Modify an old VCR TV modulator to channel 5

[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.

[Old1625]

You are pretty darned clever! And a dang sight more motivated than I am at this point to engage in such stuff. Good on ya!