Modify an old VCR TV modulator to channel 5

[wa2ise]

I modified a salvaged TV tuner and modulator from a VCR. I changed the TV modulator to move the channel from the usual ch3 or 4, to channel 5. The modulator chip is an AN3117SA, download the datasheet here http://www.digchip.com/datasheets/pa...umber=AN3117SA.

What I did was replace the saw crystal resonator (which has inside xtals for channel 3's video carrier, and channel 4's), with a 25MHz xtal salvaged from a computer board. Looks like this chip is oscillating on the 3rd overtone, which is approx 75Mhz, which is fairly close to channel 5's video carrier. Apparently close enough for a modern BPC TV set to pull it in on channel 5. Channel 5 is supposed to be on 77.25MHz.

I added a second crystal, 27MHz, to create a channel 6, on the 2nd xtal input of the chip. approx 81Mhz, apparently close enough to channel 6's 83.25Mhz.

[jeyurkon]

Clever, but what was your motivation for changing it?

By SAW I assume you mean Surface Acoustic Wave. I had never thought about trying a regular crystal in place of one.

John

[wa2ise]

Originally Posted by jeyurkon

Clever, but what was your motivation for changing it?

By SAW I assume you mean Surface Acoustic Wave. I had never thought about trying a regular crystal in place of one.

John

If I wanted to build a miniature MATV (master antenna) system, this will give me a second channel I can have along with another TV modulator at channel 3 or 4. One STB DTV converter feeding this channel 5/6 modulator, and a 2nd on the more usual 3/4.

I wasn't sure if subbing these crystals for a SAW resonator would work, but I tried it and it did work! Worked with two different xtals, so odds are it will work for someone else who has a modulator like this one.

Be aware that there is another style of TV modulator, which uses a PLL referenced off of a 4MHz xtal to make channel 3 video and sound carriers, and channel 4 video and sound, and trying to move one of those (by changing the 4MHz xtal) would yield channels with a wrong sound carrier. If you pop the cover off a TV modulator and only see one 4MHz xtal, odds are this is what you have.

[wilkes85]

You know, if you got digital cable or something, something like that would be awesome to have. The cable can be channel 3, the DVD player would be channel 4, the VCR would be channel 5, video game would be channel 6, etc.
Cuts back on distortion quite a bit.
Also, I just wish you could use a VCR on UHF.

[wa2ise]

Originally Posted by wilkes85

You know, if you got digital cable or something, something like that would be awesome to have. The cable can be channel 3, the DVD player would be channel 4, the VCR would be channel 5, video game would be channel 6, etc.

That would be nice, but these modulators didn't bother to remove the lower sideband below 1MHz below the video carrier. which means that it invades the immediately lower adjacent channel. In a VCR feeding a TV , a modulator was the only thing on the RF output jack, so it didn't matter if the lower sideband wasn't removed.

I might be able to put one of these modulators on channel 2, and a 2nd on channels 5 or 6. And a 3rd as stock, on ch 3 or 4. But you'd have to do 2, 4, 6. Without ch 2, you could do 3 and 5, or 4 and 6, or 3 and 6.

[wa2ise]

Just ordered new crystals and they arrived today. 18.432Mhz to get me on channel 2, 27.4688MHz for near channel 6, and 25.4563MHz for near channel 5. These are standard frequencies that DigiKey had in stock. About $2 a piece in oneies-twosies. I put in the ones for channel 2, and channel 6. The one for channel 2 is almost dead on, and the one for channel 6 puts me about 800KHz low. But closer than the 27Mhz crystal did (about 2.2Mhz low). These crystals are fundamentals, but I'm running them in 3rd overtone, which is close to but not exactly 3x the frequency.


You'll also notice that I trimmed the old tuner section off, just keeping the modulator portion. The two F connectors are tied together, and the modulator output connects to this link thru a 50 ohm resistor and capacitor. I can then daisy chain another modulator on channels 3 or 4 to this one. I need to skip channels, as there's no vestigial lower sideband filtering.

[wa2ise]

I decided to change the channel this CM7000 transmits on to channel 2 (as the crystal I used, 18.432MHz puts it almost dead correct for channel 2) by installing the above modulator in it. The existing modulator in the CM7000 looks to be the crystal-less synthesizer type that has channel 3 or 4 "hard wired" into PLLs, which also produce the 4.5MHz sound carriers as well (which renders changing the master timing crystal of 4MHz not possible, as the sound carrier ends up being wrong). So I disabled it (shorted its output to ground) and cut the F connector center pin solder lug loose and connected it to the blue coax cable running from the new channel 2 modulator (I removed its old F connectors). And I also picked off the baseband mono audio and baseband video that feeds the old modulator to feed to the new one. That's the black shielded cables. The orange wire is switched 5V tapped off the main board.

This new channel 2 output is then fed into a splitter along with another box outputting on channel 4, and the merged signals then feed TV sets. And it looks like a master antenna in a town with a channel 2 and a channel 4. And as soon as I can find another modifyable TV modulator that I'll put on channel 6, then I'll have 3 channels. That's as many as I can do without lower sideband filtering.

Looks like this modulator was made by Panasonic, as I see that [M] logo on the top shield.

[jeyurkon]

I wonder if you're the first to modify a CM7000. They've been out for less than a year haven't they?

John

[bgadow]

Some B+K TV Analysts will output on any channel-could something like that be modified? I'm not smart enough to know. I recently had pass through my hands a Sencore Color King IV and it would do any channel as well, though from reading the manual it seems like they accomplished this by "broadcasting" on a wide swath of channels at once.

[sunchasers]

I have a 2 year old ILO TV from Walmart that's a piece of junk. The motherboard failed and will accept only channel 2 input (no replacement parts available). I'm pretty good with a soldering iron, but weak on electronic knowledge. How difficult is it to modify a VCR to output on channel 2, instead of 3 or 4?

[wa2ise]

Originally Posted by sunchasers

I'm pretty good with a soldering iron, but weak on electronic knowledge. How difficult is it to modify a VCR to output on channel 2, instead of 3 or 4?

You'd be looking inside the VCR tuner/modulator module for something that looks like the below 4 legged creature. Many just have 4 pins without skipping a position. Usually! you want to identify the ground pin (usually 2nd from one end) and once you remove this thing from the board, install the new 2 lead crystal so one lead is on one side of the old ground pin, and the other lead the hole on the other side of the ground. If I call the ground pin 2, that means I want the crystal to connect to pins 1 and 3. Once that's done, then you may need to change the channel 3 or 4 switch position. Use a 18.432MHz crystal, which should put you on channel 2.

Some newer VCRs used a common crystal for the tuner and the modulator, those can be identified by the presense of only one crystal (usually 4MHz) and no 4 legged creature. Unfortunately, you'd be SOL if that's what you find.

[sunchasers]

The VCR tuner didn't have individual SAW resonators for channel 3 and 4. I did see an RF modulator on eBay that I can modify, but I can't find a four-pin 55.25 MHz resonator to substitute for the channel 3&4.

[wa2ise]

Originally Posted by sunchasers

The VCR tuner didn't have individual SAW resonators for channel 3 and 4. I did see an RF modulator on eBay that I can modify, but I can't find a four-pin 55.25 MHz resonator to substitute for the channel 3&4.

Use a 18.432MHz 2 legged crystal. It will operate on its 3rd harmonic, approx 55.25MHz. Of course you need to figure out which of the 4 connections to connect the 2 wire crystal to. Usually, these SAW resonators have a ground lead connecting to its case. Usually tne 2nd lead from one end. Connect the crystal to holes 1 and 3 after removing the resonator.