Adding a composite input to a B&W TV.

[ral-clan]

A week ago I picked up a B&W 5" TV with only an RF input to use as a "workshop" monitor with my VIC-20. As I expected, the picture is useable, but subject to lots of RF interference.

Also, since I have adjusted the internal pots on my VIC-20 motherboard to make it look good on a 1702 monitor, when I use the VIC with an RF modulator the colours are bad. Actually, on the B&W TV they look reversed! The background is black and the letters are white upon boot. Again, this is a useable setup for doing diagnostic stuff in the workshop, but it could be improved upon.

So, I was mulling over the idea of adding a composite input to the TV.

Firstly, I must say I know about the dangers of working inside a TV (flyback transformer and other dangerous voltages)....so this is probably not something I will try to do. But I was wondering about how possible this would be theoretically.

What are the differences between a monochrome monitor and a B&W TV? Both expect a luma signal only. The colour signals are ignored.

The difference is that the B&W TV has a tuner/RF demodulator in it, and the monitor doesn't. The monitor also is a little sharper in terms of picture quality, and might have a higher persistence phosphor coating inside the CRT. But a black & white TV is much sharper than a colour TV for text purposes.

So....inside a B&W TV the tuner module accepts a signal from the antennae input, then splits this signal into luma and audio. The internal tuner then sends the luma signal to the CRT and the audio signal to an amp and the speaker.

If it would be possible to find out where the RF modulator outputs the luma signal to the TV's circuit board, wouldn't it be possible to 'hijack' this line and route it to an externally mounted RCA jack, to which one could connect the luma signal of a computer like the VIC-20? One could also do this with the audio line.

Another reason why I think it would be great to do this, is that I have a 40/80 column board for the VIC-20, but only one monochrome monitor. If this monitor breaks I will have difficultly finding a new one....however I see RF input only B&W TVs all the time at thrift stores for $4 each. I think they might make a nice cheap monitor for such a display. I think the monitor in the original PET 2001 was a Sony B&W CRT (maybe not a TV tube, but something similar).

I did a Usenet search, and found a post by Dave Haynie (the D. Haynie?) on this subject:

Yes, no, maybe. You can't connect anything reasonable directly to the
picture tube -- don't do it. When I was 17, I got my first home
computer (an Exidy Sorcerer -- anyone recall this one), and hacked a
composite video port into my B&W TV at the time. Not a bit deal: I
drove down to the local radio supply store, got the "Sam's Photofacts"
on that model (Hitachi something-or-other), found the composite
signal. I routed the output side to an SPST switch, the common of that
switch to the input side of the signal, and the other pole of the SPST
to my circuit. It was basically just a high speed, single transistor
amplifier, and I messed around with gain on it until I got a decent
level (and luckily, didn't blow out the TV).

I also found this post on Usenet by a fellow name M. Zenier which makes me think that it's not worth it to try this hack unless you really know your way around the inside of televisions:

A problem is that TV's are cheaply built and may not have any isolation from the power line. Plug your TV in backwards and your computer blows up. (This happened to some poor soul up here in Seattle about 10 years ago, and the local club got together and scrounged up all the chips necessary to rebuild his altair)

Some older sets run with a rectifier right on the power line which feeds
everything. These are bad news. Newer sets use the Horiz. drive as a
switching power supply so most of the circuitry is isolated.

On a related thought: has anyone ever tried hooking a B&W security monitor to a VIC's luma output? These are generally high quality displays....maybe they would be sharp enough for 80 columns.

[Jeff-20]

I used to use a little BW tv. The image started out reversed and had to "warm up" slowly correcting itself to a normal image after about 5 minutes. The tv has since died.

[ral-clan]

Oh very wierd. I hope the reversed image thing is not an indication that the TV is dying! It does display TV just fine.

PS: I have noticed some little black and white 5" TVs being sold now for under $20 come with AV inputs.

[ruud]

PS: I have noticed some little black and white 5" TVs being sold now for under $20 come with AV inputs.

