tech stuff – Page 35 – Engineering Radio

The Harris Z6HD transmitter

These transmitters are good. They seem to behave in a mostly normal manner, having a few quirks now and then. This particular unit is installed at WFAS-FM in White Plains NY.

Harrsi Z6HD transmitter, WFAS-FM, White Plains, NY

I believe the reason for the installation was for the HD Radio® that was in it. The Deathstar HD Radio® exciter is in the next rack over. No further comment is needed.

On this day, the transmitter had given up, throwing a main fan fault. The fan (blower) motor had been replaced in the last six months (on a transmitter that is only five years old), so it was not that. As it turns out, the stock fuses; 10 amp, slow blow, were just a little bit underrated for the job. Harris released a service bulletin a few years ago calling for 15 amp slow blow fuses as replacements. In any case, it was an easy fix and now there is a box of 15 amp slow blow fuses in the transmitter next to the fuse holders.

Harris Z6HD transmitter front RF modules

The modules are accessible by the front and rear of the transmitter. These doors can be opened with the unit on the air since all of the high voltage is in the bottom of the transmitter where the blower is located.

The module in the middle is the IPA. Each module has two RF amps, and each RF amp has two devices (BLF-177). The devices are field replaceable, however, on the HD models, one has to make sure that the amplifiers are still linear. On the non-HD models (Z6CD), this is not a problem at all. Shorted MOSFETS will be noted by a fault of one entire power supply. Removing the bad RF module will allow the transmitter to run at somewhat reduced power. Finding the bad module may take a bit of trial and error.

The back of the transmitter has the directional coupler, low pass filter, access to the back of the analog exciter, controller, and remote control connection points.

The power supply at the bottom of the transmitter has multiple taps, each one with its own fuse. These can be a bit of a chore to work on. There is also a ribbon cable that goes from the controller to the power supply board. This is directly in the path of the cooling fan and can flop around causing the conductors in the cable to break. The result is the power supply may not come on or may show an unbalanced power supply condition (in the case of a three-phase transmitter). Very difficult to diagnose.

Harris Z6HD transmitter, on air, WFAS-FM, White Plains, NY

Here it is, running again.

Lightning season

Here in the northeast, there are seasonal variations in the types of weather phenomena encountered. Blizzards in the winter, severe thunderstorms, and the occasional tornado in the summer, at least that is the way it normally happens. This year, we have already had two thunderstorms and a stretch of unusually warm weather. My highly advanced personal weather prognostication technique consists of looking at trends, and the trend thus far this year is warmer with more storms.

When the weather RADAR looks like this, it is too late.

To that end, it is time to go around and check all of the grounding and lightning suppression methods at various transmitter sites and studios. I would rather spend a few minutes extra now than get called out in the middle of the night for an off-air emergency related to a lightning strike.

Proper grounding of all equipment, RF cables, and electrical service entrances is the minimum standard for transmitter sites. Proper grounding means a common point grounding system connected to one ground potential.

To that end, all coaxial cables that enter the building need to have their outer shields bonded to the site grounding system at the base of the tower and the entrance of the building. With an FM station where the antenna is mounted at the top of a tall tower, the coaxial cable outer jacket acts as an insulator along the length of the tower. A lightning strike on the tower will induce a very high potential on the outer conductor of an ungrounded transmission line. After entering the building, the lightning surge will find the next path to ground, which will likely be a coax switch or the transmitter cabinet. Neither of those two outcomes is desired.

Thus, it was time to ground the transmission lines at WRKI, the FM transmitter we moved last January.

Fortunately, Andrew, Cablewave, Dielectric, and others make grounding kits for various size coaxial cables. They are very easy to apply and make a solid connection between the outer conductor and the site ground.

The kit contains a copper band bonded to a ground wire, stainless steel clamp, waterproofing, tape, and a pair of bolts.

The concept of transmitter site grounding is pretty simple and inexpensive to implement. Thus, it is surprising to me how many transmitter sites, especially older sites, do not have adequate grounding. That is an accident waiting to happen.

For more on transmitter site grounding, check Nautel’s publication (.pdf) “Recommendations for Transmitter Site Preparation.”

The old humming console

We are starting to work at a new client’s studios. It is a bit like stepping into a 1980s time machine, as the newest console seems to be the Broadcast Audio console in the FM studio. I feel I should wear a wide colorful tie and part my hair in the middle when working there. There is also an older UMC console in the second production room.

A what?

Exactly.

It seems the UMC console (UMC was a Connecticut-based console manufacturer that was later sold to Broadcast Audio) was having an intermittent hum problem on all the audio buses.

After poking around under the hood for a few minutes, I decided I should begin with the basics. Checking the power supply for ripple seemed like as good a place to start as any. This console has a 30-volt and a 12-volt power supply. The 30-volt supply checked out good, but the 12-volt supply, not so much:

2.7 volts AC on the 12-volt DC power supply. That will put some hum on the audio, all right. I tried to replace the power supply main filter capacitor, but it had no effect. The regulator must also be bad and it is a Motorola part number which is likely not made anymore.

This is a pretty standard off the shelf power supply, I should be able to get one from Mouser for about $60.00 or so for a linear unit, which will be cheaper than us trying to trouble shoot and repair the old one. In the meantime, I took the 10 amp 0-30 volt bench supply and pressed it into temporary service. The console is working again, for now.

At some point, all this old, um, stuff needs to be replaced.

The open delta three phase service

Several months ago, I drove up to an FM transmitter site, looked up at the utility pole, and saw this:

Three-phase open delta is a bad hombre. Most, if not all, transmitter manufacturers will void the warranty of any transmitter connected to a service like this. What is perplexing is it appears that all three phases are available on the primary side, why would this be necessary? Perhaps it was not always so at this location. Regardless, this was the source of power for 20 KW FM transmitters since 1958 until we moved it to a new building last month.

According to a GE publication on transformers, open delta 3 phase power is undesirable because:

Although this connection delivers three-phase currents which are approximately symmetrical to a three-phase symetrical load, the currents flowing in the high voltage circuit are not equal nor are they 120 degrees apart. The maximum safe output of the bank operating in this manner is 58% of a 3 pot Wye/Delta bank. The system is grossly unbalanced, both electrostatically and electromagnetically.

Schematically, it looks like this:

Regular 3 phase delta looks like this:

Most utility companies will not hook up 3 phase delta on the customer side anymore because the “high” or “wild” leg, which as shown in the diagram runs a good deal higher than 120 volts to neutral. Hook up a high leg to a single phase 120 volt piece of equipment and wait for the power supply to blow up. Also true with 277-volt lighting circuits, as my assistant once found out with the Coke Machine in the break room. The new 3 phase service will almost invariably be 208 wye unless there is some very compelling reason, which is fine.

There are many ways to get around three phase open delta, perhaps the best is a rotary phase converter. This piece of equipment will take a 240-volt split phase and add a third leg. These legs will not be 120 degrees apart, as they would be in a true three-phase, however, they will be close enough that 3 phase motors and transformers will be happy.

This leads to an unbalanced voltage/current condition which needs to be accounted for in the design of the unit. The second way to do this is to power a three-phase generator with a split-phase motor. This will completely isolate the 3 phase equipment from the utility service and provide for true three phase power.

The downside to any motor/generator or rotary converter is moving parts and conversion inefficiencies. At any transmitter site that uses this type of equipment, either a backup power converter or a lower power split phase backup transmitter should be installed. With all mechanical things, eventually, this will need to be repaired and it would suck to be off the air while that is happening.

Regardless of any of that, this particular service is about to be disconnected permanently. Good riddance.