the filament voltage is lowered.  Once that was determined, set the voltage 1/10 of a volt higher.  I ended up running that EEV tube at 8.6 volts, which was as low as the MW50’s filament rheostat would go.

The other important thing about tubes is the break in period.  When installing a new tube, it is important to run only filament voltage for an hour or two before turning on the plate voltage.  This will allow the getter to degas the tube.  New tubes should be run at full filament voltage for about 100 hours or so before the voltage is reduced.

Tube changing procedure:

1. Remove power from transmitter, discharge all power supply caps to ground, hang the ground stick on the HV power supply.
2. Remove tube, follow manufacture’s procedures.  Most ceramic tubes come straight up out of their sockets (no twisting).
3. Inspect socket for dirt and broken finger stock.  Clean as needed.  Finger stock, particularly in the grid section, is important for transferring RF.  Broken fingers can lead to spurs and other bad things
4. Insert new tube, follow manufactures recommendations.  Ceramic tubes usually go straight down, no twisting.
5. Make all connections, remove grounding stick, half tap plate voltage supply if possible, close up transmitter
6. Turn on filaments and set voltage for manufactures recommended setting.  Wait at least 90 minutes, preferably longer.
7. Turn on plate voltage and tune transmitter.  Tune grid for maximum current and or minimum reflected power in the IPA.  PA tuning should see a marked dip in the PA current.  Tune for dip, then load for maximum power.
8. Turn off transmitter, retap plate supply for full voltage
9. Turn on transmitter and plate supply, retune for best forward power/efficiency ratio.
10. After the 100 hour mark, reduce filament voltage to 1/10 volt above performance knee.

Of course, every transmitter is slightly different.  There may not be a dip in the plate current if the transmitter is running near it’s name plate rating, in which case one would tune for maximum forward power.

This system works well, currently one of the radio stations we contract for has a BE FM20T with a 4CX15,000A that has 9 years on it, still going strong.

Behold, I do not work for Clear Channel.  I do not work for Cumulus, or Citadel.  The company I work for is not about to go bankrupt, in fact, we have purchased a brand new FM transmitter for the wretched mountain top transmitter site featured here.

Nautel V 7.5 FM transmitter

Am I happy?  Oh yes I am. New transmitter = fewer midnight phone calls thus better quality of life for me and my family.  A solid state Nautel transmitter means fewer unscheduled trips to the transmitter site and better reliability.

As a result of consolidation and smaller engineering departments, almost my entire work day is now scheduled weeks in advance.  One little hiccup in the schedule can throw things off for days, resulting in many apologies, phone calls to reschedule, and general wasted time.

This is a V 7.5 FM transmitter, very likely the last V series transmitter Nautel ever makes.  They have moved on to the NV series.

It is replacing the Broadcast Electronics FM5A, which is 24 years old.  The BE has been a good transmitter, we lost a few rectifier stacks due to lightning over the years, but over all, it has been reliable and easy to repair when problems did arise.  This transmitter will be going into standby service.

The new transmitter, it sounds awesome.

Yesterday, I threw out a transmitter.  I know there is probably some radio station out there that may have been able to use a 5 KW FM transmitter, but believe me, not that one.  There are limits to how much you can help out a fellow broadcaster.  Donating an FM transmitter that never really worked right in the first place is counter productive.

Anyway, to demostrate that I am not a total heel, here is my favorite brand of transmitter, Nautel:

Nautel V-40 transmitter (4 V-10 transmitters combined)

I like Nautel because they are rugged, reliable and good looking.  Okay, good looking is low on the list of transmitter attributes, however, you have to admit, it is good looking.  It is also good sounding.  The night we switched over from the long in the tooth BE FM30A to the Nautel V-40 I noticed a marked improvement in the station’s sound.  It was like somebody switched off the background noise generator.

