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 Post subject: Modifing a Radio Shack TV RF modulator for channels 2 and 6
PostPosted: Jul Mon 01, 2013 6:36 am 
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Joined: Aug Mon 16, 2010 7:01 pm
Posts: 146
Location: USA
I removed the SAW resonator from this modulator, and substituted 2 crystals. For channel 2 I used a 18.432MHz crystal (which seems to be a common modem frequency). For channel 6 I used a crystal I got from an old toy CB walkie talkie, 27.125MHz. Ideally I'd want 27.75MHz. But this CB crystal puts a TV channel only 1.9 MHz lower than where channel 6 should be. Most TV sets' fine tuning will let you tune this in properly. If your TV set has individual tweakable oscillator coil adjustments (like a terrete tuner) you could adjust your channel 6 a little.

If you get two of these modulators, put one on channel 2, the other on channel 6, and use a standard unmodified one on channel 4, feed them all to a 3 way splitter and then to your TV set, you have created a miniature cable head end. There's no lower sideband suppression, so channels 3 and 5 are not usable. With 3 CECBs, say one tuned to Antenna TV, RTV (retro TV) and there was a channel carrying Lassie and the Lone Ranger, you could channel hop the old fashioned way. Or use a DVD player with vintage movies. If you are feeding an old B&W TV like an Admiral bakelite console, use the luma video output, so there's no chroma subcarrier to junk up the reception (B&W TVs before 1953 often had full 4.2MHz of luma bandwidth).


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 Post subject: Re: Modifing a Radio Shack TV RF modulator for channels 2 an
PostPosted: Jul Tue 02, 2013 3:30 am 
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Joined: Dec Tue 02, 2008 3:10 pm
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Location: Ontario, Canada
That's a fun idea! I wonder how many of your neighbours you could supply a feed with? (Semi-secretly, of course)

Eric


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 Post subject: Re: Modifing a Radio Shack TV RF modulator for channels 2 an
PostPosted: Jul Tue 02, 2013 4:30 am 
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Joined: Jul Wed 08, 2009 12:17 am
Posts: 353
Location: Michigan
Quote:
If you get two of these modulators, put one on channel 2, the other on channel 6


Does the channel selector switch on pin 7 choose between the two crystals?


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 Post subject: Re: Modifing a Radio Shack TV RF modulator for channels 2 an
PostPosted: Jul Tue 02, 2013 5:30 pm 
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Joined: Jul Wed 08, 2009 12:17 am
Posts: 353
Location: Michigan
wa2ise wrote:
...If you are feeding an old B&W TV like an Admiral bakelite console, use the luma video output...


Use it to drive pin 16?


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 Post subject: Re: Modifing a Radio Shack TV RF modulator for channels 2 an
PostPosted: Jul Fri 05, 2013 8:09 pm 
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Joined: Dec Thu 06, 2007 11:54 pm
Posts: 1553
Location: Hayward, California USA
Very cool modification!

I wonder if you could put in a 15.0 or 15.08 MHz crystal and make a Channel 1 :shock: modulator?

_________________
Quote: (Antique TV collecting) always seemed to me to be a fringe hobby that only weirdos did.


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 Post subject: Re: Modifing a Radio Shack TV RF modulator for channels 2 an
PostPosted: Jul Wed 10, 2013 8:36 pm 
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Joined: Aug Mon 16, 2010 7:01 pm
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Location: USA
Yes, luma would feed pin 16, the same way a composite video signal would. Thru C4 and VR2. And the switch would select which crystal will operate the chip.

I did modify one of these to go on the old channel 1. I found I had to increase slightly C1 by a picofarad or so, and maybe C2 as well. To get this crystal to oscillate. A crystal at 15.083MHz should put you on 45.25MHz, where channel 1's picture carrier used to be. I'd make the other crystal for channel 2, which shouldn't mind the increased caps.


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 Post subject: Re: Modifing a Radio Shack TV RF modulator for channels 2 an
PostPosted: Jul Fri 12, 2013 5:12 am 
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Joined: Jul Wed 08, 2009 12:17 am
Posts: 353
Location: Michigan
For some reason I thought you were modifying the Radio Shack (Digitalstream) DTV converter box's backend modulator, and was patching a luma signal internally from some (unstated) point to pin 16. So sorry for the dumb-sounding questions. I see you mean the standalone RF modulator, which of course you can drive from any external luma source to get good B&W output.

Now that I'm on the right page, do you think it would be possible to drive pin 9 or 10 with a standard RF signal generator? Perhaps attenuated with a simple additional circuit? I have an el-cheapo that tunes from 0 to 250MHz. It would make a nice bench-test piece of equipment, to be able to sweep up the VHF band and check all the tuner circuits with one standalone piece of equipment.

Just thinking out loud...


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