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|12/12/2005 6:55 AM|
|bob p||Is it Possible to Buy a Decent Small/Medium CRT TV?|
Well, the TV in my kitchen finally died after 18 years of daily service. Because the TV sits on a room divider between the kitchen and living room, I'm looking for another small/medium CRT TV of about 19-20 inches. Its amazing how crappy the selection has become.
My criteria for a kitchen TV aren't all that stringent -- since we primarily watch things like the Weather Channel, cable news shows, or more of the TV Guide channel than I care to admit, I'd like to buy a TV that has decent geometry. I had no idea how difficult this would be.
Maybe I'm just spoiled. My "old" TV that has been in service since 1987 had a nice (aligned) "square" picture. When text or graphics was displayed onscreen, the text was clearly readable. That's not the case with the new TVs I've tried.
For some reason, just about every new CRT-based TV I've encountered has had one if not two of 3 problems:
1. Oversized screen image, where the picture size exceeds the physical limits of the CRT and the picture is clipped to the point that portions of the image are cut off of the screen. This especially noticable on channels that display scrolling text.
2. Image Tilt. Not minimal image tilt, as you'd expect when rotating a TV to face East or West, taking it out of neutral alignment with the "standard" position for calibration in the earth's magnetic field, but really bothersome tilt. Scrolling text gets clipped on one side of the screen but not the other.
3. Trapezoidal Distortion where a rectangular image is wider on one side of the screen than it is on the other.
Now I realize that no TV is going to be perfect, but I just can't understand why the current crop of TVs is so glaringly bad. Sure, I'm used to a couple of high-end TVs (35-inch Mitsubishi CRT and a 40-inch Hitachi rear projection TV), and with those you can expect to get what you pay for. But the 20-year old el-cheapo 19-inch TV that I'm replacing didn't have any of the glaring problems that the entire new crop of TVs appear to have.
Is the problem just that all of the current crop of TVs are being made in China, where QA is just non-existent? I mean, for these sorts of problems to exist, somebody just has to be doing a sloppy job of setting up the TV in the factory.
Images that are way oversized are probably caused by somebody doing a half-assed job setting the image display trim pot. Ditto for image tilt -- somebody's just doing a bad job at putting on the deflection yoke, or doing a half-assed job at adjusting the tilt trim pot (if the TV has one). (I have to admit, I don't know where the trapezoidal distortion is coming from).
I understand the logistical problems related to manufacturing a TV in the southern hemisphere for use in the northern hemisphere that relate to image tilt, but TV manufacturers are supposed to be able to apply magnetic field correction in the factory to correct for this.
Regarding oversized images, I understand the desire to provide 100% screen utilization because consumers don't like black borders around the screen. But one would think that they'd like clipped images even less.
The frustrating part of my experience is that I've bought 2 TVs in the past 2 days, and I've ended up returning both of them because they were so bad out of the box. Is this a hopeless cause?
|12/12/2005 5:05 PM|
Last year I bought a 24" Toshiba. I forget the specific model, but it's a flatscreen CRT and I think it's really nice. Iirc, there's a 20" version of it.
http://www.tacp.toshiba.com/televisions/product.asp?model=20AF45 looks like the same family. $200 msrp.
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|12/12/2005 9:29 PM|
The images are ajusted oversize purposely so the small CRT image will look larger. You can open it up and adjust the size back down to fill the screen but no further if you like. They figure if they stick a 24" picture on a 20" screen, most folks won't notice the edges are gone.
I have not noticed a tilt thing. It has nothing to do with hemisphere, all sets have a degauss ring internal that energizes briefly at power up. This keeps the earth's field out of the picture (pun intended). There is no compensation for the earth field, other than to keep the shadow mask demagnetized. Non-magnetized means non-reactive to the field.
If the yoke is skewed, not only will the image be crooked, but the trapzoidal thing will happen too.
And hopefully you have no speakers next to the set whose fields can distort the image. Doubtful in the kitchen. My old Samsung worked for years, I think I have a little RCA in the kitchen now, and it seems to work, though lately the remote rcvr is ignoring the remote. I have tested the remote into my bench sensor and it is transmitting a clean pulse train. I suspect a spider is up behind the IR window on the set.
