Modify an old VCR TV modulator to channel 5

[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

[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.