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CRT substitution

tipc

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I have to confess I know next to nothing about tubes in general. In a very broad sense it's probably a bit like working with transistors. But a crt is likely to most complex vacuum tube there is, so much complexity is involved. But in reality how difficult could it be to substitute a "close" picture tube if yours happens to be trashed? I comfort myself oftentimes when attempting what seems to be a complex task by telling myself there are a finite number of parameters, which can be very helpful. We tend to get overwhelmed by a task or tasks. To be clear though I *am not* attempting to do this at the moment, it's just a general question. It pertains to a discussion I've had with a person whose IBM 5175 got trashed in shipping, neck is cracked. I realize that companies used to specialize in repairing things along those lines (1 place in the Philly area used to cut the tube at the neck . . . and frankly I can't remember if they replaced the front of the back!!! But in any event they welded the sucker back together. Not rocket science, but probably not something a typical "hacker" would get right the first time. Maybe with some space age epoxy.

Can anyone recommend a text on something close? I personally don't know of a lot of computer monitor books. I do know of a few tv repair/theory texts. Not something I've delved into much unfortunately (not yet).
 

KC9UDX

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CRTs aren't even close to being the most complex. :)

No one repairs CRTs anymore, as far as anyone knows. The Early Television Museum is setting up a CRT rebuilding shop, but I doubt they would do that repair. When they start going I'm sure they'll have a backlog of antique tubes to work on anyway.

If you have a monochrome CRT, replacing it is no big deal. If it's colour, I wouldn't recommend that if you have no experience.

Finding a new CRT these days is next to impossible. I wouldn't replace the chassis in this case either, I'd just get a new monitor.
 

EagleTG

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All of the old CRT refurbishing companies are gone, some of their equipment has wound up in the hands of hobbyists and museums.

That said, swapping CRT's is entirely possible. Figuring out what is compatible might be a bit of a challenge. For pinouts, some of the manuals from CRT rejuvenators can provide details. Look for manuals relating to B&K and Sencore rejuvenation devices.

Also, it can be a pain to move the yoke between CRT tubes due to alignment issues. In the arcade world, the guys try to find yokes with similar impedance and move the entire CRT with yoke attached.

I'd concur, replacement of the entire chassis makes a lot more sense. You could also look for similar IBM machines to use for parts (find ones that are not working but have the part you need).
 

olePigeon

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What a shame. I could see a very limited market for some shaped glass that you could put over LCDs to use as a drop-in replacement. Probably be just as expensive as sourcing an unused CRT.
 

tipc

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well I could at least imagine something where it wouldn't be. Imagine something _like_ a lense made of fiber optic strands. In theory (some kind of theory) the light would get passed through faithfully, and image quality won't be degraded. In practice who knows. I envisioned something along those lines a number or years ago witnessing how large workstation monitors would "bloom" as they got older. I had though though that a form fitting lense affixed to the front of the tube's face, made up of millions of tiny lenses - really just bulges, or convex indentations (too tired to figure what would cause the focusing effect). Never even got to the drawing board. And alas no one cares about those old magnificent large screen crt displays anymore. They were all the howl at one point though.
 

NeXT

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My biggest problem retrofitting LCD's to devices that originally used a CRT was that in some cases the curvature of the bezels were designed exclusively around the CRT. The flatness of the LCD leaves nasty gaps or forces you to remove portions of the bezel to get a flush mounting surface. We are in the era of 3D printers now. Why not design inserts that you can fit in and mount the LCD to that? Better yet, why not design the inserts and release the models so anyone can manufacture them?

Most CRT maintenance these days outside of tube replacement which we'll just say is currently not possible to fix (you could attempt a rejuvenation cycle, if you can find the equipment), is capacitor replacement and chassis cleaning. Another portion is cataract removal.
IMHO, most VC collectors these days....suck. (Hold on kids, here comes a rant.) They don't want to get their hands dirty. They don't want to take risks. A portion of them don't even know what the hell they are doing (Read: "Nostalgia"). They hear the whisper of high voltage, read a few biased repair accounts or news articles or take the "NO USER SERVICEABLE PARTS INSIDE" note to heart and run off screaming either to hack in an LCD or searching for another identical display, wasting valuable equipment in the process and learning absolutely nothing.
 