A lot of CRTs these days come complete with yoke and permanent convergence. Yours may not be adjustable if that is the case. But you can eyeball the yoke and see if it is cocked funny. DOn't touch the convegence rings around the neck, but the wedges might be adjusted some between the tube sides and the flare of the yoke.
|12/13/2005 1:48 AM|
Thanks. I have been reading the Notes on the Troubeshooting and Repair of TV Sets web page, which contains alot of useful information.
I was really surprised to learn that the tube makers actually manufacture different picture tubes for use in the Northern and Southern hemispheres, and that they really do make corrections for this at the factory. >> link <<. According to the docs, even rotating a large screen picture tube from East to West is enough to induce tilt problems. But I'm certainly no expert, so I just have to believe what I read.
Thanks for your your help. I realize that I could probably open up the TV and access some trimpots, but I was hoping that I wouldn't have to do this with a brand new TV. FWIW, I've had this problem with premium Sony, Philips, & RCA models, and also with the el-cheapo Sylvanias and ghost manufactured store brands. Its disapointing when buying a more expensive set won't fix the oversize image problem or the image tilt problem. Spending the extra bucks to buy a more expensive NTSC set isn't especially appealing if their tuners are going to be rendered obsolete in a couple of years.
IIRC some of the reason for this is so that image shrink isn't noticable with other playback signals, like VCRs. I guess that some VCRs are known to cause image shrink on replay, so the TV makers oversize the screen as a protective measure against this.
Another question about this -- does the oversize image problem change (diminish) over the life of the set? Do they oversize the picture when the set is new because it will shrink somewhat as the CRT ages? I've also wondered why it is that the image size trimpots aren't located on the back of the set anymore -- I remember those always being there as a kid.
Let me ask you TV guys another question -- with tube amps, I'm fairly comfortable working on them because I take the standard safety precautions. (current limiting bulb in series, variac, etc.) Does the direct rail power supply on TVs require any special precautions beyond adding an isolation transformer? Even though I'm comfortable with tube amps, the 30 KV inside of a TV sounds bad enough, but the direct rail power supply seems alot more dangerous -- kind of like working on some of those old amps with direct rail power supplies.
|12/13/2005 7:43 PM|
I didn't spot the hemisphere thing, but didn't read the entire document. I have been servicing video displays professionally for about 30 years, and am so far unaware of any hemishperical differences. How would the set know whether it was facing north in the southern hemi or south in the northern hemi? Can you imagine speakers adjusted for west coast or east coast?
Usually moving the set doesn't have much effect, but it can. it is generaly a very subtle effect adn the internal degausser takes care of it.
The DTV conversion I think is now pushed back to the start of 2009, so you have a couople years yet of good old NTSC. At least they are starting to educate the public in the newspapers etc.
Again, I don't think the oversize pix is a "problem" so much as it is a strategy. Image size is an artifact of the system. Display size is adjustments to the set. In other words, the video image is "full size" ordinarily, but the set determines what you actually see on the screen.
VCRs are all but gone, I don't know why a TV maker would pay much attention to them.
Picture size has nothing to do with the CRT. Brightness and color do, but size is a matter of beam defelection. The beam is accelerated by the HV, that 30kV. The 30kV can fail, but as a rule it doesn't wear out. Then the beam is deflected by the yoke - magnetic coils around the CRT neck. Horizontal deflection is not so analogous, but vertical deflection might as well be an audio amp putting out 60Hz sawtooth waves.
The more current you drive through the deflection coils in the yoke, the more the beam is deflected, and the larger the screen picture. This is adjustable. This deflection current is predicated upon the 30kV being there. If the 30kV drops, the picture will "bloom" or get much larger - also a lot dimmer. This is because the lower HV will make the beam slower, so the given amount of deflection energy can deflect the beam further to the sides.
But the CRT has little to do with this sizing. As the electron guns wear out - cathodes become depleted - the beam gets weaker - fewer electrons - but it moves the same speed as Always and is deflected the same. SO aging CRTs change image quality but not size as a rule.
There used to be a sea of controls on the back of a set. They are now inside. The set circuits need a lot less adjusting than they once did. For example phase locked loops make horizontal and vertical hold controls unnecessary. But one very important factor is the fiddle factor. When you hide the controls, dad can't fiddle with it and fuck up the picture. Once he does, it will never be HIS fault it looks crappy, he will always blame it on RCA or whomever. They are heading this off at the pass.
