No horizontal output from IC, please help

Peter/John Smith: Exactly what are you "seeing" for horizontal pulses coming out of the chip? What does the waveform look like... please describe the amplitude, risetime, and length of the pulses that are being fed to the HDr xstr. It looks to me from your post and listing of your measurements that you might not be using a scope.... you need one for what you are trying to measure..... or take a chance and replace the chip as long as you have checked all the surrounding circuitry including the HDr, HOT,and LOPT.

-- Best Regards, Daniel Sofie Electronics Supply & Repair

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On Tue, 30 Aug 2005 07:08:58 +1000, "John Smith" put finger to keyboard and composed:

If there is a fault in the line stage, or if the B+ is excessive, many sets are designed to go into protection mode. Often you will hear a quick hiss of EHT followed by immediate shutdown.

- Franc Zabkar

All, Thankyou for your replies. BTW my name is Peter and not John Smith (silly outlook setting at the time) I'll reply here to all the posts I got so far.

Donald - I don't think the problem is a dry joint on the flyback trasformer, The joints look OK, and there is no life from the flyback if I prod it.

Daniel - Yes I am using as CRO, I borrowed a 100mghz HP from a friend, but did not spend much time in trying to figure out how to use it. If I knew where to start looking, I would probably spend more time connecting the test lead properly, and analysing. As to your question, the horizontal output from the chip (pin 19, I attached a block diagram of the IC in my original post, but it did not come through) Anyway, the Horizontal output is non existant. Ie = zero.

Franc - I think that your onto something here. On powerup, I do get a brief static pop as if the tube is firing up, but nothing else. Is there some indication to show that the set has gone to protection mode? I don't know what the "line stage" of the "B+" are, but I did find that pin

Anyway, I'll go away and prod the TV some more now.

Thanks once again for your tips & pointers (tups & punters if your a Kiwi :-). Peter peter snipped-for-privacy@engineer.com

voltage

On Wed, 31 Aug 2005 09:14:00 +1000, "John Smith" put finger to keyboard and composed:

The "line" stage is just another name for the horizontal deflection system. The B+ voltage is the main voltage (typically 100V - 150V) that supplies the line stage. You'll find it feeding the primary winding of the flyback transformer. Another way to locate it is to look for the heaviest diode on the secondary side of the switchmode transformer and an associated 160V (or higher) electrolytic cap.

There are at least two common methods of protection used in TVs. One involves sensing the amplitude of the flyback pulse from a winding of the flyback transformer. This looks to be what is happening in your case. A second method senses the beam current via a resistor in the bottom end of the flyback's HV secondary winding.

Locate and measure the test points alluded to above. If the B+ voltage is much greater than the spec, I would suspect any small electrolytic capacitors on the primary side of the switchmode transformer.

The flyback pulse that triggered the protection will disappear, but the B+ voltage will probably still be there.

Most definitely, not to mention potentially excessive X-ray emissions.

If you need to test the B+ supply on its own, you can disable the line stage by shorting the B-E junction of the line output transistor. Otherwise you can desolder its collector or its base. Then wire in a suitable dummy load across the B+ supply. I use a 240VAC 100W incandescent globe.

- Franc Zabkar

On Wed, 31 Aug 2005 20:16:18 +1000, Franc Zabkar put finger to keyboard and composed:

I should have said that another common fault which gives rise to an excessive flyback pulse amplitude is an open or partially open tuning capacitor. This is often a metallised film type (eg 10nF 1.6kV) connected between the C and E of the HOT (horiz output transistor).

- Franc Zabkar