Converting from CRT to LCD or anyone know about video signal

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Re: Converting from CRT to LCD or anyone know about video si

Postby Doug Coulter » Tue Mar 01, 2011 1:54 pm

Well, the suspect parts are (in no particular order) the output transistor (TR 11), the HV diodes (EHT) and the filament regulator (TR7, if there's no glow on the filament, but it might be ok and just hard to see -- but easy to check that filament pin with a voltmeter and see). Obviously with no filamament it won't work, and that could either be the supply, or the crt -- check that first with voltmeter on the supply and ohmmeter across the filament. I see they link to waveforms, if you've got that data too, check the collector of the horizontal output transistor for the right waveform. In general, the thing turns on and pulls to ground, then off and the collector "flies back" (well above supply voltage) due to stored energy in the flyback transformer and yoke (note they are in parallel for AC). I note there's a diode off the transformer used to make power for other things in there....that's suspect too. A lot of those failed "real leaky" and that would eat enough power to make other things not work.

In these lower voltage monitor circuits failures of the flybacks (and yokes) are comparatively rare compared to the old days of 35kv color tvs with no volt multiplier off the flyback.

The diodes can be a little hard to check as the HV ones are usually really a bunch of lv diodes in series in that package, and it may take tens of volts to turn one on forward, and tens of volts going the other way to expose any serious leakage. Note that they are using the flyback system to generate some of the on board voltages -- that's typical, but it makes things a lot harder to troubleshoot as "everything affects everything else". At the shop, we rigged up a medium voltage (say 48v) power supply and a 10k resistor to test these diodes in a sort of "in circuit" type of test, as the regular diode checker on most meters is useless for a diode that's really 30 in series in there.

I'd forgo replacing all the electrolytics unless they are bulged, got real hot (see pcb for burning) or test bad and you see a lot of ripple across one. They don't fail as much as they used to.
Usually things would sorta half-work even if they were bad -- you'd have something on screen to go by for further troubleshooting.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Converting from CRT to LCD or anyone know about video si

Postby Jerry » Tue Mar 01, 2011 3:09 pm

The output of the transistor looked somewhat OK. Pretty close to the waveform speced. I will replace it anyways. Someone had replaced it in the past when they were trying to repair it.

The HV diode is built in to the flyback.

The reason the filament is not working is because it is not getting enough power. Once I yanked the flyback the filament came up.

The rest of the diodes peak out at about 1000v PIV.
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Re: Converting from CRT to LCD or anyone know about video si

Postby Doug Coulter » Tue Mar 01, 2011 5:02 pm

Yeah, those built in diodes are a problem sometimes, but sometimes you can effectively replace them if they're leaky or shorted, by simply adding one outside the thing. In this circuit, the HV capacitor IS the crt, so you can eliminate that as a load by taking the connector off the tube and see if that makes everything else normal. I'd try that right off, since it's easy to do.
(always check the simple stuff first)

So you appear to be saying that whatever this is is dropping the 12v input down to where the CRT filament regulator can't work? It seems to be running right off the input power.
That kind of says bad flyback or yoke (or transistor or damping diode), but if the collector waveform is ok,,,,hmmm, hard to call at a distance.
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Re: Converting from CRT to LCD or anyone know about video si

Postby Jerry » Tue Mar 01, 2011 9:16 pm

I did try pulling the anode cap. No difference.

There is enough load after the HOT that its pulling the B+ down to a little over 3v. Probably going to have to replace those two components as well. I did bypass the regulator with a 10.8v supply and the Flyback section tried pulling at least 4 amps.
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Re: Converting from CRT to LCD or anyone know about video si

Postby Doug Coulter » Wed Mar 02, 2011 12:12 am

Ok..No difference on the anode means it's not that diode as a proximate issue.

4a on the flyback -- something fried medium-bad. Make sure the base drive on the output xistor goes down enough to turn that off (unlikely bad but easy to check/fix -- and failure to turn off right might be that transistor having a little collector-base issue) and if not...maybe xistor/diodes but sadly more likely an inductor (xfmr or yoke or choke) or a cap across one. Give the yoke a close visual and see if you can see something -- I've been known to get lucky, but it's starting to sound like a flyback transformer. I have a bunch with a zillion taps/windings on them, but have no idea if I have one that'd replace yours. Be nice to get a sweet old thing going swimmingly again though, especially at the basic price.
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Re: Converting from CRT to LCD or anyone know about video si

Postby Jerry » Wed Mar 02, 2011 1:04 am

The yoke looks fine as far as I can tell.

I looked up the flyback with the manufacturer and it looks like a custom job for Hantarex, they list who used it and they were the only one.

I remembered today that my cnc mill had a 9" mono crt for the display when I got it. Still had it too, two of them actually. I pulled them down and managed to find a datasheet on them and low and behold they take TTL level video plus V & H. Made up a cable and plugged it in. After some adjustment I got an image, but wonky. I remembered this was not the one I had used and grabbed the other and was able to get a good, stable image out of it.

But the damn CRT is just a touch to big to fit! Argh!!!

Back to the drawing board.

