Adding composite video/audio to an old TV

[Tim R.]

Now that analog broadcasting is gone, and virtually nothing uses the twin line or coax connectors any more, I've been toying with the idea of converting some of my TV's to accept composite video and audio.

In addition to less wires and boxes, I theoretically should get better performance by doing this instead of using an RF modulator, as there would be no signal loss due to modulation, demodulation and poor Chinese workmanship.

It seems like a fairly simple mod. Add connectors to the audio and video amplifiers, a switch to ground the stages before those sections, and a resistor between the audio jacks if the TV has mono sound.

I've searched through the forums but haven't found much about this topic. Is it as simple as I described it, or is my logic flawed?


Thanks!

-Tim

(PS - not sure where to post this question, it seems like it would belong in several categories.)

[DaveWM]

I have done it before, yes in some cases its that simple, I have pulled the last IF tube and just injected a composite video at the output of the dector, and the audio to the grid of the audio amp. Sometimes it does not work so well, maybe a phase inverter needed or just not enough signal for the gain of the video amp. I often use this approach in trouble shooting to isolate the pic issues.

I have considered it as a perm way to deal with the lack of OTA transmissoins and have come to the conclusion that its better to just work out the issue with the RF/IF and use a modulator, A well setup and aligned front end works fine, and some how I feel I am "cheating" if I ignore half the tubes or more.

[miniman82]

Worked very well on my Philco, that damn thing had a needle sharp picture. I just added an RCA jack to the video amp tube's grid, and hacked into the volume control for audio. The jacks mounted in the back, with no drilling required since the vents were already the right size.

[ctc17]

The audio is guaranteed and easy, inject it into the volume control like said.
The Video is another thing. You want to inject it into the alignment test point which is usually the grid of the video amp/output tube. It works on about half the sets I have tried it on. Sometimes the IFs output is much hotter than the composite sources.
It generally does work better, eliminates noise form the tuner and buzz from the audio.

What kind of set are you working on?

[wa2ise]

A safety note, only do this sort of mod to a TV set that has a full power transformer, for both the heaters and the B+. You don't want to do this with a "hot chassis" set.

As mentioned, audio is usually easy, inject it at the volume control. But be aware that sets using a quadrature FM detector (like a 6HZ6 dual control pentode) will need a higher amplitude audio signal than sets that used a ratio detector FM detector. You can identify such a set with a quadrature detector as one with the volume control directly feeding the audio output tube, without a triode driver stage.

Video may be harder to deal with. Video is sensitive to being inverted or not inverted. Audio doesn't care, but video does. Get it inverted and you'll have no sync, and if you do manage to recognize a picture, it will look like an old fashioned photo negative. Also the video from say a DVD player may not be high enough amplitude.

[jpdylon]

This guy has a great article on the subject. He's got it nailed.

http://ankythera.com/Projects/video_stage.htm#approach


I'm tempted to build one of these buffer amps to try it out.

[Tim R.]

Thanks for all the informative responses. Lots of good information and scenarios I hadn't thought of.

My candidates are an early 50s Hoffman EasyVision which uses circuitry typical of most TVs made then, and a mid-60s Packard Bell console TV, which I believe uses a modified RCA CTC-17 chassis. I definitely wouldn't try this on a transformerless set without the proper precautions.

Sounds like audio should be easy, but video may be a challenge. Based on what I've read in this thread, and in the article, it appears there is no clear answer to that question. It seems like the simplest approach would be to tap into the video amp and see what happens.

The article was really helpful, and I might be tempted to build one of those circuits if my Packard Bell set isn't plug-and-play, since it gets the most use.

DaveWM - you brought up a very interesting point. It does seem like cheating by eliminating various sections and tubes. Most things I connect to a TV have RF outputs. I find that the most practical way to play classic video games, watch VHS tapes, and use cable TV is through the set's antenna connectors. My main reason for the composite connection is to reduce clutter and make the TV more versatile. Permanently bypassing or disabling the RF/IF sections wouldn't sit well with me, but I would like to be able to plug my DVD player or Apple TV into the back and be ready to go.


-Tim

[DaveWM]

I understand, and have considered it as well. But with the recent availibility of cheap decent RF modulators, I decided to just no got that route.

Case in poing, most new DVD's do not even have RF out and I presume this will be the case going forward. I picked up a cheapo RF modulator from wally world for about 20$ and used it with one of my newer DVD players thad did not have the RF. Worked fine. When I was thinking of doing it I figured it would be too complicated to try and make it switchable (cuting tuner feed) without hacking up the set too much.

I can certainly see the reason for doing it, just not for me. Besides I can see myself setting up my own little cable broadcast system using a several modulators so I can still use my tuner to select them. For that I would use the high power setups like my agile 450 series.

[wa2ise]

Be aware that some TV sets had the audio output tube's cathode act like a power supply for the IF strip. That the audio output tube's B+ was around 300VDC, and the cathode biased up to around 130VDC. Which in turn fed the IF strip. That way, they didn't waste power thru resistors to get from 300VDC (mainly for the deflection circuits) to something about half that to power the audio and the IF. The audio output and the IF strip stages were effectively in series.

Also, if you have a source of S-video, you can get a much better color picture if you can inject the luma and the chroma signals to their respective points after the chroma-luma separator filters inside the color TV set. But this isn't that easy to do. Advanced experts only on that one.