Sharp 21" starts up and shuts down

[Andy]

I am trying to repair a Sharp 20MH10 21" TV with the following
symptoms:

(1) When the power button is pressed, makes a few cracking noises and
shuts down after about one second. The cracking noise comes from arcing
originating from the HV connector at top of the tube. The arcing occurs
inside the tube and reaches a length about twice the diameter of the
suction cup. There also is a blue circular spot on the screen which
gradually fades.

(2) Pressing the power button again results in the relay (that powers
the switching supply and horizontal output transistor) being switched
on for about one second, but the picture tube does not power on.

(3) After unplugging and plugging in the TV again, then the picture
tube powers up again for one second with same behaviour as (1).

(4) I tried powering the TV with the deflection coils disconnected. The
TV remains on without shutting down, but cannot leave it on for long
before one of the resistors in the switching power supply section
starts to burn. I get smaller arcing in the tube, reaching just beyond
the edges of the suction cup.

With my limited equipment (a cheap multimeter), I checked all diodes
and transistors. I also tested the SCR and 9V regulator and some
suspect resistors. I cannot find a single thing wrong with this
circuit.

The only thing that comes to mind now is the flyback transformer or the
picture tube.

Anyone have any suggestions?

 

[Andy]

I have a theory. The output voltage is too high so the HV protection
shuts down the TV. The high voltage on the output means high voltage on
the primary side on the flyback transformer. I measured 205V output
from the switching power supply being fed to the primary.

I don't have much, but a referenced four Sams Photofacts of TVs and
found that none of them had a voltage that high. The largest was from a
large screen TV that was 175V.

So I think that's my problem. I now have to check the switching power
supply. There are some resistors and a potentiometer in the feedback
network which regulates the voltage. I will check them and maybe adjust
them. I will also replace the output filter capacitor first before
adjusting any resistors.