I bought one in Poland some years ago. The original idea was that it also enabled me to watch television on a camping place (or whatever). but I completely forgot that I'm living in PAL-land and Poland uses SECAM :) But for the rest it was a perfect choice. Now I'm using an 8" LCD screen with composite, S-video and VGA input. Bought at www.Conrad.de for 200 Euro's a year ago.

About your original post: I have done that several times but that was many many years ago, 1982-1988. On itself it is as simple as feeding the signal to the input of the variable resistor of the brightness or the contrast. Sorry, cannot remember anymore which one it was.

Some months ago I discarded the magazine that mentioned this design with the thought in my mind: "Who ever is going to need that?".

[ral-clan]

About your original post: I have done that several times but that was many many years ago, 1982-1988. On itself it is as simple as feeding the signal to the input of the variable resistor of the brightness or the contrast. Sorry, cannot remember anymore which one it was.

Oh wow! That's even easier than I thought.

I wish you still had that magazine article though!

[Jeff-20]

I can't believe how difficult it is to find a small scren tv. There are tons of monitor/DVD combos, but very few options with a built in tuner.

[carlsson]

Have you tried places like the army surplus? No, not the Salvation Army, but a place that sells new stuff bought from surplus warehouses, sometimes stuff that the army no longer will use. Your trendy TV dealer probably doesn't find a profit in still offering small, basic TVs. Another option would be an electronics gadget store, they tend to have stuff that other dealers won't touch.

[saundby]

Have you tried places like the army surplus? No, not the Salvation Army, but a place that sells new stuff bought from surplus warehouses, sometimes stuff that the army no longer will use. Your trendy TV dealer probably doesn't find a profit in still offering small, basic TVs. Another option would be an electronics gadget store, they tend to have stuff that other dealers won't touch.

This last holiday season we had a lot of first-line retailers selling small B&W TVs and some color TVs at really low prices. They sold out pretty quickly in most places. They were being sold as a sort of 'get them in the door' promotional item. I considered getting one, but I already have a five inch TV that I get a good picture off of with the antenna input, and a seven inch color TV that I need to adapt a fat 12V wall wart or brick to, since my stepfather misplaced the original before he gave it to me (he only used it in his motor home, after all.)

The main concern about black and white TVs is the old ones that have a "hot chassis" design. To save money some TVs had no isolation transformer, the neutral line of the AC line cord ran straight to the TV's chassis. This problem applies to console sets primarily. Any fairly modern style portable set isn't going to have this problem, especially the smaller ones. Older "portable" TVs like ten inch and twelve inch sets with a handle recess may have a hot chassis, but the little five inchers typically don't. If you know much at all about power supplies, you can easily determine for yourself by opening up the set and taking a look. After about 1980-81 this problem goes away for portable sets (though there may be rare exceptions I'm unaware of.)

My little TV has a smaller than normal RCA-type plug inside that carries the composite image from the tuner/demodulator to the rest of the TV, so it essentially has a composite monitor input inside it already. It wouldn't be hard to put a connector in the side and put a switch inside for the signal, say a 4066 or 4016 for the signals controlled by an external switch. Or just a plain switch as mentioned by Dave Haynie.

Check inside your TV, maybe it has a setup inside like mine.

-Mark G.

[ral-clan]

Yes, I haven't tried it yet because I'm terrified of the hot chassis problem.

But seeing as this is a 5" TV made in 1986, and the fact that it is powered by a 12V wal-wart that converts AC to DC, that should guarantee no hot chassis, right? I mean, hot chassis TVs were the ones that had the AC going right inside them. i.e. console or tabletop TVs. Ones that are powered by an external walwart that converts the AC to 12V DC should have no AC in them all all, and therefore no hot-chassis. Am I correct?

[saundby]

Yes, I haven't tried it yet because I'm terrified of the hot chassis problem.

But seeing as this is a 5" TV made in 1986, and the fact that it is powered by a 12V wal-wart that converts AC to DC, that should guarantee no hot chassis, right? I mean, hot chassis TVs were the ones that had the AC going right inside them. i.e. console or tabletop TVs. Ones that are powered by an external walwart that converts the AC to 12V DC should have no AC in them all all, and therefore no hot-chassis. Am I correct?

