Repair work – Engineering Radio

The Tectrol TC91S-1465 power supply

Also know by its Nautel Part number: UG-39

These are the stock power supply for 3rd and 4th generation Nautel V series FM transmitters, which were produced in the 00’s decade starting around 2005 but were discontinued sometime around 2009. First and second generation V series transmitters used Nautel made power supplies.

The OEM PA power supplies were made by Tectrol and were designed to put out 2120 watts per unit. The V-10 transmitters have eight PA supplies, one IPA supply with an option for a hot standby IPA supply. Like all such things, occasionally they fail for various reasons.

Unfortunately for Nautel, Tectrol stopped making these supplies and no longer supports them. Nautel won’t fix them either, however, they will sell a $3,200.00 (per supply) retrofit for a new supply.

We take care of seven of these transmitters and overall, they are fairly reliable. They are not terribly old either. However, spending $28,000.00 to replace the UG-39 power supplies seems… somewhat steep. One station uses four V-10 transmitters combined to make a 40 KW transmitter. For that station, it would cost $115,000.00 to replace all of the power supplies on a transmitter that is barely 13 years old. In this time of economic instability buying a new transmitter is not an option either.

Necessity being the mother of invention; we had a few of these defective power supplies kicking around, I decided to destructively reverse engineer one and determine the failure mode or modes. Special thanks to COVID-19 for giving us lots of spare time to do things with. Pete the Bench Guy, made up a test jig with a connector and some test points. With this, he can provide 240 VAC into the unit, feed 0 to +5VDC to the control pin, thereby vary the output voltage, look for faults, get ready indicators which the transmitter uses, etc.

Thus far, we have about a ~~50% 80%~~ 90% success rate with these things. The failure modes vary from blow MOSFETS in the H bridge, bad PDM chips in the controller, fried resistors, a few other unusual things, etc. After repair, they will burn in for 24 hours in a nearby V-10 transmitter before we send the repaired unit off to wherever it is supposed to go.

Engineers hate this

Apparently, this coaxial cable has a hot spot:

7/8 inch air dielectric coax with jacket melting off

The back story:

I received a text this morning that one of our clients station “had a lot of static on it, it might be off the air.” Upon arrival, I found the Nautel VS2.5 transmitter with 0 watts forward power and an output network fault. Reset the transmitter and the forward power and reflected power increased together, triggering another output network fault. I was able to turn the transmitter power down to 100 watts, at which point it stayed on, with 50 watts reflected power. I also noted the dehydrator running continuously and 0 PSI line pressure.

Crap.

I wandered around the back of the building where the coax goes out to the tower and discovered the dripping plastic from the melted jacket. I reached up and first checked the cable to see if it was warm (it was not). Then I shook it and heard what I thought was water sloshing around inside. This is the original Andrew 7/8 inch cable from when the station signed on in 1972 or so. Very likely that further up the tower, something has chaffed through the outer jacket and shield, allowing water into the cable.

I drilled a small 5/32 inch hole at the lowest point in the cable before it enters the building. The result was a steady stream of water, which was aided by some additional pressure from a spare N2 tank. I let it drain while I ran down to town and got some lunch. I came back a half an hour later, turned the transmitter on and was satisfied to see 100 watts forward power with 1 watt reflected. I ran the transmitter up to full power for a while, then deciding discretion is the better part of valour, turned it down to half power; 820 watts which nets 8 watts reflected power.

Needless to say, the transmission line needs to be replaced as soon as possible.

The inglorious task of AM antenna array maintenance

AM radio stations are rough customers. They frequently operate on the margins, both in terms of ratings and revenue. Their transmitter plants are complex and very often have been on a reduced maintenance schedule for years, sometimes decades. Those of us that understand the operation of AM transmitter plants and all their quirky behaviours are getting older. I myself, feel less inclined to drop everything and run off to the AM transmitter site when things go awry. Seldom are such efforts rewarded, much less acknowledged. Station owners are also finding that their previous demands are unrealistic. For example, time was that any work that takes the station off the air had to be done after midnight. These days, I can tell you, I will not be working at your radio station after midnight. You can find somebody else to do that work.

Thus, today, we took this particular AM station off the air from Noon until 3 pm to diagnose and repair a problem with the four tower daytime array. Once again, this involved a shift in common point impedance and a drastic change in one tower’s current ratios.

Antenna Tuning Unit, mice have made a mess

In all fairness to the current owner, this ATU reflects years of neglect. At some point, mice made a home in here and created a mess. The ATU smells of mouse shit, piss and mothballs. It is full of mouse droppings, grass seeds and fur. All of the ATUs in this array are in similar condition.

It was warm enough that the wasps were active, if not a little bit lethargic.

This coil is being held up by the tubing that connects it to other components. When the ATU was built, no nylon or cork bushings were used between the insulators and the wall of the ATU they were mounted on. Heat cycling eventually did all of the insulators in.

Catwalks to the other towers. At least the swamp grass has been cut this year, it is only four feet tall instead of ten.

The tower bases are all elevated above the theoretical maximum water level. The ATUs are also up on stands with platforms build for maintenance access.

ATU Work "platform" — ATU Work “platform”

I cannot even blame the current owner, who has spend considerable money to make repairs and upgrades to this site. It is very difficult and very expensive to catch up with deferred maintenance. Sadly, most AM stations we encounter have similar or worse problems.

I think it is too late to save many of these AM stations. The technical issues, lack of revenue, perceived poor quality, lack of good programming are all taking their toll. At this point, the hole is so deep there is no hope of ever getting out. The FCC’s faux interest in “revitalization” followed by two years of stony indifference seems to be a final, cruel joke.

Transmitter Haiku

I wrote a little Haiku about Thanksgiving dinner:

Telephone rings
Old transmitter beckons
Dishes get cold

Not exactly the 5/7/5 of a traditional haiku, but close enough. This year, it was the nearly 30 year old Broadcast Electronics FM35A at WEBE. A set of readings from the remote control reveal; zero forward power, zero plate current and 12.8 KV plate voltage. My first assumption was some sort of drive issue; a failed exciter or IPA driver. After starting the backup transmitter and making sure that it was running stably, I spoke with the program director and told him we would be out next morning.

Upon arriving at the transmitter site, I found the BE transmitter had no filament voltage. An obvious clue, I began working backwards from the tube socket until I found this:

Broadcast Electronics FM35A filament voltage regulating transformer

This is the auto-transformer that regulates the filament voltage. Schematically, it is noted as T204 and it is in series with one side of the filament transformer. This one is burned open. The bad news; Broadcast Electronics does not stock this part, it is a special order item, the replacement part costs $2,800 dollars and it will take a few weeks to get here. The good news, after digging through our stock of old transmitter parts, I found an exact replacement:

Replacement part name plate, T204, BE FM35A

We will be installing it on Monday morning.