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.