Engineering Radio

In the previous post, the issue with the WVOS-FM transmitter was detailed: The PA feed through/bypass capacitor had arced to the PA cavity causing lots of unwanted off-air time. When I went to order the replacement parts, of course, they were not available. It seems that Broadcast Electronics changed the design of its transmitters in the late 1980s to use a different feed-through arrangement.

They were nice enough to send us a nifty retrofit kit; BE part number 959-0272 which replaces BE part number 959-0115.  If interested, the six pages of installation instructions are available here, for your reading pleasure.

The retrofitting itself was quite the job; drilling six mounting holes and one one-inch feed through hole in the PA cavity, mounting the new feed through housing, rewiring the high voltage connection to the tube and back to the HV bleeder assembly, etc.  What with all of the drilling, sawing, filing, deburring, and whatnot, I began to wonder if the transmitter would ever run again.  This is the transmitter before the modification:

This is the old high voltage feed through hole, arc mark clearly evident.

This is the modified feed through/bypass configuration.

While doing this work, I removed the tube and put a plastic sheet in the bottom of the PA cavity and around the HV parts at the bottom of the transmitter.  Somehow, getting aluminum filings in the tube socket seemed like a bad idea.  I also thoroughly vacuumed out the entire transmitter once all of the metal work was done.

I removed the Kapton capacitor plates from the old feed-through arrangement and reinstalled the Teflon insulating plates to keep the air flow out of the tube cavity going in the correct direction.  The new capacitor looks very beefy, perhaps it will never fail again.

Once the installation work was done, I brought up the transmitter first with no screen and no connection to the tube anode.  Then with the tube connected, and finally with the screen supply turned on.  The tuning needed a brief touch up but all in all, the transmitter came up and ran well with the new feed-through arrangement.

Alternate title: More blown-up stuff

This Broadcast Electronics Fm 3.5A will be thirty years old in April. We should have a party!

Unfortunately, this transmitter is not doing too well these days. The PA high voltage feed through capacitor has arced over to the PA cavity, causing the station to be off the air.

Naturally, this happened over the weekend, parts will not arrive until Tuesday at the earliest, and the station is without a backup transmitter.

Obviously, trouble shooting this was a two-person job.  Never work alone on HV equipment.  The symptom was the main circuit breaker was tripping after the HV on command.  Starting from the transformer end of the HV power supply circuit and working toward the anode of the PA tube, all of the components were tested by isolating each component then turning the HV on.   Special care was taken to discharge all components after each test.  The capacitors and bleeder resistors were reconnected at the same time.  There is too much risk involved with charged 8 KV capacitors and no way to bleed that charge to ground.  Everything worked up until the PA cavity was reconnected (without the tube), then the breaker tripped again.  Thus, the above feed-through capacitor was removed and disassembled, revealing the damage.

The question is, how long should transmitting equipment last? After all, if one were running a freight delivery company, you would not be driving around in thirty-year-old trucks, would you? No, not if you wanted to stay in business. Like all electro-mechanical equipment; transmitters, consoles, STLs, antennas, computers, etc wear out.  A smart plan would be to have a replacement schedule and be putting money into a capital equipment replacement fund.   Equipment life varies with the type.  Getting twenty years out of a main transmitter is a pretty good service life, going beyond that is pressing one’s luck.  Ten years on any one computer is a very long time.  Then there are certain transmitter manufactures that drop support on older units, which makes it difficult to keep them operating.  Owners and managers need to be cognizant of the age and condition of critical infrastructure.  As field engineers, how much time do we devote to keeping antiquated equipment running, or should we even be servicing it at all?  As independent contractors, we incur a liability whenever we touch something.  Where does the ownership’s responsibility lay in providing safe, functional equipment for their stations?  All interesting questions.