As the caption states, this is 4 V-10 transmitters combined with a ERI magic T combiner.  It is set up so that if any one transmitter fails or reduces power, the magic T combiner automatically adjusts for minimum rejected power, then the SC-1 controller turns up the other three transmitters to maintain the stations Transmitter Power Output (TPO).

TPO 28 KW

In this case the TPO  is 28 KW, which is getting into the semi serious range for an FM station.   Nautel has updated their transmitter line, which now consists of the NV series transmitter.  The differences mainly have to do with the IPA module/PA module interchangeability (not interchangeable in the V series, fully interchangeable in the NV series) and the “Advanced User Interface.”  I don’t know, fancy touch screens are optional on FM transmitters as far as I am concerned.  It’s the underlying RF generating sections that I am most concerned about.

Nautel V-40 transmitter

Another view.  Just for the useless trivia that is in it, the “V” in these transmitter names stands for “Virtuoso.”

Takes its rightful place in the world today, the scrap heap:

Gates FM5G carcass

As EDWARD I of ENGLAND once said, “A man does good business to rid himself of a turd.”

Of course, he was speaking about Scottsman John Balliol and not some old cranky FM transmitter, but I understand that feeling.  The Gates and later Harris transmitters always seemed to be somewhat less than top notch. The 5G was no exception to this rule.  The final step for tuning the transmitter was to turn off the lights in the room and look down through the screen on top to make sure there were no little arcs in the PA tuning section.  It also had a way of self oscillating, which could make for some exciting tuning.

Gates FM-5G transmitter prior to disassembly

Good bye, I will not miss you.

In one of my past jobs, I worked in a RCA town.  I worked there long after the broadcast arm of that company went out of business, however, all of the broadcast transmitters, AM, FM, TV were made by RCA.   I had an RCA FM-20ES1 which was 22 years old, built like a tank and just kept going along.  I think that transmitter was finally destroyed in a fire, caused by it’s replacement transmitter.

Old Collins, Contenental, RCA and even Broadcast Electronics transmitters had some heft to them.  Of course, not every RCA transmitter was well thought out, the amplifuze series of AM transmitters were a maintenance nightmare.

One thing that I find a little annoying is the continuing need to reboot everything at some interval.  Computers in the studio, audio vault servers and work stations, e-mail servers, files servers, network routers, and so on.  Got a problem, first thing to do is cycle the power off and on…

One of the most irritating pieces of equipment is the audio processors on one of our FM stations.  A few years ago, we purchased the whiz bang Omnia 6 processor.  Every 6 or 8 months the thing losses its mind and sounds terrible.  The station gets all bassy and the high end sounds distorted.  I have tried everything I can think of to prevent this, including installing an UPS, extra grounding, extra shielding, software updates, etc.  In the end, it just has to be rebooted, which of course, means several seconds of dead air.  Naturally, this processor is at the FM transmitter site, where it is difficult to get to.

Truth be told, when it is working, it does sound pretty good on the air, but is it $10,000 dollars better than the older Optimod 8100A?  No, it is not.

The old Orban Optimods sound pretty good as long as they are re-capped and aligned every so often.  If fact, our number one billing station has an AC format and uses an Optimod 8100A and nothing else.  Our other station in the same market uses an Optimod 8100A and a pair of Texar Audio Prisms. In the ten years I have been working for this group of radio stations, I have never had to reboot the Optimod or the Audio Prisms, they just seem to work continuously without problems. Imagine that.

I have seen this called a “retro audio chain” by some.  Nothing retro about it, a little care and feeding and I’d stack this equipment up against an Omnia 6 any day of the week and twice on Sunday.

This is a grainy video of an 8100A  in action:

That was taken in our rack room using off air audio on the rack room speakers and a cheap video camera. You get the idea.

So here is to Frank Foti and his marketing gurus that have sold all of the program directors in America on the need to “update” there air chain processors, because, you know, the Optimod, that is old skool.

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:

1. I understand you want the transmitter fixed.
2. I have done everything humanly possible to effect repairs including calling Harris on my day off to order parts and have them shipped.
3. 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.