I did borrow a little 3.5" display, 640x480 from work. It has VGA and composite in. Tried it and it couldnt sync like my other one did. But for the price of one of these panels I can replace ever component on the original circuit board...
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Re: Converting from CRT to LCD or anyone know about video si

Postby Jerry » Thu Mar 03, 2011 3:30 am

Got the little ttl adapter today. Didnt work.

I remembered I had this little box that allows you to watch TV on your regular vga monito. So I gave that a try. I could see an image but it was not stable just like the other display I had. So I messed around with some resistors. Connected the two sync line to the video line through a couple 1.5k resistors and that did it. Good stable image. Even my projector was pretty happy.

So this opens up my options quite a bit. I can just install a small vga lcd in place with an adapter.

Going to get a new projector in the next week or so. The blue polarizer in this one is going south. Thats why the center of the screen is yellowish.

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Big screen dro by macona, on Flickr
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Re: Converting from CRT to LCD or anyone know about video si

Postby Doug Coulter » Thu Mar 03, 2011 11:20 am

No problems reading that on the big screen, eh? I put a medium big screen up in my shop that has a bunch of switchable inputs. It can be an add-on screen for a couple of the computers, watch a webcam on the fusor, or do the TV/DVD thing. Very handy and I can see it from across the room without my glasses. The main issue with that one is that to keep it out of the way, it's mounted up near the ceiling and so you get neck strain looking at it for long. I need to contrive a mount that lets me pull it down lower when in that kind of use -- it's already on a pull-out/tilt/swivel, but it really needs that Z axis too. Not trivial in the space I have it in, and the weight it is.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Converting from CRT to LCD or anyone know about video si

Postby Jerry » Sun Mar 06, 2011 2:04 am

Well, the video to VGA thing didnt pan out. Cant adjust the image location and I get clipping.

So today I went down to my friends surplus place and found a 9" B&W Panasonic video monitor, made around 2000. Brought it home and hooked it up and got a nice cleat picture. So I took it apart and mounted the CRT in its place. It too was a touch to big but by trimming away a little of the frame I was able to get it in and mounted. Ran into a problem with the circuit board. It was just too big to fit where the old one was, plus there was a transformer I would have to mount someplace. I decided to look at the board and see if I could do away with the transformer. I soon realized that the thing actually ran on 12v and that I could eliminate a good chunk of board and also eliminated the big heat sink on the transistor they were using as a voltage regulator. So I removed the components in the way and marked out what I could remove and proceeded to chop it off with the bandsaw. I also chopped off the plastic base to match the board. Now it fit. I pulled off the BNC's and attached my little converter board I made and tied into the wires coming from the logic board.

It works! Screen is crisp and bright, and best of all, no burn in like all the other displays I had.

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Anilam Superwizard by macona, on Flickr

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Anilam Superwizard by macona, on Flickr
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Re: Converting from CRT to LCD or anyone know about video si

Postby Doug Coulter » Sun Mar 06, 2011 1:11 pm

Ahh, that's really sweet! I can't say how many of those I've tossed out over the years, and I was feeling bad I didn't have one to send you. Back in the day, Kaypro computers had something just like that in them, for example, in green or orange phosphor. You probably will have some burn in since that display doesn't move around much, but it may not matter a whole lot either -- just keep the brightness down most of the time and it should stay usable nearly forever. I've tossed most things with tubes in them, since on solar power, I'm not going to use them. I still have this crazy big tube collection I'd love to sell to a tube-lover someday if I can find someone to take it all and not just cherry-pick it.

I've used the bandsaw trick quite a few times here. People bring me junk, it has some useful part in it, like that power supply, and I'll just saw that off the main board and let my wife take the rest to the recycling place (she makes money at this). Hard on blades, fiberglass is pretty nasty stuff to cut, but I do have that blade welder and blade stock so the cost isn't real big.
She is finding a ton of old electronic gear in the local dumpsters -- old TV's, stereos, and so forth that people consider obsolete, and I'll grab all the cool HV parts and so on, and let her have the rest. I've gotten to where I can rip and strip a big piece of gear like that in just a few minutes -- a makita battery drill with spin-tights for bits and so on -- free self tapping screws for life. Nice little power supplies, heatsinks, and TV speakers (which I use on my radiation detectors). Anything hi tech and hard to reuse the wife takes and gets some pennies a pound for.

Dumpster diving sounds weird, but here it's kind of a social norm (or not too far out -- we call it redneck recycling), and people leave the good stuff outside the dumpster for the divers so they don't have to actually climb in -- and you can tell if there's good stuff just by driving by. I made her a really wicked sort of Shepard's crook to fish things out if needed.

Now that my wife has everyone in the neighborhood in the know, they even segregate their aluminum cans and leave them outside the dumpster in trash bags for her -- that's the big money maker, along with construction wastes (copper pipe etc). It's amazing what people toss out, including perfectly nice furniture. An especially rich source is the local college towns -- kids will toss amazingly nice things (stereos, computers, furniture etc) since they don't want to have to move it back home over the summer break, and it's "daddies money" anyway. Most of it still works!
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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