The Tectrol TC91S-1465 power supply

Also know by its Nautel Part number: UG-39

Nautel V-10 FM transmitter

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.

Tectrol TC91S-1465, aka UG-39

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.

Tectrol TC91S-1465, cover off

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.

Tectrol TC91S-1465

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 an 80% 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.


Installing a WISP on an AM broadcast tower

This is an interesting project currently underway at one of our client’s AM sites.  They have decided to go all in and create a WISP (Wireless Internet Service Provider) for the community around the AM tower.  I thought it would be interesting to explore this topic, as there are not many opportunities for AM towers to lease vertical real estate.

First a few basic ideas.  For an AM broadcaster, (aka medium wave or standard broadcast band) the entire tower is part of the transmitting antenna.  There are two types of towers; series excited and shunt excited.  A series excited tower has a base insulator, like this:

AM tower with base insulator
AM tower with base insulator

A shunt tower usually has a series of wires called a skirt, separated from the tower by standoffs, which go to the top of the tower or nearly to the top of the tower. The base of the tower is grounded, like this:

AM tower with out base insulator
AM tower with out base insulator

A shunt excited tower has distinctive advantages for co-location opportunities in that the tower itself is grounded, greatly simplifying placing additional antennas on the towers.  That is not to say that antennas can not be installed on series excited (insulated) towers, it just requires an extra step of using isolation coils.

In all cases, the tower should have a structural study done to insure that the additional antennas do not overload the tower and cause structural damage or collapse.

In this case, the tower is new and was designed for the extra load.

The plan is to create a sectorized wireless internet system using four 90 degree panels, each with three access points.  A tower mounted sixteen port switch is mounted behind the panel antennas and the switch communicates with the ground mounted router through two fiber optic cables.  A 54 volt DC supply powers the switch, access points and point to point radios mounted on the tower.  There are two fiber runs, one is for subscriber traffic and the other is for radio management.  This system is using Ubiquiti gear.

Ubiquiti 90 degree sector antennas and radios
Ubiquiti 90 degree sector antennas and radios

A word or two about Ubiquiti gear.  Ubiquiti specializes in cheap equipment manufactured in China.  That is a double edged sword.  On the plus side, if anything breaks or gets damaged by lightning or whatever; throw it out and install a new one.  On the negative side, I  have seen Ubiquiti gear do some strange things, particularly after a firmware upgrade.  The newer stuff seems to be better than the older stuff.  All that being said, as this is a brand new operation and seems to be a proof of concept, then the Ubiquiti gear will be fine to start with.

Going up
Going up

The tower crew made quick work of installing the sectorized access points.

Tower crew waiting for equipment lift
Tower crew waiting for equipment lift

Going up the face of the tower, there are the aforementioned fiber cables, the 54 VDC power cable and one backup Ethernet cable.  All of the Ethernet jumper cables used to connect the access points to the switch are UV rated, shielded Cat 5e and use shielded connectors.  This is very important on a hot AM tower.  Due to the skin effect, the shield on the shielded cable protects the interior twisted pair conductors from the high AM RF fields present on the tower.

Transtector LPU 1101-1158
Transtector LPU 1101-1158 Ethernet cable protection unit

At the base of the tower, the DC power cable and the Ethernet cable go though high quality lightning protection units.  These are Transtector 1101-1158 Ethernet and 1101-1025 48 volt outdoor DC power units.  Even though the DC power supply is 54 volts, the 48 volt LPU’s will function adequately.  The TVSS devices used in the LPU circuit are rated for 88 volts maximum continuous voltage.

Transtector 1101-1025 48 VDC lightning protection unit
Transtector 1101-1025 48 VDC lightning protection unit

In addition, I made a service loop on the DC cable with also creates an RF choke.  Several (12-14) turns of cable 18-20 inches (45 to 50 cm) in diameter act to keep the induced RF at the input terminals of the LPU low so the protection devices do not fire on high modulation peaks.  This also helps to keep the AM RF out of the 54 VDC power supply in the rack.

Making ethernet jumper cables, TIA/EIA-568B
Making ethernet jumper cables, TIA/EIA-568B

The backup Ethernet cable has a similar setup.  Regarding the Ethernet cable and induced RF, this station runs 1 KW.  As long as the shielded RJ-45 connectors are applied properly and the tower mounted switch is grounded along with the LPU, then all of the RF should be on the very outside of the cable shield (due to the skin effect).

