Mofifying a TV modulator that uses PLL chips like the LA7161 chip

[wa2ise]

The LA7161 TV modulator doesn't use crystals for channel 3 and 4, but synthesizes these carrier frequencies from a 4MHz crystal. The Data sheet
http://www.digchip.com/datasheets/pa...13/LA7161V.php is somewhat descriptive, but doesn't tell much on how to make the chip work, if you want to move it from the US channels 3 and 4 to the Japanese channels 1 and 2 (which correspond to CATV channels 95 and 96). One of the chip's pins (pin 7) is used to tell the chip which channel to create, using 4 different voltage levels. That's easy enough to provide, but what the datasheet doesn't mention (though you can see it in the internal functional block diagram) that an LC coil connected to pins 9 and 10 is used as an RF oscillator, which is PLLed by divide down circuits referenced from the 4MHz crystal. But you have to adjust this LC circuit to get it to work, and be stable. I had to change the cap of this LC circuit, which is fun to find a few pF surface mount cap and remove the old cap and solder in carefully the new cap, and use a cable ready VCR as a tuner set to channel 95 and twiddle the coil to hopefully get it to tune up. Channel 95 is in the FM broadcast band, pix carrier 91.25MHz and sound carrier 95.75MHz (you could tune the sound with an FM radio). At first I thought that the LC circuit was to create the sound subcarrier, but the sound subcarrier is actually produced by another PLL divide down chain and oscillator. This chip can supposedly do US channel 13, but I haven't gotten it to do it yet. Probably need a new LC osc circuit...

If you wanted to make this chip do PAL TV frequencies, you'd need a reference chip 4.888MHz (to get the sound subcarrier on 5.5MHz instead of 4.5MHz), and the US 4 channel would fall a little below Australian channel 3. But if Australian VCRs output on channel 3, there's little point...