View Full Version : Modified TV modulator on channel 6


wa2ise
10-27-2017, 03:26 PM
Another TV modulator (Archer 15-1273 C2S7QZ) moved to channel 6 (a little off, but that's what "fine tuning" is for) :D 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.
http://videokarma.org/attachment.php?attachmentid=195830&stc=1&d=1509131762
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).

Jon A.
10-27-2017, 04:13 PM
You've done it again Bob! :thmbsp: 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.

MadMan
10-28-2017, 12:32 AM
Noice!

kf4rca
10-28-2017, 09:04 AM
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.

wa2ise
10-28-2017, 04:00 PM
You've done it again Bob! :thmbsp: 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.