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?
- posted
17 years ago
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).
In the old days you only had to wait for the CRT heater (and longer ago - a set full of tubes) nowadays you have to wait for the front panel micro to boot up!!!
O.k. so what conclusion could one draw if this warm up period is taking very long (15 minures)?
On Mon, 06 Nov 2006 12:51:33 -0800, z Has Frothed:
Hard to tell from here. Let me get my crystal ball and I'll get back with you.
-- Pierre Salinger Memorial Hook, Line & Sinker, June 2004 COOSN-266-06-25794
What sort of TV is this? It could be that the CRT heater supply is low, I've seen this caused by dried up electrolytic capacitors. If it's a Sony TV, the CRT may be aging, causing the AKB circuit to blank the picture. Unless you're experienced working with electronics there isn't anything you can do aside from take it to a reputable tech and let them have a look.
How do you define warming up?
And the degauss thermistors.
-- pebe
When I switch the tv on in the morning (after it cooled of all night) it takes about 10 minutes until the picture comes on. (the voice does not have this delay) and this delay has become worse in the last years. It's not a Sony it's Panasonic tx-w28r3
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.
Examine the CRT PCB and see if there's any small chokes in series with the heater - if so replace with wire links and see if the CRT warms up any quicker.
In most sets today there is what you call an IK curcuit, what this does is let the guns come up to snuff equally. It keeps the picture of till they are of equal value so you don't get and of color picture.when the CRT starts to go south or weaken it takes longer. sometime if you crank up the g2 on the flyback ,just a slight tad, it will force the picture to come on sooner. but is sounds as though your crt is getting weak
Could be end of life for the tube, or a problem with the heater power supply.
That eliminates many other possibilities.
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.
Fire up the set with the back off and watch the heater in the tube.
Not likely. :)
Does the CRT heater come on immediately? You may be able to see it at the very back of the tube through the ventilation holes.
--- sam | Sci.Electronics.Repair FAQ:
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Based on what you have just said, I would have to venture the opinion that you do not have sufficient experience to be working on dangerous items such as a TV set with a switch mode power supply, and any further encouragement that we might give you to carry on trying to get to the bottom of your problem, is likely to result in injury. In view of the fact that you get sound immediately, it is unlikely that the problem is one of the psu being slow to start. Please just take it to a reputable repair shop.
Arfa
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?
The idea of this ng is for hobbists to try to fix, seems to me you are forgetting the reason for this ng. I am not going to do anything stupid, all I want to do is spray a bit of freezer spray to locate this problem, I am not even going to touch any connection, so what can go wrong? I will not take it to a repair shop, if I can't fix it I will simply buy a new one.
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