Base of AM tower with WISP equipment installed
Base of AM tower with WISP equipment installed

This principal also applies to lightning strikes.  Although lightning is DC voltage, it has a very fast rise time, which makes it behave like AC on the initial impulse of the strike. The voltage induced on the shield of the cable will not effect the twisted pairs found deeper within the Ethernet cable.  Of course, all bets are off if there is a direct strike on a piece of equipment.

AM stations running powers more than 1 KW, Superior Essex makes armored shielded cable called BBDG (the new trade name is EnduraGain OSP).  This cable comes with a heliax like copper shield with an optional aluminum spiral armor.  This cable looks very robust.

Enduragain OSP armored shielded Category cable
Enduragain OSP armored shielded Category cable

On series excited towers (those with an insulated base) fiber optic cable can be used to cross the base insulator without any problems, as long as there is not any metal in the cable (armor or aerial messenger).

LBA Group TC-300 tower lighting choke
LBA Group TC-300 tower lighting choke. 180 turns #12 AWG enamel wire on 6 inch coil form.

DC power can cross the base insulator using something called a “Tower Lighting Choke.”  This device is a set of coils wound around a form which passes the DC power but keeps the AM RF from following the DC power cable to ground.  These work relatively well, however, lightning protection units still need to be installed before the DC power supply.

BE AM tuning network

Occasional reader Scott asked for a picture of the inside of a BE AM output tuning network.  I figured it might be helpful to make a short post about it.

These things are pretty simple; a T network with a capacitive leg to ground.

BE AM Output tuning network
BE AM Output tuning network

This particular unit is for 1230 KHz.  I believe the capacitor is frequency determined and they may also use larger inductors for lower frequencies.

BE AM output tuning network schematic
BE AM output tuning network schematic

The inductors are Kintronic LV-15-20 (15uH 20 amp) and the capacitor is 0.0018 uF CDE 6KV 5.6 amp.

The issue with this particular unit is dirt.  The inductors have round metal plates that roll along the inductor coil to make the variable inductor tap.  Dirt has accumulated on the coil turns and on the inside of the plates.  This, in turn, causes arcing anytime the Tune or Load controls are moved.  A through cleaning should take care of the problem.

More AM retuning work

Working on another old AM station, this one is a simple Class C one tower on 1230 KHz.

Broadcast Electronics AM Output Tuning Network
Broadcast Electronics AM Output Tuning Network

The main problem today was this BE AM output network unit between the BE AM1A and the ATU.  This site has had some dirt difficulties over the years and the internal parts of this tuning unit arc at full power.  I attempted to drive the ATU directly with the transmitter, which was a no-go.

Gates Radio 1 KW AM ATU, circa 1947
Gates Radio 1 KW AM ATU, circa 1947

I took a look at the ATU, which is a pretty standard Gates 1 KW ATU from the late forties or early fifties.  I have seen perhaps dozens of these things.

My first thought was that over the years, likely due to changes in the ground system, the base impedance has shifted away from its licensed values.  However, a quick measurement of the base impedance shows it to be exactly at the licensed value, 17.3 ohms.  The tower is 67 degrees tall, so that impedance value is right in the theoretical norm.

I  measured the input to the ATU, which showed 38 ohms with about 7 ohms capacitive reactance.  I can only surmise that it has always been this way.  The transmitter in use before the BE AM1A was a Harris/Gates Radio BC-1G.  That model transmitter will drive anything including an open transmission line.

Retuned ATU input; 49 ohms resistive, 0 ohms reactance
Retuned ATU input; 49 ohms resistive, 0 ohms reactance

Having the bridge on hand, I decided to retune the ATU for a better match. I put the bridge on the input terminals of the ATU and set it to 50 j0.  Using the remote control, I turned the transmitter off and on while making small adjustments to the output strap on the coil until the resistance was 49 ohms with zero reactance.  I would have gotten it to 50 ohms, but the strap on the output side of the coil would not stretch far enough to reach the proper spot on the coil.

Now the transmitter will run into the ATU directly at full power with about three watts reflected.   The BE AM output matching network unit has been removed for cleaning and repairs.  I will reinstall it once those repairs are completed.