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Old 10-28-2017, 03:00 PM
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wa2ise wa2ise is offline
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Join Date: Dec 2002
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Quote:
Originally Posted by Jon A. View Post
You've done it again Bob! Do you have schematics for these mods? ...
There's several similar TV modulator chips out there, so a schematic would be of limited value. However, if you have some electronics experience, you should be able to do these mods. One thing to look for in a TV modulator module is a rectangular metal cased device next to the chip. Size about 1 cm long, 1/4 inch wide, and 3mm tall. Has 4 leads, one's a ground, two pins to one side of this ground pin are the channel 3 and ch 4 resonators. Other side of the ground is a common. All these non ground pins connect to the chip. If you don't see this device, the modulator chip is a different type and would be harder to work with. But if you do find this device: Remove this device, and install a crystal one lead to the common, and the other lead to one of the ch3 or ch4 pins (whichever physically fits best). Use a crystal that has a 3rd overtone that puts it within a MHz of the desired new TV channel video carrier frequency. I used 82.7MHz crystal to get me about a half MHz to channel 6, 83.25MHz. I've also used crystals 18.432MHz (used for UART RS232 modems and such) to get channel 2 (only about 25KHz off, note that the resulting frequency will be close to but not exactly 3 times the marked frequency on the crystal). Another UART crystal 25.8048 gets you close to channel 5. And another UART frequency 27.648 gets you close to channel 6. You want fundamental crystals for these, so they will operate at the 3rd overtone to give the channel carriers you want. Looks like Mouser has these and Digikey has most of these. To test, tune your TV set to the new channel, and if you don't see your channel, switch the channel selector switch to the other channel selection. Once you know which selection works, hardwire it after removing the switch. As I mentioned above, it seems best to make the modulator a single channel unit, as a 2nd crystal made the modulator noisy.

As for the signal strength mod, some chips have a single RF output for both video and sound carriers, others have separate output pins for these. And the outputs may have resistors going to ground or B+ supply, I'd leave those alone (probably provides a DC bias to an emitter or a collector of an output transistor). Look for resistors of a few hundred ohms that are in the path to the RF output F connector, likely with coupling caps to block DC to the output. Replace with resistors 1/10 the resistance. Any resistors going to ground leave alone. There will likely be small coils, I'm not sure what they do (maybe part of a low pass filter to remove the modulator's 2nd harmonic?) as my playing with them had little to no effect. There's likely a 4.5MHz "IF" can elsewhere, you don't need to touch it.
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Last edited by wa2ise; 10-28-2017 at 03:21 PM.
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