CRT TV picture problem

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tpny

Joined May 6, 2012
220
Hi my crt tv's picture went black. Sound is ok. On screen display channel numbers and volumn bars are scrunched together in a line midway down the screen. Is it a vertical deflection problem?.. Please see attached picture. Any advice, comment is most appreciated.
 

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vk6zgo

Joined Jul 21, 2012
677
You could have lost the "boost HT" which is generated by an overwind on the horizontal output Transformer.
It usually looks like about +160v.

If it is "a bit less" (around 100v) when you look at it with a DMM,it is still can be a faulty filter cap on that supply.

I had this problem with my Mother-in-law's TV,& didn't have an Oscilloscope.
The DMM (Fluke 77) saw the unfiltered 15kHz halfwaves as DC of a lower voltage.
 

absf

Joined Dec 29, 2010
1,968
It can also be a bad/open vertical height preset.
Or a bad service switch OR it is in the wrong position.

Allen
 

tom_s

Joined Jun 27, 2014
288
my money's on a dry/cold joint

if you tap the tv on the side (NOT hard), does it come back up?

edit: yes, the tv has to be on and looking at the screen while manipulating :)
 

MaxHeadRoom

Joined Jul 18, 2013
28,576
As with the majority of problems in any circuit, a schematic is required and LOGICAL fault finding using a meter and/or 'scope.
Anything else is pure speculation!
Max.
 

MaxHeadRoom

Joined Jul 18, 2013
28,576
I would start by taking voltages around the TA8427 IC vert output, the output side is 14.5dc with a 55v peak pulse to drive the vertical scan coils, if no 14.5 check the vert fuse.
It also helps if you have a scope, look for leakage in the electrolytic caps in that stage.
Also in the Troubleshooting it shows primary points to test for vert problems.
Max.
 
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ian field

Joined Oct 27, 2012
6,536
I would start by taking voltages around the TA8427 IC vert output, the output side is 14.5dc with a 55v peak pulse to drive the vertical scan coils, if no 14.5 check the vert fuse.
It also helps if you have a scope, look for leakage in the electrolytic caps in that stage.
Also in the Troubleshooting it shows primary points to test for vert problems.
Max.
I'd have a look at D301 - it is after all; a flyback output - and I think that's what D301 does.

Switching speeds and voltages are a bit milder than in the horizontal flyback, but I'd go for something a bit quicker than a 1N4005. Frame rate is only 50/60Hz but the retrace is faster than that.

Once or twice I've been caught out by V-out diodes - they can check OK and still not work. Test by substitution is best.

Many V-out chips have a high voltage headroom for the sawtooth generator to improve linearity, usually 30V or more - that means probably a high value resistor lurking somewhere just waiting to go high or open.
 

ian field

Joined Oct 27, 2012
6,536
D301 appears part of the vertical phase detection signal sent to the base QA14 in the video section.
Max.
Nope - it goes from the fuse on the 28V supply, pretty much where I'd expect a V-flyback diode to be.

The 220uF cap on the cathode side is probably the bootstrap for the O/P stage bias chain - that can cause trouble too, but probably not down to a fuzzy line.
 
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