EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: JohnnyMalaria on June 10, 2018, 06:38:33 pm
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Hi,
My primary (and only) TV is a 1997 27" Sony Trinitron. :)
There's a problem with the red channel. It seems to have become non-linear in its response such that colors with lots of red look okay but other colors take on a greenish/bluish/greyish appearance.
Attached is a still image I found on line for a film I'm watching right now and a version where I have modified the red response curve so that it looks more quadratic than linear. The third is a photo from the same film as seen on the TV. The white balance of the latter isn't right and should be more like the second (modified) one.
Any ideas if this is the gun failing or simply a discrete component that needs replacing?
I really do not want to get a new TV. I have yet to find a modern TV less than 40" that I can stomach to watch due to all the dreadful artifacts. Plasmas are great but are no longer made and used ones are too big. And temporal resolution on digital TVs sucks.
Thanks.
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The symptom may be caused by a dozen reasons.
If you can find a proper PAL/NTSC signal generator
you **MUST** check the proper DECODING of the RGB matrix first.
(aka DEMODULATION)
Then you pass on to the video AMP and finally think the CRT.
WITHOUT the proper bar signal demodulated it will not be easy to guess
UNLESS someone else had this very same issue.
You need one of this HANDY Function Generators to have the correct outputs
More precisely the PAL and NTSC CHROMA signals
Paul
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Thanks. I have a few DV and DVCAM devices that can generate NTSC color bars. I'll start there.
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list the make & model number so a schematic can be located.
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It may help to have the output of the DEMODULATION matrix
with a proper BAR pattern
But it may be easier with a device that can SHUT OFF EACH COMPONENT
e.g. a proper PAL/NTSC generator can manipulate each component per si
that helps a lot in case of complicated issues with the delay line
Nowadays these old generators may be found very cheap
as they are not very used in digital sets without the CRT and the demodulation matrix
Paul
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Flip the service switch and set the bias for each gun, then set the individual drives or screen voltages, depending on the circuit.
This adjusts offset and gain for each colour. It's something you have to do constantly when fixing CRT TV's.
I'd start there before troubleshooting the electronics.
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list the make & model number so a schematic can be located.
It's a Sony Trinitron KV27S22 (NTSC)
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I found a service manual for a KV27S42 (I think) and also my model. So I can get into the service mode which is fun to rummage around in. There's an interesting difference. The '42 has cut and drive settings for R, G, and B but my model only has them for G and B. However, the manual is pretty clear about where the appropriate test points are to hook up a scope and it givesthe adjustment values for waveforms from different test signals.
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The likely causes of colour loss are as mentioned are many. Was the failure sudden or did it gradually change? The following is a short list of several possibilities.
1. Worst possibility is the relevant CRT colour cathode emission is failing.
2. Video driver failing. Check the video driver collector voltages with a meter for similar readings on no signal and a neutral coloured, best guess, full brightness picture that you can find. Try several pictures to get a ball park figure although a scope will tell you more.
3. An IC earlier in the video chain is losing an output. Again a scope will show all.
4. A Coupling capacitors in the video chain has ceased to be a capacitor. This does happen despite being the low voltage part of the set.
5. Capacitors associated with the 'cathode/gun area' go leaky.
Hopefully the problem is not number 1.
Best of luck.
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Yep the CRT will be hard to find a match. Possible but likely to be a used tube
If you are in short of cash to buy a proper PAL/NTSC generator
and you want to give a long shot...
You may try the following:
- ARRANGE a RED PATTERN - no matter how
- it may by a recording of some red surface or digitally generated red image still
- FEED THIS RECORDED RED IMAGE on the RGB ONLY (pure video) input
- DO NOT USE the YPx entries as they are modulated carriers
- USE THE PLAIN RGB DEMODULATED INPUT - RED ONLY
- now you can measure the OUTPUT of EACH VIDEO AMP directly in the TUBE
- the tube collar neck usually holds 3 AMPS - 3 BJT transistors - which drive each gun
there you will have e pretty good response - once is PURE RED - only ONE GUN
should be active and a pretty flat response is required.
a CHEAP LONG SHOT - but will give you answers.
And mainly if the RED GUN is working properly
That would also be achieved by the Generator using a "RED" pattern
Paul
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Thank you.
It only has composite input.
