Tag Archives: tv engineering

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.