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Old 2nd February 2004, 01:52 PM   #81
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Join Date: Feb 2004
Location: Gloucestershire
Hi all, first post here.
I've been following your posts here for some time.
I've decided to plump for a RPTV as I don't have the space to run a projector.
Rather than using a gutted TFT and bright light, is there anyway to perhaps recycle a CRT monitor? Most people will probably have a spare 15" or 17" CRT knocking around the house that could use a new lease of life, and to be honest turning it into an (almost) HD RPTV is one heck of a way to see in your retirement.
So if someone could see their way to giving it some thought I'm sure a number of us lurkers would be grateful.
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Old 2nd February 2004, 06:59 PM   #82
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Default CRT ect

David, great pricure, it seems that RPTVs are much slimmer than they used to be! As for reusing the existing case and lenses ect, well in theory there is no reason why you can't as the focal length of the lens will be correct. However, you will only be able to do this if the lens is designed to project an image from a flat CRT tube, e.g. are the tubes in the RPTV flat or curved? THe liquid you saw was probably coolent. If the CRT's in the TV have a flat serface you will be able to reuse then lens, however if they are curved you will have a problem as a LCD is flat, so the lens only part of the image will be in facus. e.g. the center and towards the edge out of focus. This assumes that the lens is the type that is directley in front of the CRT tubes. I would need an image to work out what is going on if there is an aditional sphere in the light path. I would assume that it would not be too difficualt to put in an lcd if the lens is ok, just put it where the CRT used to be and put a light soure behind, depending on how close the lens is you may not need a fresnel. Also some projection lens are only designed to project an image, e.g. from the CRT that is exactley behind the lens, so you may have limits to the size of the LCD you can use, e.g. up to 5"x4", however the lens may work if you use a fresnall, tests are the way to go ahead to see if it works, bin the thoery books and get some first hand experience/practice is always what I have said!

If you post some pics of the lenses we may be able to help/advise.

Speedwolf, i'm not sure if you could recycle a CRT monitor as there are several problems that you would need to overcome.

1) CRT monitors are much brighter than a standard monitor and are full of cooling liquid to control the heat. Although you can turn the brightness of a CRT monitor up you often sacrafice colour/contrast.

2) Most CRT monitors are curved on the surface, and most lenses are designed to project an image that is flat, in very broad/loose terms. So you may have focusing problems.

3) You may be able to adopt a fresnel+lens approach though brigntness will be an issue, the image will be dim/colour will be poor

4) You may end up with an image that is flipped/upside down unless you have the right lens (though this would depend on you light path.


Here's my advice for an easy to make/good quality RPTV


The best approach I can see for building a rear projection tv, at a good price with out having to do any real work is as follows:

Bits you need:

1) An OHP, must have a triplet lens!
2) A projection panel - with video input, and 2mill cols +, 640x480 mis good as the screen will I guess not be any larger than 60" plus
3) A piece of glass the same size as the screen you want
4) Some mirrors
5)


As it is rear projection and the distance from the projector/bulb will be small a low powered bulb is all that is required, e.g. standard 32v 400w OHP bulb will be more than ample!!!!, don't worry about the light colour, I will come to this.



The OHP will be mounted at the bottom of the case (imagin you are looking at the frot of the RPTV. The ohp will be mounted on it's side so the light source/bottom is at the bottom left of the TV. The mirro/Lens will be in the center. Rotate the OHP so the llight projects out towards the center back of the tv. Now mount a mirror there that reflects the image onto the ground glass screen. If you look at the diagram David kindly posted above, the lens of the OHP will be where the crt tube is so the light is projected towards the morror then onto the screen. Now mount the projection panel onto the ohp so the image is the correct way around on the screen, build an enclosure around it. The advantage of doing this is as follows,:

1) OHP's are cheap to buy used,
2) Projection panels can be bought easilly on Ebay, and have video inputs and remotes ect
3) You don't have to take anything appart!!
4) No real heat problems as Projection panels are designed to be on OHP's for long periods at a time.
5) THe enclosure will reduce the noise of the OHP fan
6) It will work!!, this is basically what I used to do when I was hired to set up a rear projection presentation using 16mm projectors (Oh the good old days!) , and even if you can't be bothered to finish it, you still have an undammaged OHP and projection panel for some other projects!!

From what I remember, projection panels have either video or S-video inputs. Now this means that it is easy to correct any colour problms due to bulb temperatue, here's how.

The process

VIDEO---> Convert to RGB individual signals----->Adjust each colour where neccessory, chrome levels and brightness/contrast------> convert back to S-Video or Video.

Easy, mount the controls for the circuit on the fron of the tv for easy colour ajustment. THere are various circuits on the internet for converting video to RGB and adjusting each level, and best of all they are made from easy to find/cheap conponents and the circuits are easy to build and require no adjusting. If any one wants any more details on these circuits, just post a question, and I will look for some links for you, I just can't remember the site at the moment. Failing that, I'll look through my collection of circuits for one, I know I have one somewhere.

Sorry had to type this post rather quickly and don't have time to grammer check/spell check, so you will have to excuse all spelling/grammer mistakes this time.


