Proton WT-3650 intermittently bright crt.

[David Farber]

This Proton WT-3650 television is about eleven years old. The CRT has a 16X9
aspect ratio. The symptom was that the power relay would just click, click,
click, upon power on. I found and replaced two shorted diodes in the
switching power supply. I also checked for cold solder joints and bad caps.
None of the caps was even close to being out of spec.

Now the picture and sound work fine but occasionally there will be a bright
flash on the screen. As it warms up, it gets worse. Sometimes the picture
will just come on as though the screen voltage were turned way up and you'll
see retrace lines. I've tapped and jiggled all the boards but can't seem to
find anything that will make the problem appear or disappear (depending on
which state it is in) with a tap of a screwdriver.

Does this sound more like a CRT problem, an intermittent flyback, or
something else?

Thanks for your reply.

 

[Art]

What effect do you get when tapping lightly on the CRT neck or on the actual
CRT Board? Possible intermittant short within the CRT or maybe just falty
connections on the CRT Drive Board.

 

[David Farber]

What effect do you get when tapping lightly on the CRT neck or on the actual
CRT Board? Possible intermittant short within the CRT or maybe just falty
connections on the CRT Drive Board.

No change when tapping the CRT or drive board. I checked the G2 screen
voltage and when the picture is good, it's about 340 volts, when it goes
bright, the voltage drops to about 300 volts. I first checked the voltage
with my x1K probe because I didn't know how off this measurement was going
to be. Then when I switched to my regular probes, it measured less than half
that voltage, about 140 volts. I checked the calibration of the x1K probe
and it was accurate to 100 volts which of course makes comparing these
readings almost meaningless. More interesting than the actual numbers is
that when using the regular probes, the screen got brighter just by checking
the G2 voltage. So this must be a very high impedance source or maybe this
is an indication of what is wrong.

Thanks for your reply.

 

[David Farber]

(depending



No change when tapping the CRT or drive board. I checked the G2 screen
voltage and when the picture is good, it's about 340 volts, when it goes
bright, the voltage drops to about 300 volts. I first checked the voltage
with my x1K probe because I didn't know how off this measurement was going
to be. Then when I switched to my regular probes, it measured less than half
that voltage, about 140 volts. I checked the calibration of the x1K probe
and it was accurate to 100 volts which of course makes comparing these
readings almost meaningless. More interesting than the actual numbers is
that when using the regular probes, the screen got brighter just by checking
the G2 voltage. So this must be a very high impedance source or maybe this
is an indication of what is wrong.

Thanks for your reply.

The problem turned out to be an intermittent NPN regulator transistor, Q901,
BC547. It is located on the CRT board. It supplies a low voltage to the
emitters of the RGB drive transistors. The voltage drop across the B-E
junction went from .6 volts to .9 volts and that's what tipped me off.