"z" wrote in message
oups.com...

James Sweet wrote:
z wrote:
ian field wrote:

"z" wrote in message
oups.com...

boardjunkie wrote:

z wrote:

Anyone knows which circuit in a tv set is resposible for the
brief delay between switching the set on and the appearance
of an image on the screen?

Tube cathodes heating up via the filament (heater).

O.k. so what conclusion could one draw if this warm up
period is taking very long (15 minures)?


One possible cause is high value start up resistors in the PSU going
very
high, this can mean the PSU doesn't start immediately but can be got
going
by a random mains spike.

Some designs use the charging current through a small high voltage
electrolytic to kick-start the PSU.

Another capacitor related problem is whichever electrolytic holds the
PSU
error feedback voltage goes high ESR giving a false excess voltage
error
voltage - this causes the PSU to idle at too low O/P to start the set -
in
both capacitor related faults, the capacitor in question gradually
warms up
causing the ESR to decrease, at some point the capacitor may then get
close
enough to normal operation for the set to start.


O.k, this is useful, I will spray some freezer spray to see if I can
locate a faulty electrolytic or resistor. Which area of the chasis
should I look at? The power supply area is it near where the ht cable
which goes into the crt (the suction pad at the other end), there seems
to be one very big electrolitic there rated at 400volts and quite a few

smaller ones near it. The other circuits are for example the one on
the neck of the tube, and several circuits are inside metal boxes
(like the tuner).



Turn off the lights and watch the neck of the tube, does the heater glow
appear normal after 10-15 seconds or does it take a while? If the heater
is glowing and still no picture, you can eliminate the heater circuit as
a culprit.

Yes, it glows after 15 seconds, this seems fine, btw which is the
heater circuit, is it the one on the neck of the tube?


If you need to ask a question like that, you probably shouldn't be messing
with the potentially dangerous voltages around a CRT!

The heater is supplied by two of the pins in the connector on the end of the
tube neck, and in a TV usually gets its power from an extra winding on the
line O/P transformer.