Tag: FM transmitters

This is a neat piece of kit, designed to save those late-night/early morning callouts, which is the ultimate goal of all broadcast engineers, or at least it should be.  This seems like a really good idea, however, BE has discontinued the product line, and the last manual update is from 2000.

A small four-port coax switch is located next to the power supply transformer.  This is controlled by the circuit board.  The circuit board senses a loss of excitation by detecting a forward power level below the threshold set on the board. The power sample comes from the exciter forward power remote metering terminals.  Thus, it can be used with any exciter(s) that have a remote forward power sample.

The idea is to use the RF fault function output of the FX-30 (later FX-50) exciter to automatically switch from a faulted exciter to one that is working.  Finally, it can be hooked to a remote control for manual switching.  The unused exciter is muted and routed to a dummy load mounted on the back of the unit.

An alternate configuration would be to route the backup exciter to the backup transmitter instead of the dummy load.  This would create the best redundancy on a limited equipment budget.  It also has a battery bank designed to hold the last state of the unit through a power outage.  As we have a good-sized UPS powering the remote control, STLs, and satellite receivers, the batteries are not needed.

On the face of it, a pretty good idea.  I have had a few exciters fail over the years, which normally means the backup transmitter is placed in service by remote.

I did download the manual, but since it is currently listed on the BE website, it’s probably not a good idea to post the schematic.  Suffice to say, it is a tad bit complicated what with all the CMOS logic and that.  It is very possible to duplicate the functions of this equipment with a simple RF forward power sample and set a failure threshold with a comparator circuit.  Hook that to a small four-port coax switch and a couple of RF mute/un-mute commands to each exciter and: Viola!  Automatic exciter switching!

Perhaps a good rainy day project.

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.

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.

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.

Here it is, running again.