Tag Archives: comark

The Focus Supply

In a high powered IOT type transmitter, there is a very important component to the operation of the tube. It is known as the Focus.

The focus’ job is to help focus the beam into the collector inside the IOT, and to prevent scatter. This is done with a very large electro magnet, known as a “Focus Coil”.  The Focus Coil is similar to that which is in your CRT TV. To power this coil, it uses a large DC power supply, known commonly as a focus supply. About two months ago, we had an issue where one of the tubes would not turn on, after a power hit. I drove out to notice the focus supply breaker was tripped. It reset, and everything went on its way. The next power hit we took, the same breaker tripped. OK this time it was time to take the supply out.  This supply is 1998 vintage to the first digital transmitter out here.

Focus supply with the cover removed and the power supply taken out.
Focus supply with the cover removed and the power supply taken out.

So on the bench it goes, and time to pull out the actual supply part.

The actual power supply
The actual power supply

Upon removing the supply, something obvious comes to view to me.
The three large capacitors look discolored.

A closer look at the discolored caps.
A closer look at the discolored caps.

You can see how dark they are. So I marked them for the polarity, and removed them. As you can see from this removed capacitor, that these are definitely shot.

Blown capacitor
Blown capacitor

(Please don’t mind the dirty sink).

Fortunately, I had already ordered these caps as replacements, so I had the three brand new ones standing by. Popped them in , cleaned up the terminal strip…

Running supply.
Running supply.

As you can see she’s purring like a kitten…

Operating transmitter with the repaired supply.
Operating transmitter with the repaired supply.

And the transmitter is running smoothly with the new supply.

No more tripped breakers.

The Tale of the “Watchband”

When you are dealing with any medium or high level RF transmission system like we have, it is good practice to have a “line sweep done”. A trained technician connects up equipment to the transmission line to measure the line impedance and a TDR (Time-Domain Reflectometer) to determine if there are any “fail points” in the transmission systems. The TDR can determine if any point in the transmission line is starting to degrade. In just about all transmission systems using rigid line, there are many connection points in the system. Where the pieces connect to each other, and when they bend, in “elbows”. The lines are fabricated with an internal conductor (a “center conductor”) that is usually made out of copper pipe. This is the pipe that is the conduit for the RF energy, with the outer being the shield, like in any coax. This internal copper pipe is connected with a device known as a bullet.

A sample picture of a bullet. Image courtesy ERI.
A sample picture of a bullet. Image courtesy ERI.

This bullet is designed to connect the two pieces of pipe together. It makes the connection to the pipe using a small wirewround spring contact, known as a “Watchband”. This watchband allows the bullet to move within the pipe (remember these pipes are outdoors, so there is expansion and contraction with weather).
Anyways, when we had the recent inspection, our inspector found an anomaly 637′ up the tower. So we had a crew come in and “break open” the transmission line. At the piece where the line was broken open, we found one of the watchbands had become dislodged and deformed.

A deformed and broken watchband spring from 4" coax.
A deformed and broken watchband spring from 4″ coax.

This deformation causes poor contact and can even cause arcing. The watch band was replaced and the line was reassembled. Because this line is our backup, I had to wait until Saturday to “test drive” it.

Main (left) and AUX (labelled "V4"). V4 is on the air, main is into the dummy load to keep the tubes warm.
Main (left) and AUX (labelled “V4”). V4 is on the air, main is into the dummy load to keep the tubes warm.

One of the luxeries of working where I do, is we have two separate transmission systems. The primary puts out around 40kW., the backup puts out around 10kW. The two systems are 100% separate. Separate transmission lines, RF sections and antennas.

Our primary, top left, standby white hanging down, bottom left.
Our primary, top left, standby white hanging down, bottom left.