Question on how to modify an old TV with AV inputs

[Aussie Bloke]

Hi all. I have a technical question.

I've got a late 1970s Princess (Australian brand) colour TV with only a VHF tuner and a 1990s portable B&W Radio Shack Portavision TV set and I want to modify them by fitting AV input sockets to them so I can directly input video to them.

However I'm not very certain on how to go about it as my technical skills are a bit limited. My wild guess is running them off the audio and video outputs of the tuner module, is that correct?

Anyways if anyone can give me directions I'd most appreciate it.

Cheers
Troy

[steerpike2]

Many (most?) TV sets of around that time (before switched mode power supplies became common) used a live chassis. I.e, circuit 'ground' is tied directly to one side of the AC mains. You *cannot* add AV inputs to these sets without adding a mains isolating transformer (240v:240V) which is heavy and expensive if big enough for a large colour TV.
Still, if you want to proceed.... :-)
The tuner module (i assume you mean the silver metal box) probably only puts out the video- and sound intermediate frequencies - no good for AV in.

Audio you can easily tap into the 'hot' end of the volume control potentiometer - UNLESS it uses DC controlled volume, (look for a TBA120 chip - a comon one that does the sound detection - you could tap into its output pin)
Video is trickier - does the set use ICs, or is it all discrete transistors? If it uses ICs, check the part numbers until you find the one that does the composite video processing - ONE of its pins will be a suitable input. (if it used the TBA line of chips - I can give u pin numbers to tack into)

[AndrewM]

An easier alternative to modifying the sets is to use a VHF RF modulator to convert the video to rf. These are available from various sources including Ebay. If you don't mind experimenting a bit, older VCR's have VHF RF modulators in them that can be re-engineered into a converter box. The only drawback with a RF modulator is a loss of video quality.

If you must have a direct composite video input, you will have to do a fair bit of circuit tracing to find a suitable injection point for the video. As mentioned in the previous post, any video IC's in the set will help provide some clues as to where to do this.

Your Princess is quite elderly and will most likely have several IC's and transistors in the video stages. I'm not sure who the original maker of the chassis is as I think Princess was a brand name only. If the chassis can be identified, I may be able to help with some circuit information.