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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

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Old 24th Dec 2006, 12:32 pm   #1
ballistic
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Default Sony Trinitron KVD2912U (CRT?)

When I adjust the screen voltage to reduce the flyback lines, the TV takes longer and longer to display a picture when you turn it on... to the extent that if you get rid of the lines completely, the picture never appears.

Not good.

I'm hoping some other magical control can compensate for this??

Last edited by Mike Phelan; 15th Oct 2007 at 12:02 pm. Reason: More meaningful title
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Old 24th Dec 2006, 6:40 pm   #2
dsergeant
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Default Re: TV adjustments - another quick NOOb question

Symptom of a worn tube. Most sets monitor the beam current during the turn on period and wait until it has reached a certain value before unblanking the video. This data is also used for black-level correction on the three guns.
As the tube ages and you have a weak gun the beam current on that gun will never reach the turn-on point. If you monitor the cathode voltages with a scope you will see the weak gun sat at a much lower voltage than the others - the correction has to set it at a lower level to get the same current.
Adjusting the screen voltage allows you to increase the levels up to a point where it will come on. Up to a point the black-level compensation will result in a reasonable colour balance until the weak gun can be compensated no more.

Only cure I am afraid is a new tube (or you can try rejuvenating it with mixed and unpredictable results.)

Dave
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Old 25th Dec 2006, 11:01 am   #3
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Default Re: TV adjustments - another quick NOOb question

Spot on Dave. The 2912 suffered from a very short tube life. Many of these sets were scrapped after only two years when the 29" tube rapidly became low emission. I have never seen a 'full emission' picture on one of these sets!
Regards John.
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Old 25th Dec 2006, 7:41 pm   #4
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Default Re: TV adjustments - another quick NOOb question

The 27" Sony tubes seemed to have a decent life. A few of the very early ones suffered a similar problem but this was sorted out.

The 29" tubes did have a problem...I used to pull 2912s off the tip weekly and every single one, without exception, had a flat tube. I gave up trying to find a decent one and that was probably just as well due to the incredible weight of the set.

I never managed to disable the blanking circuit in these models. I have no doubt this could be done. It will be interesting to see if anyone comes up with the answer.

Regards John.
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Old 25th Dec 2006, 11:52 pm   #5
Danny
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Default Re: TV adjustments - another quick NOOb question

Hi
Shouldn't say this, but short the heater series resistor out, and it will usually come on but it will look sick but will prove the fault.

I've had other Sonys with this fault, including some of the newer wide screen sets. Not saying they are all bad but certain ones do suffer.
I remember these tubes had poor life expectency - you will not be able to reactivate it, though.
I've tried all methods, and they all failed to come up.
The resistor is a safety device so don't leave it shorted and sell the set.

Danny
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Old 2nd Jan 2007, 12:58 pm   #6
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Default Re: TV adjustments - another quick NOOb question

Hi
If you want to prove the point (for your own interest only), connect three high-value resistors (say 100k) to each cathode on the crt base and connect the free ends together. Then you can sit back and watch a muddy green picture!
I agree with Danny - rejuvenation just doesn't seem to work with these Trinitrons, and shorting the resistor out offers little improvement.

Sony didn't seem too bothered - when Hitachi brought out a duff tube in 1980 - the 'instavision' directly heated cathode on the CBP220 - they were replacing them FOC for six years - and we got paid for doing the job!

Glyn
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Old 2nd Jan 2007, 10:04 pm   #7
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Default Re: TV adjustments - another quick NOOb question

Hello again,
I am working from memory here but I think the tube heater supply to the Sony KV2912 is derived from the line output transformer in the normal way but is rectified by a small diode mounted on the tube base but not smoothed.
This ‘half’ AC/DC arrangement appears to have an odd effect with some valve and crt heaters resulting in them dropping less volts than is required as they age, more so if supplied at line rate.

Many years ago [1965] Ferguson produced a 16” mains portable using a special ‘folded’ version of the very popular 950 chassis. In an attempt to reduce dissipation four valve heaters were wired in series with the H.T. supply. The set had a strange warm up sequence producing a raster and then slowly as the I.F. and video output valves warmed up, a picture and sound.

As the set aged the warm up time became longer and longer finally resulting in a service call. Observation would prove that the four valves in series with the H.T. were hardly glowing at all. The only way to get it working was to replace all four valves but these would work perfectly normally in a conventional series circuit.
Ferguson recalled huge numbers of these for modification to normal mains dropper configuration.

I don’t know the tube heater voltage for the crt used in the Sony but would imagine it to be the usual 6.3v. It would be interesting to disconnect the heater pins on the tube base and feed it separately with a 6.3v supply and observe the results.

I never managed to improve the crts by reactivation. The cathodes were probably well poisoned by then. I would add that some of these sets were very clean and appeared to have had little use. The 21” version also had a somewhat premature life, but nothing like as bad as the 29” version.

Regards John.
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