Ah, you're absolutely correct! No worries, there's no hot chassis here.

You'll have about 12-14kV on the anode line, about 1.5-2kv on the focus line, some moderately high voltages on the deflection and gates for the tube, but all at a low enough current level they won't kill you unless you soak yourself in brine for a while. Stay away from the CRT and you're fine, it'll all be low voltage stuff.

If you've got an o-scope you can poke around and see where the signal is. Otherwise, look for an RCA-style connector inside between boards or modules, or a short piece of coax cable. If you have another monitor, one way to look for a composite signal is to take an old RCA cable and turn it into a probe on one end. Plug it into the other monitor, tie the shield to ground inside the little TV, then go looking for a signal on the low voltage boards in the little TV with the center conductor of the cable/probe.

-Mark G.

[ral-clan]

Just a question though: let's say I find the luma input on the 5"TV circuit board and I route that to the outside of the TV case.

Could it be possible that (due to this being a hack) there would be voltage present on this jack, so when I plug the VIC luma output into it, the voltage would come back along this wire and fry something in my VIC?

[saundby]

The difference between a luma signal and the system ground won't go above about a volt. In fact, you're more likely to find a luma+sync signal than a plain luma signal, in other words, the complete black and white composite signal. Typically these signals are less than a volt peak to peak compared to ground.

Bringing it out of the case with, say, an RCA connector won't present any voltage problems, just tie the outer conductor to the system ground, preferably the signal ground if you can tie to it, but the chassis ground will do (though if you get a high amount of interference or other signal problems, you'll probably want to try finding a better ground since you may have set up a 'ground loop' if you tie to the chassis ground. Not something to worry much about unless you do get a bad signal.)

By tying together the grounds of the two systems, you get rid of any voltage differentials between the two systems. This is why good connectors provide a ground connection before any other signal gets connected.

You could put a low value resistor in the signal line on the input if you really want to, but it'd be more for protection of the TV than for anything outside it. This would attenuate the video signal a bit, though, and wouldn't provide too much in the way of protection, so it's probably not worth bothering. So long as you have clean connections inside the TV, I wouldn't worry about bringing out any of the video signal lines. The clean connections will make sure that you don't have, say, some stray strands of wire that are going to brush up against something with more oomph.

-Mark G.

[ral-clan]

Thank you very much for your knowledge. I've searched the 'net for info like this pretty deeply, to no avail. As you probably have surmised, I understand the basics of what's going on inside a TV, but am only a novice.

In fact, you educated me on the fact that a composite signal is as little as a sync+luma. I though composite only referred to chroma+luma.

So thanks.

Sounds like a good place to tap into the composite signal line (since my TV doesn't have one of those mini-rca jacks inside) is right before the brightness or contrast pots, like Ruud said above. That is assuming that a full composite B&W signal is sent through these pots and not just a luma signal, which is later merged with the sync signal.

[saundby]

The difference between a luma signal and the system ground won't go above about a volt. In fact, you're more likely to find a luma+sync signal than a plain luma signal, in other words, the complete black and white composite signal. Typically these signals are less than a volt peak to peak compared to ground.

Bringing it out of the case with, say, an RCA connector won't present any voltage problems, just tie the outer conductor to the system ground, preferably the signal ground if you can tie to it, but the chassis ground will do (though if you get a high amount of interference or other signal problems, you'll probably want to try finding a better ground since you may have set up a 'ground loop' if you tie to the chassis ground. Not something to worry much about unless you do get a bad signal.)

By tying together the grounds of the two systems, you get rid of any voltage differentials between the two systems. This is why good connectors provide a ground connection before any other signal gets connected.

You could put a low value resistor in the signal line on the input if you really want to, but it'd be more for protection of the TV than for anything outside it. This would attenuate the video signal a bit, though, and wouldn't provide too much in the way of protection, so it's probably not worth bothering. So long as you have clean connections inside the TV, I wouldn't worry about bringing out any of the video signal lines. The clean connections will make sure that you don't have, say, some stray strands of wire that are going to brush up against something with more oomph.

-Mark G.