Safety is important. USE AN ISO. A lot of sets these days use switchers, so other than the switcher circuit itself, everything is isolated. But simpler sets are direct mains rectified, and the chassis is hot. Clip your scope ground to chassis and POOF you killed a rectifier in the power supply. Not to mention you burned off a ground trace somewhere.
The 30kV is not so much lethal as it will cause you to break your arm jerking it away once you touch it. And there is this - the circuit is not where the charge is. The CRT itself is a capacitor and it is where the charge lurks. The outside coating of the CRT is conductive and MUST be grounded. There is usually a strap or spring stretched across the back of it to ground it. The HV cup on the side of the CRT connects to a similar coating inside. The CRT glass is the dielectric. I don't know what it measures today, but 40-50 years ago 500pf was a common cap. Of course that was also only 15-19kV black and white.
With the set unplugged from the wall, ground a clip lead and hook it to a small screwdriver. SLide that up under the HV cup and touch the contact. You will hear the SNAP of discharge. Then it will be safe to remove the connection. The wire will have no charge on it, but reground teh cup tip, because the CRT will remember some charge.
The focus pin of the CRT could have a few thousand volts on it too. The rest of the circuit runs up to maybe 200v mostly.
SOme sets can be run up on the variac, some cannot.
|12/15/2005 8:44 PM|
thanks, Enzo, for your helpful post.
i'm happy to report that i've got at least part of the problem solved -- i found an el-cheapo TV that doesn't have any of the problems that i had been complaining about earlier (extreme oversize and image tilt). after returning a bunch of defective sets i finally got one out of the box that had decent picture geometry and image size, so its a keeper.
i guess its just my luck that the only TV that i could find with a decent image has had its plastic case scuffed on both sides at the factory before it went into the box. well, after going through all of the trouble to finally find a TV set that doesn't have a bad image, i'm willing to settle on a set that's got scuffs on both sides of the case. what's really odd about this situation is that i had tried a bunch of 19 to 20-inch sets at all price ranges, and the best image alignment came from the cheapest 19-inch set i had encountered -- a cheep sylvania that retails for $90 and actually cost me $50 as a loss leader at a big store Christmas sale. go figure.
regarding the TV that needs to be fixed -- its an old sears unit from 1987. the board on it bears a Toshiba silkscreen and model number, so it shouldn't be too hard to find the Sams information on that at the library. it has a heatsink at the PS section of the board has a live voltage warning sticker on it. as i understand it, if you don't use a current limiter that direct 120 VAC is alot more dangerous than the HiV on the tube, but I'll do my best to stay away from both of them.
thanks also for your tips on discharging the HiV circuit. just so i have this correct, you're connecting the ground on your discharge probe to the metal straps or wires that span across the back of the CRT and contact the DAG coating, and the tip of your probe to the HiV connection under the "suction cup", right?
i was just wondering, since we at Ampage tend to make fun of the guys who use the BFS (Big Screwdriver) approach to discharge the PS caps on their tube amps, and recommend discharge through a high impedance resistor... is it considered a faux pas to discharge the CRT using the BFS? interestingly, i have a friend who used to be an engineer in the TV/radio division at Motorola who said that they used the BFS technique on the TVs in the motorola lab back in the day.
|12/16/2005 4:12 PM|
Worse than 120VAC, it is rectified up to 170VDC, or if a switcher, probably doubled to 340VDC. Lethal. Yes, more dangerous than the HV. I can't think of any current limiting scheme that would allow operation of the set for testing at the same time as limitimg things to where they were safe to touch.
You don't want the discharge your filter caps with a screwdriver. Aside from taking a divot out of the tool, you also run the risk of burning out the internal connection from the foils to the posts.
But discharging a film cap - that .047/600v for example - with a screwdriver is fine. The big filter can deliver a large current, but the film cap cannot. Hey, the filter might be a thousand times larger capacitance!
But the CRT doesn't store a lot of current, just a lot of voltage. SO the spark is impressive, but it won't do much work. In the CRT there are no internal wires to melt. The cup anode connects directly to the internal aquadag.
Yes, ground to the strap - which should be bonded to chassis anyway - and poke up under the suction cup.
Toshiba did make a lot of Sears TVs, among other builders. Sams should cover it. But you should also be able to look it up in Sams as the Sears model. You needn't figure out which Toshiba it matches. The Photofact file will just be labelled for both.
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