Ringing a LOPT

A step by step procedure to test for shorted turns, within a line output transformer.

Note

A very simple but effective “How To Ring a Line Output Transformer Guide”. For those that require much more detail look in the Technical library, there is a good document on this.

For this test I used a known “good” 1957 Bush TV62 LOPT.

lopt1

Step One

The LOPT must be out of circuit to do this test. Connect the scopes calibration point to one side of the LOPT (TC connection), and connect the scope probe to the overwind

lopt2

Step Two

I then set the scope to display just one trace, set Chan 1 to 50mV/Div with the timebase set to 50us/DiV and trigger set to auto. The scope then displays a ring trace of a healthy LOPT (see below)

lopt3

Step Three

Now you’re probably thinking, “ok so that’s what a good LOPT’s waveform looks like, but what about a LOPT with a shorted turn?”, “what would that look like”. To simulate a shorted turn we just need to wrap a single piece of wire around the core of the LOPT (see below)

lopt4

Step Four

As you can see, the scopes waveform now shows how a shorted turn would look. The ring is clearly dampened (see below)

lopt5

Step Five

Now lets remove the wire, as you can see the LOPT ring returns to normal.  (see Below)

lopt6

Hope this helped and as I say it is a quick check but by no means definitive proof. There are conditions when ringing will not suffice, I recommend reading the documentation in the Technical Library.


Real World Examples

Below is an example from a recent Colour TV chassis restoration, a BRC 3000. In this case R907 was very hot, indication that excessive current was being drawn. I tried replacing the Line output transistor but this did not resolve the issue so I removed the EHT transformer and did the ring as detailed here.

As you can see below it revealed a very poor ring, indicating shorted turns.

ring1
new_txs

new_ehtG11 LOPT

loptringdig

 

 

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