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.

[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.

[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

[wa2ise]

Originally Posted by 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

Oh, some people were trying to reactivate the digital audio output jack, but it seems that the firmware has it disabled. And there are missing driver circuits. I made that jack a mono audio output, by the simple expedient of a pair of 10K resistors adding the left and the right channels together.

And I figured out how to make this box be a B&W only source for our vintage pre color TV broadcast standard (before RCA's NTSC color, which came out late 1953) sets. Many of those sets could present the entire 4MHz of B&W video on their CRT screens. And the chroma subcarrier would show up as a crawling fine checkerboard pattern on such sets. This switch just removes the chroma subcarrier from the composite video, and that feeds the TV modulator, both the OEM one, and my replacement.

You can see that I did this to the CM7000 a few posts ago while I had it apart, the green and yellow wires.

[wa2ise]

I did a 2nd CM7000 CECB with a modulator salvaged from another VCR. This chip was marked "3129", which I couldn't find any info on. But it had a similar 4 legged 2 channel crystal SAW TV frequency module next to it, like the other one I modified earlier. But this 3129 chip looks to have separate video and sound carrier output pins, and it uses resistors and caps to merge them together. I changed the resistors to get more signal strength. The video carrier should be about 14dB stronger than the sound carrier. That works out such that the video resistor is 1/4 that used for the sound.

This box became a channel 2 source, used a 18.432Mhz crystal to get me 3x which is very close to what channel 2's RF carrier frequency should be. Only 46KHz off. Which most TVs will fine tune into.

Noticed some audio buzz on the TV set when the bright white menu or info on-screen-display is activated. First tried reducing the video level feeding the modulator chip, but later I found that the sound carrier LC tank circuit was slightly off frequency, so I tweaked it and most of that buzz went away. Only time the buzz happens is with the menus, but not with program material, so I'll forget about it.

Just got some 28MHz fundamental crystals, and they put me at 84MHz, which turns out to be one of the frequencies CATV systems use for channel 6, and the BPC set easily tunes it, and so does a Panasonic VCR I have here also has no problem tuning it as well.

[wa2ise]

Originally Posted by wa2ise

Just got some 28MHz fundamental crystals, and they put me at 84MHz, which turns out to be one of the frequencies CATV systems use for channel 6, and the BPC set easily tunes it, and so does a Panasonic VCR I have here also has no problem tuning it as well.

Hooked up a digitally tuned FM radio broadcast receiver to see what frequency the sound carrier landed on. Found it at 88.3MHz, which means that the picture carrier, 4.5Mhz lower, must be at 83.8MHz. Which is a little less than 3X the 28MHz fundamental frequency. This means that the TV modulator chip is running this crystal at the 3rd overtone, which is close to but not exactly 3X. Some hard core technical info, http://tonga.globat.com/~ko4bb.com/T...with_notes.pdf, page 59, shows crystals have a fundamental, a 3rd overtone, 5th overtone, and maybe more. That's where I got the below diagram. Anyway, in my case, a 28Mhz fundamental crystal looks to have a 3rd overtone at 83.8MHz, which is 2.9929 times the fundamental. As CATV systems use a range of frequencies for channel 6, from 83.25 to 85.25 and digital tuners need to hunt thru this range, and my 83.8MHz is in this range.

So why didn't I just get a 3rd overtone crystal made at the right frequency? I could, but that costs a lot more than using commonly available cheap crystals DigiKey stocks.

[wa2ise]

I'm sure that this thread is pretty well played out, but there was one last problem I was chasing after for the past few weeks with the use of some of the TV modulator chips. In the CM7000. THese chips and associated circuitry I salvaged from old VCRs all ran off of +5V, and the CM7000 has a 5V supply, so naturally I used it. Well, turns out that one of the chips had good power supply ripple rejection (PSRR), but two other chips did not. And it seems that the CM7000 draws irregular bursts of current while it tries to cope with bad or non-existent reception on a DTV signal. Which makes the 5V and the 12V supplies bounce up and down around 50mV. The power supply tightly regulates the 3.3V supply, and the 5V and 12V not really regulated (feedback taken off of only the 3.3V). Well, that much ripple doesn't bother digital circuits, and analog audio and video circuits (mostly op-amps), but it made some of the TV modulator chip outputs flicker on the TV set screen. At first thought I had a poorly performing power supply board, and changed a few caps on it. Then I swapped power supply boards on both my CM7000's, and the problem stayed with the TV modulator, and not the power supply board.

Well, to make a short story longer, I decided to forget using the 5V supply, but to grab a 7805 regulator chip and use the 12V and make it into a clean 5V. Problem solved!

For added fun, you could extend this TV modulator mod trick to create a channel 1 for your late 40's TV sets that had a channel 1 on the dial, you need a crystal that puts the video carrier at 45.25MHz. You'll want to exercise some care that this channel 1 signal doesn't get into the DTV's IF strip, so keep it all out of the tuner can, and not route it to the RF output jack inside the tuner can.

Or you could produce a TV set IF signal at 45.75MHz, to feed directly into a TV set's IF strip (disconnect the tuner front end coax cable that fed the TV set IF, and have the modified TV modulator feed the IF strip instead. This would let you do minimal mods to a TV set, but let you hide the converter box inside the TV set, and make the TV remote controllable.

[wa2ise]

Had some trouble moving a TV modulator to operate on the old channel 1 frequency, 44.25 picture carrier, or 47.75 if I wanted to inject it directly into a modern 41-48 MHz video IF strip. From a huge stash of old police radio xtals the closest frequencies I could find was 45.3 for channel 1 (only 50KHz off) and 45.638MHz (112Khz off, a little more problematic in that there'd be no fine tuning to dial it in to fit the IF strip filter better).

Before, the modulator osc circuit wouldn't run at frequencies this low, but I increased the loading caps (caps from the xtal to ground). From about 4pF to 8pF. Now it runs. Seems that lower frequency xtals need bigger loading caps. Likely to yield a similar reactance at the new freq like that with the old cap at the old freq. Those of you who have experience in xtal osc circuits would recognize this, probably better than I would. Some hard core engineering info: http://www.analogzone.com/hft_0102.pdf
Figs 5 and 6 show that larger caps shift the osc freq range down. Crudely speaking, this would make a circuit that worked at a higher xtal frequency work at a lower freq xtal.

[wa2ise]

Set the digital cable box to 3, and modify the above Channel master to channel 2 or 5. Then use the TV's channel tuner to pick which service you want to use.