Hi A.k,

A flyback need B+ voltage and a pulse to base of HOT. The secondary winding output of flyback also should not have any shorted components. A shorted pc tube also can cause the flyback to malfunction. Flyback is a high voltage device and by checking the internal diode only is not adequate. This means it can checked to be good but fail when under load. If there is presence of B+ voltage and pulse to base of HOT then chances is high the flyback could be the problem. Make sure also the flyback primary winding is not shorted and open circuit.

Jestine

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I was searching "How to test the internal diode of EHT in a CRT TV" and came across your post. This thread had started in 2010 and your last reply was on Dec 2019. I hope you are still available to reply questions. Here is my question with a little history.

I had a very old 21" color TV that didn't have AV inputs. To connect to SET TOP BOX, I identified the video and audio input points on the motherboard. I had also disconnected the internal SMPS and connected external SMPS. It worked without any problem for over two years.

The problem started recently. The picture will suddenly go blank for few minutes and then start on its own. I thought it may have the external SMPS promlem and replaced with new SMPS. However as soon I connected the new SMPS and switched on the TV, I heard a low blast sound and the display went blank. The screen didn't pull hairs on my hand when I touched the screen indicating the there is no anode voltage.

Since then I have checked most of the components and associated voltages and all seems to be OK. Sound part is working. All voltages on the picture tube pins are OK. My last doubt is about anode voltage. As I don't have anode voltage prob, I want to be sure that internal diodes in the EHT are OK.

Instead of two analog meters, I had used a 21 volt DC source to check the voltage between EHT's 7th pin and the anode. This voltage is about 4.5 volts. Reversing the meter leads gives no reading indicating high internal resistance.

What conclusion can I draw from these readings?

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Hi Patrick,

Probably arcing in the flyback. You need to open the TV and check.

Jestine

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HI Yahiya,

Check if there is waveform going into the base of HOT.

Jestine

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Hi James,

Have you checked if the B+ voltage to the flyback good?

Jestine

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Hi Virgilio,

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https://www.jestineyong.com/resources-2/

Jestine

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