FM transmitters – Page 15 – Engineering Radio

Harris FM25-K

Old blue, I like to call them, the Harris 1980s 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 backup transmitter. When we arrived, we found the HV power supply feed through the insulator at E1 arced over and broken. Again, fortunately, this station’s management believes in stocking spare parts and a replacement was on hand.

Harris FM25K HV power supply feed through insulator

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) were 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 the 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.

The sad story of WCVR

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 downhill. Over the next decade, the station transferred 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. The transmitter site was raided before the transfer and things like spare parts, a backup transmitter, and a 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 throughout the country as a 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 a 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 doing.

Next, step-by-step troubleshooting of the High Voltage power supply. Step one, resistance checks on the HV transformer and filter chokes to the 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 with the HV power supply RF filter 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 make 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 months 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, VT

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.

Filament Voltage Management

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.

The 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 an accident, I did it by maintaining the filament voltage and 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 intensities 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 by 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 the 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:

Remove power from transmitter, discharge all power supply caps to ground, hang the ground stick on the HV power supply.
Remove the tube, and follow manufacturer’s procedures. Most ceramic tubes come straight up out of their sockets (no twisting).
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
Insert new tube, follow manufacturer’s recommendations. Ceramic tubes usually go straight down, no twisting.
Make all connections, remove grounding stick, half tap plate voltage supply if possible, close up transmitter
Turn on filaments and set voltage for manufacturers’ recommended setting. Wait at least 90 minutes, preferably longer.
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.
Turn off transmitter, retap plate supply for full voltage
Turn on transmitter and plate supply, retune for best forward power/efficiency ratio.
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 its 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.

Nautel V-40 FM transmitter

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 counterproductive.

Anyway, to demonstrate 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 an 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 station’s Transmitter Power Output (TPO).

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.

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