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...

[wa2ise]

Success.

Finally convinced the LA7161 chip to modulate onto channel 13. Seems VCRs in Taiwan use channel 13 for their outputs, same channel 13 in the USA. What I did was to parallel with the adjustable coil that was used originally for channel 3 or 4, and I had moved to cable channel 95 (same as Japan's channel 1) by reducing the capacitance (I series-ed a pair of 10pF or so SMT caps), and wired the control pin (as mentioned in my previous post) to make the chip do Japan channel 1. Okay, according to the data sheet, this same control voltage along with a new 270K resistor from the crystal osc pin to Vcc will tell the chip to do channel 13. But you have to get the LC circuit to be resonant at channel 13 (around 210MHz). So paralleling the above adjustable coil with another smaller coil should do it. I pulled a few likely looking candidate coils out of a dead VCR tuner/IF module and futzed around. Using a test lead point (made of brass, which makes inductors lower, inverse of ferrite rods) I got channel 13 or appear momentarily. Which meant I was halfway home (anyone who ever messed with VHF LC circuits will know what I mean). So reducing the number of turns, spreading or squeezing the coil to get it on resonance eventually happened. And I used the old adjustable coil to fine tune it in. The shield cover posed a problem, as the new coil was on the back of the board, nearly touching the shield. So I just cut a hole in the shield to avoid detuning the coil. Oh, there would be a slight amount of leakage, but not enough to worry about.

[Electronic M]

Very cool! Now all it needs is a tuned antenna, and low power RF amp designed around common TV tubes to boost the signal to a level that can be received ~200' away...Then it would be a fun ~20$ alternate to buying a Blonder Tongue agile modulator.

[JBL GUY]

Very interesting.

Thanks for the information and the picture.