Tales of disaster – Page 13 – Engineering Radio

What the inside of a ceramic vacuum tube looks like

In case you have wondered it yourself:

This is an EIMAC 4CX3500A which came out of a Harris HT5 transmitter. As you can see the ceramic cracked in half. When I arrived at the transmitter site, the unit was on full plate voltage, with no plate current, and no overload lights. I figured it might be something with the tube, so I tried to pull it out, but only the top half came. One of those “Ah ha” moments.

Fortunately, there was a working spare at the transmitter site and we got back on the air relatively quickly. That, in and of itself is amazing considering the building that this transmitter lived in. One of those abandoned former studio sites with the transmitter jammed into a back room somewhere. To get to it, one has to dodge pigeons, beware of rats and wade through piles of garbage.

It is a little bit hard to tell in this photograph, but there are two “cages” which are the Screen and Grid. The post in the center is the filament/cathode and the top detached part is the plate/anode. In an FM transmitter, the exciter is coupled to the grid, and the screen accelerates electrons toward the plate and therefore controls the power, the plate collects the electrons and is coupled to the output stages and the antenna. Good stuff.

BE AM6A power supply

Another picture from my collection, this one is the back side of a power supply module from a Broadcast Electronics AM6A transmitter:

It happened during power up from 1 KW to 5 KW and it was quite loud, as I was standing right next to the transmitter. The exploded part is a 0.1 uf capacitor that looks like an add-on. In fact, some of the other power supplies don’t have it. It also took out the 20 amp slow blow fuse.

I like the exploded look of the board, kind of like on The Road Runner, when Wyle E. Coyote looks into a box and something explodes.

This is the only problem I have had with this particular transmitter.

Harmonic Filter for BE FM-30T

Another example from my blown-up shit collection, pictures archive:

Burned out harmonic filter, BE FM-30T transmitter — Burned-out harmonic filter, BE FM-30T transmitter

The harmonic filter from a Broadcast Electronics FM-30T. This actually started in the bullet connector to the 3-inch hardline on the output side of the filter.

Burned out 3 inch hard line section — Burned out 3-inch hard-line section

Again, I did not install this myself, someone else did. Cutting 3-inch hard line is pretty straightforward. When using a field flange, the outer and inner conductors are cut flush. Both conductors should be de-burred and filed smoothly. It only takes a little thing to start an arc with 30 KW of FM power, so once again, attention to detail is key to avoiding these things.

Fortunately, BE sent along replacement parts for the harmonic filter and the line section was replaced.

BE AM5E power supply problem

This is from my burned-out shit collection, pictures section:

It is a power supply from a Broadcast Electronics AM5E transmitter. Here is another view:

Broadcast Electronics AM5E power supply mating connector

As you can see, there was a small fire started in the mating connector for the transmitter wiring harness. I did not install this unit so I have no way to know for sure what happened, but I suspect that the mating connector was not pushed all the way in during installation. In this business, really in all engineering fields, it is the little details that will catch up with you.

I know that one of the stations I used to work at had a fire at their electrical service panel at the FM transmitter site after they installed a new transmitter. This happened after I departed for greener pastures. In any case, it is very important to torque the connections on any service disconnect or circuit breaker to the panel manufacturer’s specifications. I also check the lugs every so often with a Fluke 62 mini IR temperature meter. Any loose connections will show up as hot spots, which can be fixed before the fire breaks out.

All current-carrying electrical connections should be double-checked for solid connections before the transmitter is turned on, then check periodically thereafter for heat buildup and or heat damage.