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Old 08-17-2014, 08:28 PM
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wa2ise wa2ise is offline
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Join Date: Dec 2002
Location: USA
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Quote:
Originally Posted by old_tv_nut View Post
The simple analog ch 3/4 units could probably be modded by changing a crystal,...
I've done it. Some VCR TV modulators get the carrier frequency from the tuner circuit's reference frequency, but the ones useful to us usually use a small SAW resonator. These look like a small flat metal rectangle. Remove it carefully (to not damage the circuit board traces). The pinout is usually 4 pins in a row, a drive pin, ground, then the last two pins are for channel 3 and 4 (I don't remember which order). I've used a 18.432MHz crystal (a small one about 1cm tall and 2/10 inch pin spacing for the leads) to replace it. It will run at 3 times that frequency, which is close to channel 2's. One lead goes to the driver, and the other one feeds one of the connections for channel 3 or 4. Then tune the TV to channel 2 and play with the ch 3/4 switch to see which way it needs to be to get a signal.

See if you can read the chip number off the modulator chip. and maybe you can find a data sheet, try http://www.digchip.com/. Googling it will likely give you dead leads, as various Chinese web sites fool the Google spiders into thinking datasheets exist at their web sites. If you luck out and find a data sheet, it should have a sample circuit that will show the video carrier and sound carrier output pins, and then you can trace out the circuit in your VCR modulator. They usually have attenuation resistors from each output feeding another resistor to ground, remove the to ground resistor, and strap resistors 1/10 the ohms across the other two, this should make the output stronger. If it's surface mount, I've salvaged resistors off junk boards (easy if you use a pair of soldering irons, one tip per end of the resistor to melt both solder joints), and in this case it's easiest to just stack the new resistor atop the old one and solder it down. This for the bigger ones, like 1/10 inch long, forget the modern micro ones...
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