I can create digital images and use my DSLR to send a composite signal to the TV.
The service menu allows each gun to be turned on/off individually. One of the tests in the manual is to turn off B and G and look at a color bar with a scope. It should give a staircase with equal height steps.
Full saturation reds look fine. e.g., the on-screen display has all of the primary and secondary colors and they look good as do full saturation reds on my satellite receiver's display.
The problem seemed to develop over a period of a week or so. The first symptom was when the picture abruptly changed to a bright, full screen image (like a graphic in a news broadcast). The brightness would drop slightly after a second or two. In the case of white, it would start slightly orange then become white. I noticed the red issue a week or so later. Everything remained that way for about 6 months until last week when the problem worsened.
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Then record a CLEAR RED ONLY SIGNAL - use a digital still RED (RGB)
record that and plug as composite.
It will be DEMODULATED but you WILL have as much as possible
only the RED GUN active. The better the RED the better results
Just like so to G and B in case
Paul
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A weak CRT is very likely. Sony CRTs were never known for long life, even when these sets were not that old.
You can measure the voltage on the three cathodes (lower voltage = higher cathode current). If the voltage on the red cathode is higher than the other two, it might be a drive signal problem. If it's lower, then it's probably already driving the red as hard as it can. I believe that model has automatic CRT bias (AKB), so it will automatically adjust the drive signals. The AKB circuit will blank the picture if it can't achieve balance. Does the picture blink on and off at all? That's a a classic symptom of a bad CRT. Turning up the screen (G2) voltage might help for a while.
You could try swapping the drive signals between the red and blue (or green) cathodes on the CRT socket with some jumper wires. If the red is still weak, then it's the CRT.
If you really love the set, most Sony 27" CRTs are interchangeable, so finding a donor shouldn't be hard.
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One more tip is that you can rule out a color decoding error by turning the color control all the way down, or better yet, connect the Y output from a DVD player to the video input jack. Looking at a B&W picture is far more revealing in this case. Don't even bother with color bars until you know you have a good B&W picture.
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If you happen to be in the Seattle area I may have a 27" Sony CRT you can have, there's a mid 90s set in the shed over at my mom's place with a bad flyback that I harvested some other parts from years ago. The pins on the tube got bent up but I think the vacuum is still intact so I could try straightening them. I've been tempted to throw it out for years but I just hate to throw away a good CRT.
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Unfortunately, it sounds likely to be a worn out CRT, based on many years doing TV repairs back in the CRT era.
The red gun was generally the first one to go (especially on Sony Trinitrons), because the lower efficiency of the red phosphor required a higher beam current on the red gun compared to green and blue. If it hasn't worn out too badly, an adjustment of the red gun drive/bias controls might improve it.
You MIGHT be able to wring a bit more life out of it by installing a "brightener" into the CRT heater circuit. This is basically a small step-up transformer that increases the applied voltage, running the cathodes hotter than normal, and (temporarily) increasing electron emission from worn out cathodes.
https://www.suburban-electronics.com/display/SEW-123/CRT-BRIGHTENER (https://www.suburban-electronics.com/display/SEW-123/CRT-BRIGHTENER)
After installing the brightener, you will need to adjust the RGB drive and bias controls to get a proper greyscale balance.
Another possibility is to get hold of a CRT tester/rejuvenator (used units at hamfests or on eBay sell for next to nothing these days), and give it a shot. This isn't without risk of destroying whatever remaining emission you have, so if you go this route, start with the mildest "cleaning" function, and only use the "restore" or "rejuvenate" as a last resort.
The real cure is obviously a new CRT. But NOS tubes are becoming hard to find, and all the rebuilders have shut down.
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My experience is that Sony tubes do not respond well to brighteners and rejuvenators. I remember brightening a 21" Trinitron computer monitor about 20 years ago by adding another turn of wire around the flyback core and putting that in series with the heater. It looked great for a couple hours but then rapidly started dimming back down.
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where in usa?
I'm in Maine, and been tripping over a Sony xx inch, that I had kept dormant for 13 years. Very little use. I kept it around for possible analog uses, became an ISF calibrator, so wanted to keep a crt phosphor reference.. Component/composit inputs, no hdmi, vertical squeeze for amathromorphic sp? tapes.
Pay shipping and logistics (box, etc) and is yours.