I will try and post some diagrams of what I mean, can anyone help me with a good site to host some picures, i'm a bit lost when it comes to hosting/posting pictures to these pages ect.
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Old 3rd February 2004, 01:26 AM   #83
dbc105 is offline dbc105  United States
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I had thought about using an OHP and a high res panel but was not sure about being able to get the OHP mirror far enough away from the rear of the screen. Which OHP have triplet lenses? I will try to remember to take a camera home this weekend to take pictures of the inside of the RPTV. The CRT is flat on the surface facing the lense so that should not be a problem. There is the concern of getting a lcd small enough. The CRT is about 5" x 4" at the face. Hey Ace, what is the width and hight of the lilliput? jr I can host a few pics for you.
David
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Old 3rd February 2004, 01:30 AM   #84
dbc105 is offline dbc105  United States
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oh yea, jr if you run up on the links for that RGB curcuit please post them. Thanks.
David

ps. did anyone see half time of the superbowl game last night? that would have been something on a big screen! LOL!
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Old 3rd February 2004, 02:49 PM   #85
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WOW, it's been a while since last time I posted on this thread, I've been sidetracked by other fun projects, any way I've got a 60" panasonic and my overhead setup, My biggest problem is that the focal lengh is shorter then my OHP triplett lens (not using the OHP mirror), I think I could lay the projector on it's side and use the OHP mirror, but then it would have to stick out the back of the TV enclosure, I took one of my lenses apart and focused it by hand, but the image ends up being too small (about 40") oh well I've got the origianl tv lenses I've yet to try.
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Old 3rd February 2004, 04:16 PM   #86
dbc105 is offline dbc105  United States
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ds21, do you have any pics or drawings you can post to show us what you have? I have 2 other bad RPTVs in the basement. The one I have been discussing seems to be more suited due to it's design to setting up with a small lcd and using one of the tv lenses. One of the others I believe may be well suited to gut it out and build a shelf to mount an OHP into an let it display to the original screen. All 3 have 40" wide screens so what you are getting may work out real good for me. On the third one the press board bottom of the case seems to have been wet and is sagging badly. I think I will rob the mirror and lenses and screen and trash the rest of it.
David
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Old 3rd February 2004, 05:12 PM   #87
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dbc105, it's basically configured like the drawing you put up, to take out the gut was something like 3 screws and 2-3 ribbon cables then the whole unit slid out. For the hand focus experiment I just took the top lens of the triplet and faced the concave side toward the screen. The easiest way, I've found, to experement with various lens configurations is to get a measurment from where your lcd sit to a point on the mirror, then a measurment from that point on the mirror to the screen, then add the two measurments, then what I did was just take a candle in a dark room and position it that many inches away from a wall then start holding you lenses inbetween and just move them around and play with them, that way you can see what makes them work. I now know about how far away/apart the triplet lenses need to be I just need to make some kind of mounts for them.
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Old 5th February 2004, 11:42 PM   #88
ebswift is offline ebswift  Australia
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Although I'm new to this forum and to DIY projectors in general I'm really interested in this project. It seems the only practical way to get DIY big screen TV for general use. The projector stuff is nice, but it seems like you've got to get the room darker and use brighter bulbs. This thread should be made permanent because it's the only recent thread I can see focusing on RPTV.

At this stage of the game I'm looking at the materials that can be used for the RP screen. I've skimmed through some commercial stuff, and for the price it seems I could get the whole TV from the local retailer. I've got access to normal projectors so I can test various materials before running off to build the projector itself (based on designs from this forum of course). Basically if screen material isn't readily available I'll forget the idea of RPTV.
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Old 6th February 2004, 11:10 AM   #89
dbc105 is offline dbc105  United States
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ebswift, going about it the way I am I think should save me a considerable amount of money. I have 3 RPTV's, all bad that I picked up from a buddy that works on TV's. on the ones I dicide not to use I plan to remove the screen, mirror and lenses and trash the rest. This should net me the most hard to find items. check around in your area for someone that does TV repair and see if you can find a bad one you can pick just to get it out of their way. I am also considering the idea of an OHP mounted in the RPTV box focusing on the rear of the screen. Since my screen is only 40" wide there may be enough room to place the OHP behind the screen and only use the mirror on the OHP. This is based of the suggestion that JR made in his post but I am thinking on the smalleer screen I may not need the added focal length in which case I can project right onto the screen. the room light is what got me interested in the RPTV. I will need to be able to use mine all the time. if you have any ideas or thoughts post them. I had not considered the OHP in the tv before JR's post, but now I am thinking it is to simple to pass up.
David
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Old 8th February 2004, 12:02 PM   #90
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Default Lots of things

Here are some links on how to make Video to RGV/S-Video converts and vice versa:

http://www.epanorama.net/links/videocircuits.html

This next one looks great, you could replace the resistors with variable resistors to ajust each colour. It looks cheap too, though remmeber this is designed for PAL, not NTSC, so you would need to adjust it for NTSC ect.

http://www.nexusuk.org/projects/rgb2svid/index.php


I'll post some more links for RGB colour correcting circuits when I can find out what they are again, I did have some myself that I used to use, but I have found that some of the components are not made any more, so they are of no real use anymore - pitty.

As for my idea of using an OHP in a RPTV application, here is a (rather poor) diagram of what I mean, a spectial thanks is due to Davide here for hosting the image for me!

Click the image to open in full size.

I hope the diagram gives an idea of what I thought of, as for image size, many OHPs these days have very short throw lens, so this should not be a problem, if focal length is a problem, then you could use a zoom projection lens off ebay insted, they are fairly cheap.
A triplet lens is a lens that has 3 elements, the idea is the lens corrects colour distortion, whch effects cheap/OHP single element lens, most good ohps these days would come with 3 or more elements lens I would guess.
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