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

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, ...

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.

... but I did find that pin
20 on the chip is called the
"High voltage detection/shut down input", so I could trace back through the
schematic to see where it goes (I guess).

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.

But how can I test for excessive voltage?

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.

Correct me if I am wrong, but if the chip goes into protection mode, then
the excessive voltage is likely to be shut down as well?

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

I could, disconnect pin 20, but don't I then run the risk of frying more of
the set?

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

I remember reading an article where someone used an auto transformer to
bring the mains up slowly,
but I don't have one, and it's not likely to work on a switch mode power
supply anyway.

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
--
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