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Modified TV modulator on channel 6
Another TV modulator (Archer 15-1273 C2S7QZ) moved to channel 6 (a little off, but that's what "fine tuning" is for) Used a 82.7MHz crystal I salvaged from a flip phone. 550 KHz low, but TV sets handle it fine. This replaced a 2 channel SAW resonator.
I tried using a second crystal so I could have channel 5, but as it's not completely switched out of the oscillator circuit, it created noise on the selected channel. So I went with only one crystal and thus one channel. I also changed an attenuation resistor (was 680 ohms, now 33 ohms) to get about 13dB more signal strength. So I could split the RF signal to multiple sets if I want and have decent signal to noise. You could do this without changing the crystal. I also changed the half wave rectifier diode with a bridge, so the transformer doesn't end up with DC current bias (tends to cause core saturation).
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Last edited by wa2ise; 10-27-2017 at 02:33 PM. |
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You've done it again Bob! Do you have schematics for these mods? Also, what's the modulator model? Looks like an older one.
Edit: Oops, just noticed the model number. Last edited by Jon A.; 10-27-2017 at 03:16 PM. |
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Noice!
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The design specification for a TV's AFC spec is + or - 1 MHz. (Believe this is an EIA spec.)
If that modulator uses the Motorola MC1374 chip, they work best thru low band with no problem and somewhat into midband with reduced output. Most designs I have seen around this chip feature LC frequency control.
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Quote:
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. |
Audiokarma |
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