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KC9UDX

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Heh :) I was just thinking the other day how all my life I've scoffed at "No user serviceable parts inside. Pilot lamp soldered in place".

All the lenses in the world can't un-quantise an LCD.

If you're going to use an LCD monitor, why bother with the CRT housing anyway?

I've got an Amdek monitor that twenty years ago I thought was worthless :( so I carefully broke the tube to leave behind only the face of it. I mounted a surveillance camera inside.
 

retrogear

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Back to CRT tube substitution. A new tube would be rare and cost a fortune. Many later CRT tubes included the yoke as part of the assembly. Swapping used color tubes would require a matching yoke as well.
1st qualifier is of course diagonal screen size as well as length of the neck. The distance from the cathode to the screen face as well as how wide the beam has to sweep should match because then the high voltage and yoke waveforms would also match providing the yoke impedances match. As someone said, pinouts of the sockets would have to match and when I used to test CRT's with my Sencore tester, it seems one or two sockets matched many of them. The other big factor is the cathode gun characteristics, which require a certain grid to cathode bias, screen grid acceleration and focus grid potential. The gun assembly is what got removed from the neck and replaced on rebuilt tubes. The tube # used to have something like a 3 letter code to designate the gun type. Anyway, the first two or 3 digits of the tube# were screen size followed by an alphanumeric code of the gun type followed by the number of active pins minus 1. More of my dormant brain cells are churning ...

Larry G
 
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KC9UDX

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I'm not sure that they sell new ones anymore. The last time I bought a new CRT from one of them for close to $1000 an LCD showed up with a note that this was the "new CRT replacement kit."
 

larryniven

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Jun 24, 2014
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What if we rephrase the question a little bit, how hard is it to reverse engineer the input/output signal from the pins of the CRT and then reproduce it with software on a LCD?
 

KC9UDX

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That makes no sense to me. Just abandon the monitor and use an LCD VGA.

CRTs don't have input/output. They have cathodes and grids. It just wouldn't make the slightest bit of sense to try and reverse that only to adapt to something that would plug right in where the monitor normally plugs in anyway.

It would be like putting a mic on a harpsichord, using FFT to isolate the notes and then fire solenoids to push the keys on a piano next to the harpsichord.
 

Chuck(G)

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I'm sure someone has thought of that. :) Rube Goldberg lives. Still playing a piano is very different from playing a harpsichord. A piano is touch-sensitive, a harpsichord not so much...

It's still hard to beat a monochrome CRT though--the resolution is mostly limited by the bandwidth of the circuitry driving the CRT.
 

KC9UDX

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I take it you've played a harpsichord! I haven't in thirty-five years. I'd like to play one again someday. I've got nothing to play anymore though :(

I did unearth a large book of classical piano music last night. I'll have to see if I can still play any of that stuff. Probably not.
 

Chuck(G)

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Built one (a Zuckermann double kit) back in 1975 (at least that's what the nameboard says), but the last decade or two, I've gotten to be more sociable playing strings and brasswinds.
 

andy

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In my experience, similar CRTs are very often compatible if you're willing to swap the yokes and do all the necessary setup. Just about any 14" monitor CRT with the same diameter neck has a good chance of working. Even CRTs with a "permanently" attached yoke can be used. There is always a way to remove the yoke from a donor CRT (even if it means destroying the yoke to get it off).

Sometimes the gun isn't compatible with the yoke which makes it impossible to achieve proper convergence at the edges. There can also be differences in the mounting brackets which make installation impossible. Both of these problems become obvious pretty quickly. Fortunately, 14" VGA monitors are almost free when you see them, so it doesn't hurt to try several until you find the right one.

If you give up on the monitor, I would be interested in buying it since I can probably make it work.
 

vwestlife

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A lot of vintage computer monitors were really just tunerless TV sets, making CRT replacement trivial -- find a CRT from a similar TV set and swap it in. Even if the monitor uses a higher scanning rate than the broadcast TV standard, it can be made to work; a CRT is just a piece of glass and metal -- it's up to the electronics that drive it to handle whatever scanning rate is being used.

For example, a lot of older low-end VGA monitors still only used a television-grade CRT, with a large dot pitch and non-anti-glare screen; they just increased the scanning rate to get it to display a VGA signal, causing a grainy image and lots of eyestrain (for example, the IBM 8512 and Tandy VGM-220).
 
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