Ringing a LOPT
Now step-by-step example test for shorted turns within a lopt using an oscilloscope
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.
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
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)
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)
As you can see, the scopes waveform now shows how a shorted turn would look. The ring is clearly dampened (see below)
Now lets remove the wire, as you can see the LOPT ring returns to normal. (see Below)
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 at the start of this article.
Other 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.
Using specifically designed equipment
This is quite a sophisticated tester and no my goto device when checking a LOPT. It can not only test out of circuit, but in circuit too. Rather than having to isolate components, allegedly you can test the entire output section for both Valved and silicon stages. Also perform peak-to-peak drive voltage checks. All a little more sophisticated than the Go-No-Go of my Jabco (see below) or a simply ringing with a scope.