Looking for some help to build an RF Modulator schematic from 1985 I found online :)

Thread Starter

ausys

Joined May 1, 2022
8
Hey all, I am looking for some help to build an RF Modulator schematic from 1985. I was searching for a way to wirelessly transmit a UHF/VHF TV signal to an old portable TV I have and found a schematic on a website called Petlibrary (https://petlibrary.tripod.com/rfmod.htm).

My idea was to use an old digital set-top box with a small antenna to pick up the free-to-air signal, or perhaps a composite input, then modulate it, and again transmit it with an antenna to my portable TV. (This will also require modifying the box to run on a battery but I will get there eventually.)

It seems to be a detailed article but I don't have much experience modifying or building circuits on single layer prototype boards, nor do I have any experience with winding inductors. I will mention that I have done some research on replacement components and have seemed to found a decent list. If anyone could guide me on both what input I should use, and how I should lay out the circuit without the power circuit within (denoted by * on the website) it would be much appreciated! :)
 

bassbindevil

Joined Jan 23, 2014
422
Tin box RF modulators were a common off-the-shelf item in the days when most TVs didn't have baseband video inputs. It should be easy to find one on ebay, though when I looked just now, it was mostly "plastic box" modulators from more recent decades, which are very commonly found in thrift stores for a few bucks. (There's also a good chance of finding them inside sufficiently old VCRs, game consoles and TV set-top boxes.)
 

Thread Starter

ausys

Joined May 1, 2022
8
Tin box RF modulators were a common off-the-shelf item in the days when most TVs didn't have baseband video inputs. It should be easy to find one on ebay, though when I looked just now, it was mostly "plastic box" modulators from more recent decades, which are very commonly found in thrift stores for a few bucks. (There's also a good chance of finding them inside sufficiently old VCRs, game consoles and TV set-top boxes.)
I found similar devices when looking on ebay as well, makes it a bit hard to find a good device. Thrift stores are a good idea, I will have a look around for some near me that sell electronics and will hopefully get lucky! I do have a set-top box lying around, I believe it says RF Loop on the antenna ports however, is that any different to what I need?
 

bassbindevil

Joined Jan 23, 2014
422
Hard to say; RF loop sounds like a UHF antenna, so I'm not sure what that would be for. Here's an example of the sort of RF modulator I frequently see in thrift stores:
https://www.ebay.com/itm/125278870869
Now, re-reading your original post, you want to do this wirelessly. These gizmos are all intended to be connected directly to the TV's antenna inputs, so output levels could be too low to reach far wirelessly. A comment on the following Instructible said they only got 10 to 20 feet.
https://www.instructables.com/How-to-Make-a-Simple-but-effective-tv-transmitter-/
But, there might be an attenuator in the antenna output of those boxes that could be removed or shorted to increase output power. (That's true of some car FM modulators.) Or, easier, use a cable TV booster amplifier, if you can lay your hands on one.

There have been magazine projects for TV transmitters. Nuts & Volts August 2003, and the kit was available from North Country Radio:
https://web.archive.org/web/20130105044844/http://www.northcountryradio.com/Kitpages/lptvx.htm
Ramsey sold a TV transmitter kit, the TV6C. Electronics Now magazine may have published the plans for that in the '90s, or possibly back in the '80s when the magazine was called Radio-Electronics.
 
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Thread Starter

ausys

Joined May 1, 2022
8
Can't say I have come across any of those devices in thrift stores, they seem to be a bit less common in Australia. I may have to build my own from the article in my original post because of the scarcity of these devices, and any help on single layer circuit boards would be appreciated as well. 10 to 20 feet would be fine initially, but I agree a TV booster or plans for one would be a good addition to increase range and should be easy enough to build.
 

bassbindevil

Joined Jan 23, 2014
422
Do you need audio as well as video? The schematic you linked to only does the video. As did a transmitter I found in Electronics Now, December 1993 "Build A Miniature Video Transmitter".
https://worldradiohistory.com/Archive-Radio-Electronics/90s/1993/EN-1993-12.pdf
Double-sided, but it looks reasonably easy to make as a one-off. Print both sides at scale, drill the holes through the paper, then join the dots with ink/paint, or cover the copper with packing tape and cut away the areas that need etching.
 

Thread Starter

ausys

Joined May 1, 2022
8
Audio would definitely be nice to have, would it be as simple as using an am/fm transmitter and getting to the same frequency as the video? Will have to do some research on the double sided process as it sounds a bit out of my depth for the time being.
 

bassbindevil

Joined Jan 23, 2014
422
NTSC audio is transmitted using FM on a separate frequency about 4.5 MHz away from the AM video signal. I found what looks like the schematic and other details of the Ramsey TV6C transmitter kit, most likely scanned from Electronics Now magazine. Some people on forums said it didn't work well or at all when they tried building one.
http://electronics-diy.com/electronic_schematic.php?id=851

But, Australia uses a different type of analogue TV, PAL B, with audio offset by 5.5 MHz. This transmitter looks much simpler, although the description of the RF transformer used for the audio section is vague, so some trial-and-error and instrumentation might be required to make it work.
http://electronics-diy.com/electronic_schematic.php?id=846.

Searching Elektor turns up one from way back in 1976, but video only.
https://www.elektormagazine.com/magazine/elektor-197607/57694
And this one from 1990 includes audio, but it's based on a TDA5660P chip that will be harder to find and cost more than a complete RF modulator.
https://www.elektormagazine.com/magazine/elektor-199003/32089
page 41 in this barely legible scanned copy:
https://worldradiohistory.com/UK/Elektor/90s/Elektor-1990-03.pdf
 

Thread Starter

ausys

Joined May 1, 2022
8
Ah a signal offset would make sense. The Australian circuit is pretty vague hey, will definitely have to do some research on that one. The Elektor Modulator looks much nicer in comparison to the others, and is much more detailed. I didn't find the document to be too illegible, but you are correct in saying the TDA5660P chip is hard to come by! A quick google search rendered out of stock results and as you said the chip was more expensive than an entire unit. I may have to do some digging for modern schematics of RF Modulators like the ones on ebay. Otherwise I can separate the transmitters and use the original one I linked for video, and a kit FM transmitter I have laying around.
 

debe

Joined Sep 21, 2010
1,294
Why would you even bother to build one. Theres plenty to be salvaged from old VCRs for free & they work great. Ive scraped lots of VCRs as i no longer repair them & thrown out all of the workshop manuals & spare parts.
 

Thread Starter

ausys

Joined May 1, 2022
8
Why would you even bother to build one. Theres plenty to be salvaged from old VCRs for free & they work great. Ive scraped lots of VCRs as i no longer repair them & thrown out all of the workshop manuals & spare parts.
Building one off of my original link is not only cheaper than buying a VCR just to scrap it, but is also more interesting than doing so. I see that you're in SA, it may be different for you but I haven't seen any free VCRs where I live in VIC, nor ones that are cheaper than a brand new RF Modulator from ebay, even in thrift stores.
 

bassbindevil

Joined Jan 23, 2014
422
Asking on some local online buy and sell or Freecycle group might get you a VCR or old game console or satellite set-top box. I don't see many VCRs even in thrift stores these days, but I'll bet there's a bunch gathering dust in attics and basements. Possibly stacks of them at local electronics repair shops. Or asking nicely at a recycling yard might persuade them to put some aside for you.
 
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